Sensitivity and specificity

Simple, Reliable Sepsis Screening Tool Improves Mortality in Trauma Patients

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Abstract

Learning Objectives The recent update to the Surviving Sepsis Campaign Guidelines advocates for the use of routine sepsis screening. We have previously shown that sepsis screening improves outcomes in surgical patients but data are lacking in the trauma population. The purpose of this prospective, observational study was to determine the utility of a novel screening tool for the early identification of sepsis in trauma patients.

Methods Patients admitted to the trauma service were screened for sepsis twice daily using our sepsis screening tool which assesses a patient’s white blood cell count, respiratory rate, temperature, and heart rate and assigns a numeric score (0 to 4) for each. Patients with a score of ≥4 screened positive. ACCP/SCCM Consensus definitions were used to diagnose sepsis. Sensitivity and specificity were calculated using a standard two by two table. Chi square analysis was used for categorical data. Values are expressed as mean ± standard deviation.

Results 10,201 screens were completed on 1,637 trauma patients admitted to a Level 1 Trauma Center. The incidence of sepsis was 7.3%. The sepsis screening tool had a sensitivity of 92.5%, specificity of 97.4%, positive predictive value of 73.5% and negative predictive value of 99.4%. There was no difference in the age (43.3±18.15 vs. 42.7±19.1, p= 0.74) or gender (74.1% male vs. 68.7% male, p=0.22) of patients with sepsis and without sepsis. Patients that developed sepsis had higher injury severity scores (23.3±12.3 vs.12.7±9.3, p<0.0001), increased hospital length of stay (31.1±26.7 vs. 8.5±8.4, p <0.0001), increased intensive care unit (ICU) length of stay (16.4±16.3 vs. 1.7±3.9, p<0.0001), and fewer ICU free days (14.7±19.4 vs. 6.9±7.1, p <0.0001). Thirty-day ICU mortality decreased from 13% to 8% (p=0.08) after implementing the sepsis screening tool.

Conclusions Sepsis in trauma patients has a significant impact on patient outcomes. Our sepsis screening tool accurately identifies sepsis in trauma patients with a high sensitivity and specificity. The implementation of routine screening was associated with improved ICU mortality rates.

Hopes Raised for Early Pancreatic Cancer Detection.

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Scientists at the Johns Hopkins University School of Medicine say a simple blood test based on detection of tiny epigenetic alterations may reveal the earliest signs of pancreatic cancer. The findings of their research, if confirmed, they add, could be an important step in reducing mortality from the cancer, which has an overall five-year survival rate of less than 5% and has seen few improvements in survival over the last three decades.

“While far from perfect, we think we have found an early detection marker for pancreatic cancer that may allow us to locate and attack the disease at a much earlier stage than we usually do,” explains Nita Ahuja, M.D., an associate professor of surgery, oncology and urology at the Johns Hopkins and leader of the study (“Novel Methylation Biomarker Panel for the Early Detection of Pancreatic Cancer”) described online this month in the journal Clinical Cancer Research.

For their study, Dr. Ahuja and her colleagues focused on two genes.

“We used a nanoparticle-enabled MOB (Methylation On Beads) technology to detect early-stage pancreatic cancers by analyzing DNA methylation in patient serum,” wrote the investigators in their journal article. “We identified two novel genes, BNC1 (92%) and ADAMTS1 (68%), that showed a high frequency of methylation in pancreas cancers (n=143), up to 100% in PanIN-3 and 97% in Stage I invasive cancers.”

Together, BNC1 and ADAMTS1 were detectable in 81% of blood samples from 42 people with early-stage pancreatic cancer, but not in patients without the disease or in patients with a history of pancreatitis, a risk factor for pancreatic cancer.

Dr. Ahuja’s team found that, in pancreatic cancer cells, it appears that chemical alterations to BNC1 and ADAMTS1, i.e., epigenetic modifications that alter the way the genes function without changing the underlying DNA sequence, silence the genes and prevent them from making their protein product, the role of which is not well understood. These alterations are caused by the addition of a methyl group to the DNA.

