Evaluation of the Xpert® MTB/RIF assay and microscopy for the diagnosis of Mycobacterium tuberculosis in Namibia

The clinician, the lab and the patient: Understanding lab diagnostics to eradicate tuberculosis

BACKGROUND

Diagnostic modalities for diagnosing Tuberculosis caused by Mycobacterium group of organisms include mainly AFB smear by Ziehl Neelsen carbol fuchsin smear microscopy, GeneXpert (CB NAAT) molecular method, Line probe assay (Molecular method) and AFB culture (Liquid automated systems and solid media) methods.

METHODS

This study was initiated to understand and prioritize TB lab diagnosis, with reference to selection of lab diagnostic tests and its order of preference for MTC and NTM/MOTT closely associating it with the TB irradication program initiated by the Government of India.

RESULT AND CONCLUSION

The results and discussion bring to light the importance of each test and the purpose of their requisition. When diagnosis is handled half heartedly eradication of TB becomes a challenge. All the efforts including planning, resources in the form of manpower, infrastructure, finances, education, time etc., would be hampered. This challenge is not only for India but the globe. For countries harboring TB, Correct diagnostic request and timely diagnosis and treatment is the key to eradication of tuberculosis.

Diagnostic Accuracy of Sputum Microscopy in Comparison With GeneXpert in Pulmonary Tuberculosis

Background

Tuberculosis (TB) is a major health problem. In Pakistan, the diagnosis of pulmonary tuberculosis mainly relies on acid-fast bacilli (AFB) smear microscopy and Xpert® MTB/RIF assay (Cepheid Inc., Sunnyvale, CA) - a nucleic acid amplification test - where available. There is a wide variation in the reported sensitivity of Ziehl-Neelsen (ZN) smear microscopy across previous studies. This study aimed to determine the accuracy of sputum ZN smear microscopy in diagnosing pulmonary tuberculosis as compared with the sputum GeneXpert (Xpert MTB/RIF assay) as the reference test.

Materials and methods

This is a retrospective cross-sectional study conducted in the outpatient department of the Pakistan Institute of Medical Sciences. This study included 326 patients, aged 12 years and above, who had their sputum samples tested for ZN smear microscopy and GeneXpert during the period January to June 2019. Patients' demographic details, sputum ZN smear microscopy, and GeneXpert test results were collected for data analysis. A case of pulmonary tuberculosis was defined as a patient with positive sputum GeneXpert test result.

Results

Out of the 326 patients, GeneXpert detected MTB deoxyribonucleic acid (DNA) in 50 patients and ZN smear microscopy detected AFB in 30 patients. There was a marginal male predominance among GeneXpert positive cases. Adolescents were the least affected age group. The sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy of ZN smear microscopy were 60%, 100%, 100%, 93.24%, and 93.87%, respectively. The positive likelihood ratio was infinite whereas the negative likelihood ratio was 0.4. The area under curve (AUC) for ZN smear microscopy was 0.800 and receiver operating characteristic (ROC) curve analysis revealed a diagonal straight line closer to the left upper corner.

Conclusion

Sputum ZN smear microscopy is a highly specific but moderately sensitive test for the diagnosis of pulmonary tuberculosis. This study recommends the sputum GeneXpert MTB/RIF test to avoid a missed diagnosis of smear-negative pulmonary TB.

Evaluation of gene xpert for routine diagnosis of HIV-associated tuberculosis in Nigeria: A prospective cohort study

BACKGROUND

Xpert MTB/Rif (Xpert) is described as a game changer in tuberculosis (TB) control. We evaluated the impact of Xpert on diagnosis, time to treatment, and treatment outcome among patients with HIV associated TB in Nigeria.

METHODS

Adults with HIV being evaluated for pulmonary TB (PTB) were consecutively enrolled into the study cohort. At baseline, expectorated sputa were examined using Xpert and smear microscopy for Mycobacterium tuberculosis (MTB) and acid fast bacilli, respectively. Patients diagnosed with TB were followed-up until 6 months post TB diagnosis. TB was defined as sputum positive by smear microscopy, Xpert detection of MTB (bacteriologically confirmed case), or clinician diagnosed TB with initiation of full TB treatment (clinical diagnosis). Time to treatment was time from first clinic presentation for TB evaluation to initiation of TB treatment. We examined the proportion PTB patients with a positive Xpert result and compared time to TB treatment and outcome of TB treatment in patients based on sputum test results.

RESULTS

A total of 310 adults with HIV were enrolled. The median CD4 cell count was 242 (interquartile range (IQR) 120-425) cells/mm3 and 88.1% were receiving antiretroviral therapy (ART). PTB was diagnosed in 76 (24.5%) patients, with 71 (93.4%) being bacteriologically confirmed. Among patients with PTB, 56 (73.7%) were Xpert positive. Median time to treatment was 5 (IQR 2-8) days and 12 (IQR 5-35) days in patient with and without Xpert positive results, respectively; p = 0.005. Overall 73.1% had symptom free survival at 6 months post PTB treatment initiation with no significant differences observed based on TB test method. 10 (14.9%) died within 6 months of TB treatment initiation. In analysis adjusted for age, sex, and mode of diagnosis (Xpert positive or negative), only ART use independently predicted mortality (AOR 0.10; 95% CI 0.01-0.93).

CONCLUSION

The use of Xpert for routine care reduced time to PTB treatment, but did not improve survival in patients with HIV treated for susceptible PTB.