IM27.{7,9-11} | Tuberculosis Diagnostic Testing — Summary & Reflection

KEY TAKEAWAYS

TB diagnostic testing under NTEP 2020 follows a hierarchy:

First-line: CBNAAT (Xpert MTB/RIF) — first test for ALL presumptive TB; detects MTB + RIF resistance simultaneously in ~2 hours; 88% sensitivity (smear-positive), ~67% (smear-negative); specimen: sputum, CSF, pleural fluid, FNAC, BAL.

Mantoux (TST/PPD): Intradermal 5 TU PPD; read induration at 72 hours; thresholds: ≥5 mm (HIV/immunosuppressed), ≥10 mm (India contacts/high-risk), ≥15 mm (low-risk). Critical pitfalls: false-positive (BCG, NTM); false-negative (anergy — HIV CD4 <200, miliary TB, severe malnutrition, window period). Does NOT diagnose active TB.

AFB smear: Positive grades — scanty (1–9/100 fields), 1+ (10–99), 2+ (1–10/field), 3+ (>10/field). Cannot speciate or test drug sensitivity — CBNAAT or culture required for both. ZN stain most common; Auramine-O fluorescent for high-volume labs.

Line probe assay (LPA): MTBDRplus — first-line resistance (rpoB, katG, inhA); MTBDRsl — second-line (gyrA/B, rrs, tlyA). Indicates pre-XDR/XDR-TB; required for all RR-TB before MDR regimen prescription.

Culture (LJ/MGIT): Gold standard; LJ takes 4–8 weeks; MGIT 10–14 days. Species identification + phenotypic DST. Indications: CBNAAT-negative high suspicion, NTM exclusion, treatment failure, XDR characterisation.

Serology: NEVER use. Commercial TB antibody tests are banned under NTEP — highly variable sensitivity/specificity, no role in diagnosis.

Special tests: Pleural ADA >40 IU/L (~92% sensitivity/specificity for TB pleuritis); CSF: lymphocytic pleocytosis + raised protein + low glucose + elevated ADA + CBNAAT; IGRA (private sector, LTBI before immunosuppression, BCG-vaccinated contacts).

REFLECT

Return to the opening scenario — the sputum smear report showing 1+ AFB in front of you. You now know that this result, while positive for acid-fast bacilli, tells you almost nothing actionable beyond confirming bacillary presence. Before writing the prescription, you need CBNAAT to confirm M. tuberculosis and screen for rifampicin resistance. You need HIV serology, fasting glucose, LFTs, and creatinine as baseline investigations. You need to register this patient on Nikshay. And you need to identify and screen household contacts. The diagnostic encounter is not complete when the first positive test result arrives — it is complete when you have enough information to prescribe safely, monitor systematically, prevent resistance, and protect contacts. What one step would you add to your personal practice after studying this module that you were not doing before?