Using MOB, the researchers were able to single out in the blood even the smallest strands of DNA of those two genes with their added methyl groups. The technique uses nanoparticle magnets to latch on to the few molecules being shed by the tumors, which are enough to signal the presence of pancreatic cancer in the body, the researchers found.

Dr. Ahuja says the practical value of any blood test for cancer markers depends critically on its sensitivity (meaning the proportion of tumors it detects) and its specificity (meaning how many of the positive results are false alarms). The specificity of this new pair of markers is 85%, meaning 15% would be false alarms. She hopes further research will help refine the test, possibly by adding another gene or two, in order to go over 90% in both sensitivity and specificity.

Detection of Tuberculosis in HIV-Infected and -Uninfected African Adults Using Whole Blood RNA Expression Signatures: A Case-Control Study.

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Abstract

Background

A major impediment to tuberculosis control in Africa is the difficulty in diagnosing active tuberculosis (TB), particularly in the context of HIV infection. We hypothesized that a unique host blood RNA transcriptional signature would distinguish TB from other diseases (OD) in HIV-infected and -uninfected patients, and that this could be the basis of a simple diagnostic test.

Methods and Findings

Adult case-control cohorts were established in South Africa and Malawi of HIV-infected or -uninfected individuals consisting of 584 patients with either TB (confirmed by culture ofMycobacterium tuberculosis [M.TB] from sputum or tissue sample in a patient under investigation for TB), OD (i.e., TB was considered in the differential diagnosis but then excluded), or healthy individuals with latent TB infection (LTBI). Individuals were randomized into training (80%) and test (20%) cohorts. Blood transcriptional profiles were assessed and minimal sets of significantly differentially expressed transcripts distinguishing TB from LTBI and OD were identified in the training cohort. A 27 transcript signature distinguished TB from LTBI and a 44 transcript signature distinguished TB from OD. To evaluate our signatures, we used a novel computational method to calculate a disease risk score (DRS) for each patient. The classification based on this score was first evaluated in the test cohort, and then validated in an independent publically available dataset (GSE19491).

In our test cohort, the DRS classified TB from LTBI (sensitivity 95%, 95% CI [87–100]; specificity 90%, 95% CI [80–97]) and TB from OD (sensitivity 93%, 95% CI [83–100]; specificity 88%, 95% CI [74–97]). In the independent validation cohort, TB patients were distinguished both from LTBI individuals (sensitivity 95%, 95% CI [85–100]; specificity 94%, 95% CI [84–100]) and OD patients (sensitivity 100%, 95% CI [100–100]; specificity 96%, 95% CI [93–100]).

Limitations of our study include the use of only culture confirmed TB patients, and the potential that TB may have been misdiagnosed in a small proportion of OD patients despite the extensive clinical investigation used to assign each patient to their diagnostic group.

Conclusions

In our study, blood transcriptional signatures distinguished TB from other conditions prevalent in HIV-infected and -uninfected African adults. Our DRS, based on these signatures, could be developed as a test for TB suitable for use in HIV endemic countries. Further evaluation of the performance of the signatures and DRS in prospective populations of patients with symptoms consistent with TB will be needed to define their clinical value under operational conditions.

Discussion

We have identified a host blood transcriptomic signature that distinguishes TB from a wide range of OD prevalent in HIV-infected and -uninfected African patients. We found that patients with TB can be distinguished from LTBI with only 27 transcripts and from OD with 44 transcripts. Our findings appear robust as the results are reproducible in both HIV-infected and -uninfected cohorts, in different geographic locations, and in an independent TB patient dataset. The high sensitivity and specificity of the signatures in distinguishing TB from OD, even in the HIV-infected patients that have differing levels of T cell depletion and a wide spectrum of opportunistic infections as well as HIV-related complications, suggests that the signatures are promising biomarkers of TB. The relatively small number of transcripts in our signatures may increase the potential for using transcriptional profiling as a clinical diagnostic tool from a single peripheral blood sample (i.e., using a multiplex assay [35],[36]).

