Xpert MTB/Rif assay valuable option for rapid detection of lymph node TB

Tuberculosis is the leading infectious disease killer, ranked ninth among the overall death causes globally. In 2020, the global incidence of TB was 9.9 million. 14% of the incident cases worldwide represent s extrapulmonary TB. WHO's 2020 statistics for India provide an estimated incidence of 2.59 million, of which 29% are extrapulmonary. Lymph node TB is the most frequent form of extrapulmonary tuberculosis in India.

Molecular diagnostic tests, including polymerase chain reaction (PCR), which detects mycobacterial DNA, help in early microbiological confirmation before treatment initiation. Xpert MTB/Rif assay or Xpert MTB/Rif assay TB is a PCR-based rapid diagnostic test with high specificity and sensitivity in smear-positive TB cases. In India, standards for tuberculosis care recommend microscopy/culture/CBNAAT/molecular test/histopathology examination and drug sensitivity testing on suitable specimens from the presumed involvement sites for all patients with presumptive extrapulmonary TB.

Against the above background, Nithya Haridas, Associate Professor, Amrita Institute of Medical Sciences, Amrita Vishwa Vidya Peetham, Kochi, Kerala, India, and colleagues aimed to examine the use of Xpert MTB/Rif assay in lymph node tuberculosis.

The study included 212 patients who underwent lymph node samples and were subjected to Xpert MTB/Rif assay. The authors noted Xpert MTB/Rif assay result, demographic profile, smear acid-fast bacillus (AFB), AFB culture results, and histopathology/cytology findings.

The study included one hundred seventy-nine patients whose samples were subjected to AFB smear, AFB culture, histopathological examination, and Xpert MTB/Rif assay. AFB culture was done in 183 patients, and histopathology assessment of lymph node specimens was performed in 209 patients. Xpert MTB/Rif assay and AFB smear examination were done in all patients.

The study led to the following findings:

• Based on the CRS, 81 cases were diagnosed as tuberculous lymphadenitis. Among the 81 tuberculous lymphadenitis cases, 62 showed a positive Xpert MTB/Rif assay report, while 19 patients had a negative Xpert MTB/Rif assay result.
• Xpert MTB/Rif assay has a sensitivity of 71.83% and a specificity of 98.97% for smear-negative samples compared to CRS.
• Xpert MTB/Rif assay has a sensitivity of 75.61%, a specificity of 98.97%, a positive predictive value of 98.41%, and a negative predictive value of 82.76% against CRS. It has a high accuracy of 88.27%.
• Xpert MTb-Rif assay did not detect any rifampicin-resistant case in this study. One patient had indeterminate rifampicin resistance, which was sensitive to all first-line drugs in drug susceptibility tests.
• First-line drug sensitivity was done in 24 of the 56 culture-positive specimens. Fourteen patients showed M. tuberculosis sensitive to all first-line drugs. Mono-resistance to rifampicin, ethambutol, and streptomycin was seen in one patient each.
• Two patients had pyrazinamide resistance. While one case of multidrug resistance was seen (rifampicin, isoniazid, and streptomycin), various combinations of resistance were seen to first-line drugs in four patients - isoniazid and pyrazinamide, rifampicin and ethambutol and isoniazid and streptomycin.

To conclude, Xpert MTB/Rif assay is a helpful test for rapid diagnosis of lymph node TB.

Reference:

Haridas N, Mehta A, Kunoor A, Khan S. Retrospective observational study to compare Xpert MTB/RIF assay with other diagnostic tests in lymph node tuberculosis. Indian J Public Health 2022;66, Suppl S1:17-21