The accuracy of tuberculous meningitis diagnostic tests using Bayesian latent class analysis

Low diagnostic performance of thick blood smears of 50 µl in comparison with direct examination of 10 µl blood and the leukoconcentration technique of 5ml blood among loiasis-suspected patients with low microfilaremia in Gabon, Central Africa, using the STARD-BLCM guidelines

BACKGROUND

The aim of this study was to determine performance indicators of thick blood smears of 50 µl (TBS-50), following the Standards for the Reporting of Diagnostic Accuracy Studies-Bayesian Latent Class Model (STARD-BLCM) guidelines. TBS-50 was compared with two common parasitological techniques-direct examination of 10 µl blood and a leukoconcentration of 5 ml-for the diagnosis of microfilaremic loiasis.

METHODS

The study population was recruited among patients of the Department of Parasitology-Mycology-Tropical Medicine over a period of 1 year. Age, sex, symptoms, and eosinophilia variables were recorded from laboratory registers and medical files. Direct examination of 10 µl of blood, TBS-50, and the leukoconcentration technique with 5 ml of blood were performed for each patient. The classical formula and BLCM were used to determine the diagnostic accuracy of the three techniques as well as the prevalence of microfilaremic loiasis. Three models were built within the framework of BLCM-the BLCM model I and alternative models II and III-for sensitivity analysis.

RESULTS

In total, 191 patients consented to be included. The direct blood examination and TBS-50 yielded comparable qualitative and quantitative results. Hence, they are reported together. The prevalence of Loa loa microfilaremia was 9.4% (95% CI 5.7-14.5; n = 18/191) with direct blood examination/TBS-50 and 12.6% [8.2-18.1] (n = 24/191) for leukoconcentration. Comparing TBS-50 with the leukoconcentration method using the classical formula, the sensitivity was 75.0% [53.3-90.2], specificity was 100.0% [97.8-100.0], the positive predictive value was 100.0% [81.5-100.0], and the negative predictive value was 96.5% [92.6-98.7]. The prevalence of microfilaremic loiasis was estimated at 9.7% [6.2-13.7] using BLCM model I. The outputs of BLCM model I showed sensitivity of 78.9% [65.3-90.3], specificity of 100.0% [99.3-100.0], a positive predictive value of 99.1% [87.2-100.0], and a negative predictive value of 93.0% [87.3-97.7] for direct blood examination/TBS-50.

CONCLUSIONS

TBS-50 demonstrates low sensitivity relative to two other techniques. In one in five cases, the result will be falsely declared negative using these methods. However, this method can be deployed with limited funds.

A novel diagnostic model for tuberculous meningitis using Bayesian latent class analysis

BACKGROUND

Diagnosis of tuberculous meningitis (TBM) is hampered by the lack of a gold standard. Current microbiological tests lack sensitivity and clinical diagnostic approaches are subjective. We therefore built a diagnostic model that can be used before microbiological test results are known.

METHODS

We included 659 individuals aged [Formula: see text] years with suspected brain infections from a prospective observational study conducted in Vietnam. We fitted a logistic regression diagnostic model for TBM status, with unknown values estimated via a latent class model on three mycobacterial tests: Ziehl-Neelsen smear, Mycobacterial culture, and GeneXpert. We additionally re-evaluated mycobacterial test performance, estimated individual mycobacillary burden, and quantified the reduction in TBM risk after confirmatory tests were negative. We also fitted a simplified model and developed a scoring table for early screening. All models were compared and validated internally.

RESULTS

Participants with HIV, miliary TB, long symptom duration, and high cerebrospinal fluid (CSF) lymphocyte count were more likely to have TBM. HIV and higher CSF protein were associated with higher mycobacillary burden. In the simplified model, HIV infection, clinical symptoms with long duration, and clinical or radiological evidence of extra-neural TB were associated with TBM At the cutpoints based on Youden's Index, the sensitivity and specificity in diagnosing TBM for our full and simplified models were 86.0% and 79.0%, and 88.0% and 75.0% respectively.