The major challenge for diagnosis of TB in Africa is how to distinguish this disease from the range of other conditions that show similar symptoms in countries where TB and HIV are co-endemic. Previous TB biomarker studies have focused on distinguishing patients with TB from healthy controls, or from LTBI [21],[22],[24], or have used other disease controls that may not represent the “real world” disease spectra from which TB should be clinically differentiated [19],[25]. Furthermore, these TB biomarker studies have also excluded HIV co-infected patients who are the group that most need new diagnostics. Our study design should ensure that our signatures are applicable in TB/HIV endemic countries as we recruited patients with TB concurrently with patients with a range of conditions that present with similar clinical features to TB, as well as recruiting both HIV-infected and -uninfected individuals.

We have identified separate signatures for distinguishing TB/OD and TB/LTBI, which only overlap in three transcripts. In practice the clinical applications of these signatures might be distinct as the TB/LTBI signature would be of value in contact screening, where the concern is distinguishing active disease from previous exposure in minimally symptomatic individuals. The TB/OD signature would be of most value in evaluating symptomatic patients presenting to medical services with symptoms of TB. We have also explored whether a single signature might be used to distinguish TB from both LTBI and OD. The combined signature showed lower performance to the separate TB/LTBI and TB/OD signatures. Further exploration of the operational performance of a combined signature or separate signatures is needed to establish the best strategy.

Although our signatures and DRS distinguished the majority of patients with TB from those with LTBI or OD, a proportion of patients were not correctly classified. There is increasing recognition that TB and LTBI may represent a dynamically evolving continuum, particularly in HIV-infected patients and thus failure to culture M.TB is not absolute proof that TB is not present. Some false assignment by our current “gold standard” is to be expected as noted by post mortem studies at which undiagnosed TB is confirmed [14],[15]. All patients in the OD group presented with symptoms for which TB was included in the differential diagnosis, and it is possible that TB may have been misdiagnosed in a small proportion of OD patients despite the extensive clinical investigation used to assign each patient to each diagnostic group. Some improvement in sensitivity and specificity of our DRS may also be achieved by weighting the signal from the most discriminatory transcripts, and this could be explored in subsequent refinements of the method.

A major concern in using transcriptional signatures as a clinical diagnostic tool in resource poor settings is the complexity, as well as cost, of the current methodologies. Our results have shown that transcriptional signatures can be used to distinguish TB from OD in an African setting. We explored the feasibility of a simplified method for disease categorization that may facilitate development of a diagnostic test based on our signatures. Our DRS provides a new approach that enables the use of multi-transcript signatures for individual disease risk assignment without the requirement for complex analysis. Our method could be used to develop a simple test in which the transcripts comprising the diagnostic signature (separated into those that are either up- or down-regulated in TB relative to controls) are each measured using a suitable detection system [35], and the combined signature used to identify each patient’s risk of TB. For example, a simple test using the TB/OD signature probes that show increased transcript expression in TB relative to OD could be located in a single well or tube, and those probes that show reduced transcript expression in TB located in a second well or tube. Binding of RNA from a patient’s blood to these probes could be detected as a combined signal from each tube using one of the aforementioned detection systems. To allow normalization, expression of up- or down-regulated transcripts in an individual patient could be compared with that of housekeeping genes, which do not show variation between healthy and disease states. There are methods for rapid detection of multi-transcript signatures including lateral flow reverse transcription (RT)-PCR based systems, nano-pore technology [37], nano-particle enzyme linked detection [38],[39], and detection using nano-wires and electrical impedance [40]. Some of these may be suitable for direct analysis of multiple transcript signatures in blood and at a relatively low cost.

While this study provides a proof of principle that relatively small numbers of RNA transcripts can be used to discriminate active TB from latent TB infection and OD in Africa, limitations remain that need to be addressed in order to translate these results into a clinical test. One such limitation is that our study has not assessed performance of our DRS in patients treated for TB solely on the basis of clinical suspicion, without any microbiological confirmation. Amongst these “probable/possible” patients with TB, there is no gold standard to evaluate any new biomarker. Exclusion of probable/possible patients with TB may have produced better estimates of sensitivity and specificity than would be achieved in a prospective “all comers” study including the entire cohort of patients in whom TB is included in the differential diagnosis. Thus, further evaluation using a prospective population based study in which the decision whether and when to initiate TB treatment is evaluated against the new biomarker is required. Future studies will also be required to refine the use of these biomarkers in a clinical decision process either as an initial screening tool, or in conjunction with more detailed culture based diagnostics.

From a clinical perspective a simple transcriptome-based test that reliably diagnoses or excludes TB in the majority of patients undergoing investigation for suspected TB, using a single blood sample, would be of great value, allowing scarce hospital resources to be focused on the small proportion of patients where the result was indeterminate. The challenge for the academic research community and for industry is to develop innovative methods to translate multi-transcript signatures into simple, cheap tests for TB suitable for use in African health facilities.

SOURCE: PLOS

Diagnostic Accuracy of Quantitative PCR (Xpert MTB/RIF) for Tuberculous Meningitis in a High Burden Setting: A Prospective Study.

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Background

Tuberculous meningitis (TBM) is difficult to diagnose promptly. The utility of the Xpert MTB/RIF test for the diagnosis of TBM remains unclear, and the effect of host- and sample-related factors on test performance is unknown. This study sought to evaluate the sensitivity and specificity of Xpert MTB/RIF for the diagnosis of TBM.

Methods and Findings

235 South-African patients with a meningeal-like illness were categorised as having definite (culture or Amplicor PCR positive), probable (anti-TBM treatment initiated but microbiological confirmation lacking), or non-TBM. Xpert MTB/RIF accuracy was evaluated using 1 ml of uncentrifuged and, when available, 3 ml of centrifuged cerebrospinal fluid (CSF). To evaluate the incremental value of MTB/RIF over a clinically based diagnosis, test accuracy was compared to a clinical score (CS) derived using basic clinical and laboratory information.

Of 204 evaluable patients (of whom 87% were HIV-infected), 59 had definite TBM, 64 probable TBM, and 81 non-TBM. Overall sensitivity and specificity (95% CI) were 62% (48%–75%) and 95% (87%–99%), respectively. The sensitivity of Xpert MTB/RIF was significantly better than that of smear microscopy (62% versus 12%; p = 0.001) and significantly better than that of the CS (62% versus 30%; p = 0.001; C statistic 85% [79%–92%]). Xpert MTB/RIF sensitivity was higher when centrifuged versus uncentrifuged samples were used (82% [62%–94%] versus 47% [31%–61%]; p = 0.004). The combination of CS and Xpert MTB/RIF (Xpert MTB/RIF performed if CS<8) performed as well as Xpert MTB/RIF alone but with a ~10% reduction in test usage. This overall pattern of results remained unchanged when the definite and probable TBM groups were combined. Xpert MTB/RIF was not useful in identifying TBM among HIV-uninfected individuals, although the sample was small. There was no evidence of PCR inhibition, and the limit of detection was ~80 colony forming units per millilitre. Study limitations included a predominantly HIV-infected cohort and the limited number of culture-positive CSF samples.

Conclusions

Xpert MTB/RIF may be a good rule-in test for the diagnosis of TBM in HIV-infected individuals from a tuberculosis-endemic setting, particularly when a centrifuged CSF pellet is used. Further studies are required to confirm these findings in different settings.