CONCLUSION

Our diagnostic model shows reliable performance and can be developed as a decision assistant for clinicians to detect patients at high risk of TBM. Diagnosis of tuberculous meningitis is hampered by the lack of gold standard. We developed a diagnostic model using latent class analysis, combining confirmatory test results and risk factors. Models were accurate, well-calibrated, and can support both clinical practice and research.

Estimating sensitivity and specificity of diagnostic tests using latent class models that account for conditional dependence between tests: a simulation study

BACKGROUND

Latent class models are increasingly used to estimate the sensitivity and specificity of diagnostic tests in the absence of a gold standard, and are commonly fitted using Bayesian methods. These models allow us to account for 'conditional dependence' between two or more diagnostic tests, meaning that the results from tests are correlated even after conditioning on the person's true disease status. The challenge is that it is not always clear to researchers whether conditional dependence exists between tests and whether it exists in all or just some latent classes. Despite the increasingly widespread use of latent class models to estimate diagnostic test accuracy, the impact of the conditional dependence structure chosen on the estimates of sensitivity and specificity remains poorly investigated.

METHODS

A simulation study and a reanalysis of a published case study are used to highlight the impact of the conditional dependence structure chosen on estimates of sensitivity and specificity. We describe and implement three latent class random-effect models with differing conditional dependence structures, as well as a conditional independence model and a model that assumes perfect test accuracy. We assess the bias and coverage of each model in estimating sensitivity and specificity across different data generating mechanisms.

RESULTS

The findings highlight that assuming conditional independence between tests within a latent class, where conditional dependence exists, results in biased estimates of sensitivity and specificity and poor coverage. The simulations also reiterate the substantial bias in estimates of sensitivity and specificity when incorrectly assuming a reference test is perfect. The motivating example of tests for Melioidosis highlights these biases in practice with important differences found in estimated test accuracy under different model choices.

CONCLUSIONS

We have illustrated that misspecification of the conditional dependence structure leads to biased estimates of sensitivity and specificity when there is a correlation between tests. Due to the minimal loss in precision seen by using a more general model, we recommend accounting for conditional dependence even if researchers are unsure of its presence or it is only expected at minimal levels.

Chronic Pulmonary Aspergillosis Situation among Post Tuberculosis Patients in Vietnam: An Observational Study

This study provides a brief view of chronic pulmonary aspergillosis (CPA) in the post-tuberculosis treatment community in Vietnam, a high burden tuberculosis (TB) country. In three months in late 2019, 70 post-TB patients managed at Vietnam National Lung Hospital were enrolled. Of these, 38 (54.3%) had CPA. The male/female ratio was 3/1 (28 males and ten females). CPA patients had a mean age of 59 ± 2.3 years (95%CI 54.4-63.6). The mean Body mass index (BMI) was 19.0 ± 0.5 (18.0-20.0) and 16 of 38 (42.1%) patients had concurrent diseases, the most common of which were chronic obstructive pulmonary disease (COPD) and diabetes. Twenty-six patients (68.4%) developed hemoptysis, 21 (55.3%) breathlessness, and weight loss was seen in 30 (78.9%). Anaemia was seen in 15 (39.5%) and 27 of 38 (71.1%) patients had an elevated C-reactive protein (CRP). The most common radiological findings were multiple cavities (52.6%) and pleural thickening (42.7%), followed by aspergilloma (29.0%) and non-specific infiltrates. There were five of 38 patients (13.2%) with a cavity containing a fungal ball on the chest X-ray, but when the high resolution computed tomography (HRCT) was examined, the number of patients with fungal balls rose to 11 (28.9%). Overall, 34 of 38 (89.5%) cases had an elevated Aspergillus IgG with an optical density ≥ 1, and in 2 cases, it was 0.9-1.0 (5%), borderline positive. In nine patients (23.7%) Aspergillus fumigatus was cultured from sputum. CPA is an under-recognised problem in Vietnam and other high burden TB countries, requiring a different diagnostic approach and treatment and careful management. HRCT and Aspergillus IgG serum test are recommended as initial diagnostic tools for CPA diagnosis.