Discussion

Although the utility profile and accuracy of Xpert MTB/RIF has been well characterised in sputum samples, there are hardly any data to guide its utility and implementation for TBM. This is critical as the rollout of Xpert MTB/RIF means that quantitative PCR is now available in many high burden settings, and data are urgently required to guide appropriate and relevant usage of this technology in biological fluids other than sputum. That Xpert MTB/RIF performs poorly in fluids from some compartments, e.g., the pleural space, highlights the need for such data [27]. The key findings of this study were as follows: (1) Xpert MTB/RIF is likely a good rule-in test for the diagnosis of TBM in HIV-infected patients; (2) centrifugation of the sample improved sensitivity in this context to almost 80%; (3) among HIV-infected patients, Xpert MTB/RIF performed significantly better than the widely available same-day alternative tests, i.e., smear microscopy, which suggests that prompt diagnosis of TBM is potentially achievable in the majority of patients in this setting; (4) the diagnostic value of Xpert MTB/RIF for HIV-infected patients is clinically meaningful given that it performed significantly better than hypothetical decision-making based on clinical characteristics and basic laboratory data (the CS); and (5) when combined with the CS, Xpert MTB/RIF test usage could be reduced by only a modest ~10% whilst retaining similar sensitivity and specificity compared to using Xpert MTB/RIF alone. This last finding informs clinical practice in resource-poor settings. Finally, we quantified the limit of detection of the assay, its relationship to bacterial load, and the impact of PCR inhibition. These data require reproduction in HIV-uninfected and non-TB-endemic populations.

There are limited data about Xpert MTB/RIF performance in TBM [28]. Published data include only small numbers of microbiologically proven TBM cases (range of 0 to 23) [29]–[32], often in a case-control design with a non-uniform reference standard, and often CSF-associated data were published as part of a laboratory-based evaluation of extrapulmonary TB samples, usually including samples from countries with low TB prevalence. Furthermore, there are no studies from high burden settings, and technical performance evaluations, including bacterial load studies, threshold level of detection, and impact of PCR inhibition, have hitherto not been undertaken.

Xpert MTB/RIF sensitivity was as high as 80% when a centrifuged CSF sample from an HIV-infected patient was used. This suggests that Xpert MTB/RIF, at least in an HIV-endemic environment, represents a possible new standard of care for the diagnosis of TBM. Sensitivity was considerably better than in previous studies using commercially available or non-standardised PCR tools [9],[32]–[34]. The ostensibly better performance is likely related to a combination of centrifugation (and hence concentration of bacilli) and technical aspects, including a more efficient standardised extraction protocol, fractionation of mycobacteria by a pre-sonication step, and a nested PCR protocol, thus maximising amplification. However, possibly higher bacterial loads in HIV-infected patients may have also played a role. Our findings have practical relevance because they imply that at least 3 ml of CSF should be set aside and centrifuged, and re-suspended in phosphate-buffered saline, before being run on the Xpert MTB/RIF. This high-sensitivity and potentially rapid diagnosis in most cases is likely to benefit HIV-infected patients suspected of having M.tb., as diagnostic and treatment delay is associated with higher mortality [35]–[37]. Impact-related studies are now required to verify this hypothesis. It is noteworthy that a second sample improved sensitivity minimally. These data suggest that, at least in an HIV-endemic setting, using a second cartridge is unlikely to give further benefit. However, larger studies are required to confirm this possibility.

Similar to the findings when using sputum, the level of detection of Xpert MTB/RIF was between 80 and 100 colony forming units per millilitre. This explains the sub-optimal sensitivity of Xpert MTB/RIF compared to culture, where the detection threshold is as low as 1–10 organisms per millilitre [38]. We did not find a correlation between TTP and Xpert MTB/RIF CT values, as has been shown in sputum [39]. In contrast to previous PCR-based studies [40],[41], we found that CSF had a minimal inhibitory effect on the PCR reaction when compared to sputum. This may be due to the wash step incorporated into the assay that removes extracellular debris. We did not find a difference in TTP between the Xpert MTB/RIF–positive samples from centrifuged versus uncentrifuged CSF. This may be due to a type two statistical error, as the sample numbers were small.

There were three patients who were culture negative but Xpert MTB/RIF positive, i.e., Xpert MTB/RIF positive in the non-TB group. Our previous work has shown that such cases (Xpert MTB/RIF positive but culture negative) are likely to be true TB positives, and this is corroborated by high specificity obtained in large sputum-based studies where a significant minority of the patients had had previous TB [11]. If these culture-negative Xpert MTB/RIF–positive individuals are hypothetically designated definite-TB cases, then the overall case detection rate improves by a further ~10%.

The proper and meaningful value of a test lies in its ability to influence patient management through its incremental value over pre-test probability, or to have an impact on decision-making based on logical clinical judgement (based upon clinical features and basic laboratory parameters). We therefore derived a CS, hitherto unavailable for HIV-endemic settings, to evaluate Xpert MTB/RIF utility in clinical practice. Xpert MTB/RIF had significantly better performance outcomes than the clinical prediction rule (using a rule-in cut point, so appropriate comparisons could be made). Furthermore, hypothetically combining the CS with Xpert MTB/RIF resulted only in a modest ~10% reduction in test usage, but still maintained high sensitivity and specificity. These data suggest that clinical algorithms or scoring systems to limit test usage are unlikely to be significantly useful in resource-poor settings.

There are several limitations of our study. We could not determine the impact of Xpert MTB/RIF (time and proportion of patients initiated on treatment) compared to a smear microscopy/empiric treatment-based strategy given our study design and the fact that management decisions were not based on Xpert MTB/RIF results. However, this was because Xpert MTB/RIF had not yet been endorsed by the World Health Organization when the study commenced, had not been validated for use in CSF, and had been used as a research tool only (thus, for ethical reasons, study samples were evaluated only several weeks later). Although the confidence intervals of some of our estimates are wide (because of limited sample numbers), this is to our knowledge the largest diagnostic study undertaken in TBM (based on the number of microbiologically proven TBM cases; n = 59). This reflects the challenge and difficulty in performing such studies in resource-poor settings. It is possible that the Xpert MTB/RIF performs much better in HIV-infected individuals because of a possibly higher bacterial load, and thus our findings need to be confirmed in other settings. Given the small number of HIV-uninfected patients, we were unable to meaningfully compare this sub-group. The CS was developed to assess only incremental value above basic clinical and CSF parameters. The CS and the combination of CS plus Xpert MTB/RIF need prospective and independent validation. The non-significant difference in sensitivity between the paired centrifuged and non-centrifuged samples may reflect a type two statistical error, as the number of culture-positive paired samples was limited. Lastly, there were nine patients who could not be categorised within our defined groups and were excluded from the analysis.

In conclusion, Xpert MTB/RIF may be a good rule-in test for the diagnosis of TBM in HIV-infected individuals in a TB-endemic setting, particularly when a centrifuged CSF pellet is used. A second Xpert MTB/RIF test had minimal incremental benefit. Smear microscopy and the CS, when combined with Xpert MTB/RIF, only modestly minimised test usage in a resource-poor setting. Further studies are now required in non-HIV-endemic settings, and using validated scoring systems, to evaluate the impact of Xpert MTB/RIF on diagnostic accuracy, and morbidity and mortality in patients with TBM.

Source:PLOS

Utility of the Xpert MTB/RIF Assay for Diagnosis of Tuberculous Meningitis.

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Tuberculous meningitis (TBM) is characterized by copious cerebrospinal fluid (CSF) inflammation and yet few Mycobacterium tuberculosis. This combination creates a disease that is notoriously difficult to definitively diagnose. In this week’s issue of PLOS Medicine, Patel and colleagues report the diagnostic performance of the GeneXpert system’s Xpert MTB/RIF assay for the diagnosis of TBM in a cohort of 204 South African, predominantly HIV-infected, adults presenting with suspected meningitis of whom 59 had definitive TBM [1]. The Xpert MTB/RIF assay’s overall sensitivity was 62%, and specificity was 95%. The performance was better using larger volumes of centrifuged CSF among HIV-infected persons with sensitivity of approximately 80% and excellent specificity for microbiologically confirmed TBM. Xpert MTB/RIF performance was less impressive using uncentrifuged CSF with a sensitivity of ≤50%, and Xpert MTB/RIF performance was negligible in HIV-uninfected persons.

What Is GeneXpert?

The GeneXpert System (Cepheid) is a single use cartridge-based real-time PCR fully automated system that performs sample decontamination, sonication, automated nucleic acid amplification, and fluorescence-based quantitative PCR [2]–[4]. The Xpert MTB/RIF assay, developed by David Alland, detects M. tuberculosis DNA in approximately 2 hours with minimal hands-on time[3]. This new technology was endorsed by the World Health Organization in December 2010, and as of June 30, 2013, a total of 1,402 GeneXpert instruments and over 3 million Xpert MTB/RIF cartridges have been procured in the public sector in 88 countries [5]. The concessional pricing is US$9.98 per cartridge for 145 low- and middle-income countries [6]. The same GeneXpert platform also can be used for a variety of US Food and Drug Administration (FDA)-approved testing (e.g., influenza, Clostridium difficile, methicillin-resistant Staphylococcus aureus).

What Is the Performance of Xpert MTB/RIF?

There is a rapidly emerging literature regarding the performance of the Xpert MTB/RIF assay. Fundamentally, the sensitivity depends on the burden of organisms and thereby the target DNA present in the specimen. The published Xpert MTB/RIF detection threshold is approximately 100–130 colony forming units (cfu)/ml of sample [2],[3]. Patel and colleagues observed a similar threshold of 80–100 cfu/ml of CSF in this study [1]. In comparison, the detection threshold is <10 cfu/ml for mycobacterial liquid culture and is >5,000 cfu/ml for Ziehl-Neelsen staining for acid fast bacilli (AFB) via standard microscopy in sputum [7]–[9]. In real world clinical terms, this means 98%–99% detection by Xpert MTB/RIF of AFB smear-positive pulmonary TB, and approximately 75% detection of smear-negative, culture-positive pulmonary TB [3],[10],[11].

The threshold of detection is a key principle. The Xpert MTB/RIF test performs better when there is a larger burden of infectious organisms present in the specimen being tested. Yet, TB meningitis is a paucibacillary condition with few organisms. More organisms are likely present when the host is immunocompromised, or when a larger input volume is used for the test. Thus specimen centrifugation can compensate and should improve diagnostic yield, as demonstrated by a 35% improvement in sensitivity in this study [1].

The prior data on Xpert MTB/RIF testing of CSF are limited. In India, the Xpert MTB/RIF assay detected two of seven culture-positive specimens using an input volume of ~1 ml [12]. In an Italian study, 11 of 13 TBM patients were detected by Xpert MTB/RIF using an input volume of 2 ml into the cartridge without the standard N-acetyl-L-cysteine-sodium hydroxide (1% NALC-NaOH) decontamination and mucolytic step (i.e., GeneXpert Sample Reagent) [13]. This sample reagent was designed for sputum samples. Numerous commercially available PCR assays exist for other pathogens (e.g., herpes simplex PCR) without such a decontamination step [13], and the necessity of using the sample reagent for non-bloody CSF is unclear.

Public Health Significance

Although two commercial TB PCR tests previously existed [14], the innovation is that the GeneXpert platform is fully automated and is being rolled out in low- and middle-income countries. Thus, GeneXpert is an actual technology that can be—and is being—widely used globally. However, immediate implementation of a US$10 Xpert MTB/RIF assay for all cases of meningitis is unwise and unsustainable. Further research is needed on how best to incorporate the Xpert MTB/RIF test into diagnostic testing for meningitis, to ensure that it is a cost-effective intervention that improves health and does not waste resources.

Patel and colleagues modeled a clinical score to predict who had such high pretest probability of TBM that Xpert MTB/RIF was unnecessary to perform [1]. Yet health systems also need the opposite, a clinical score or algorithm to identify who has such low pretest probability that they do not require testing. Several investigators have developed meningitis diagnostic algorithms, yet broader validation is needed [15]–[17]. Ordering comprehensive testing of all the available diagnostic tests for every patient with suspected meningitis, including Xpert MTB/RIF testing, is 3–4-fold more expensive, without any better diagnostic yield than a targeted stepwise approach[15].

Key Principles of TB Meningitis Diagnosis

Despite molecular diagnostics, there remain a number of key pieces of information that inform clinicians as to the likelihood of a TBM diagnosis, so as to target testing in a cost-effective manner. The first is history. TBM is a subacute illness. Symptoms <6 days are atypical for TBM, yet near universal for bacterial meningitis [16]. Second is the immunology of the patient. Immunosuppression due to HIV/AIDS or age (e.g., infants, elderly) are key drivers of TBM, and immunosuppression increases the bacillary burden of M. tuberculosis organisms, likely increasing the diagnostic yield of molecular testing. In the current study, the GeneXpert performed poorly among HIV-uninfected persons, and the performance in children is unknown. Third is the CSF profile. TBM is classically a lymphocytic meningitis (i.e., >30%–50% lymphocytes in >90% of persons [16],[18]); with a low CSF glucose of <60% of serum glucose or an absolute CSF glucose concentration <2·2 mmol/l (<40 mg/dl) in >92%–95% [18]–[20].

In HIV-infected adults, the clinical history and CSF profile overlap extensively with meningitis due to Cryptococcus neoformans, and cryptococcal meningitis is overall the most common meningitis etiology in adults in sub-Saharan Africa [15]. Thus before a US$10 Xpert MTB/RIF test is performed for a less common condition, a US$2 cryptococcal antigen lateral flow assay should likely be performed for a more frequent condition [15].

If there is insufficient CSF volume available for testing (i.e., ❤ ml), in a clinically stable patient treated presumptively for bacterial meningitis a repeated lumbar puncture in 48 hours is likely a better strategy than sub-optimal Xpert MTB/RIF testing using a limited volume. A repeat lumbar puncture can collect a sufficiently large volume as well as reassess CSF glucose. At 48 hours, the CSF glucose should have risen by >100% of the initial level in treated bacterial meningitis[16]. Persistently low CSF glucose levels at 48 hours coupled with excluding cryptococcal meningitis should prompt Xpert MTB/RIF testing and/or empiric anti-TB therapy [16].

Xpert MTB/RIF appears to be a highly useful test to “rule in” the diagnosis of TBM, yet the clinical acumen of physicians remains a necessity for the wise use of any new diagnostic test. Careful application of these new diagnostic tools should improve clinicians’ ability to deliver timely, cost-effective care to patients with suspected TBM throughout the world, an approach that future studies should systematically evaluate.

Source:PLOS

Imaging in Acute Cholecystitis.

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Cholescintigraphy is slightly more accurate, but ultrasound is more readily available.

Ultrasound (US) and cholescintigraphy (e.g., hepatobiliary iminodiacetic acid [HIDA] scanning) are used widely to evaluate patients with suspected acute cholecystitis. Additionally, emergency department clinicians sometimes order computed tomography (CT) as the initial test, especially when they are considering both biliary and nonbiliary causes of abdominal pain.

To address the diagnostic accuracy of imaging tests for acute cholecystitis, researchers performed a meta-analysis of 57 studies with explicitly stated criteria for positive tests and with surgery and clinical follow-up as reference standards. Cholescintigraphy was evaluated in 40 studies, and US was evaluated in 26 studies; CT and magnetic resonance imaging were evaluated in only 1 and 3 studies, respectively. For cholescintigraphy, sensitivity was 96% and specificity was 90%; nonvisualization of the gallbladder was the usual criterion for a positive test. For US, sensitivity was 81% and specificity was 83%; criteria for a positive test varied widely, from simple presence of gallstones to combinations of additional findings (e.g., wall thickening, distention, pericholecystic fluid, sonographic Murphy sign).

Comment: Although US might be less accurate than cholescintigraphy, it remains a reasonable initial procedure, because it is simple and inexpensive to perform. The investigators’ figures for sensitivity and specificity of US should be considered only approximations, given the varying criteria for positive tests across studies. Cholescintigraphy should be done when US results are ambiguous. Despite widespread use of CT for initial evaluation of abdominal pain, its accuracy for acute cholecystitis has not been studied adequately.

Source:

Journal Watch General Medicine