Tuberculosis (TB) Diagnosis: Methods and Procedures

TB diagnosis requires a combination of clinical, microbiologic, and radiographic findings.

Diagnostic Steps

1. History & Physical: Chronic cough (>2 wks), fever, night sweats, weight loss; lung findings, lymphadenopathy.
2. Sputum Smear Microscopy: Ziehl-Neelsen or auramine-rhodamine stain for acid-fast bacilli (AFB). Rapid, low-cost, low sensitivity (~50–60%).
3. Sputum Culture (Gold Standard): Lowenstein-Jensen (4–8 wks) or MGIT liquid media (1–3 wks). Allows species ID and drug susceptibility testing (DST).
4. Chest X-ray: Upper lobe infiltrates, cavities, miliary pattern; not specific.
5. Tuberculin Skin Test (TST/Mantoux): Intradermal PPD; induration ≥10 mm (5 mm high-risk) = latent TB infection (LTBI). BCG can false-positive.
6. IGRA (QuantiFERON-TB Gold, T-SPOT.TB): Blood test measuring IFN-γ response to TB antigens; no BCG cross-reactivity; preferred for BCG-vaccinated.
7. Molecular (PCR/Xpert MTB/RIF Ultra): Rapid (2 hrs) detection of M. tuberculosis DNA + rifampin resistance. WHO-recommended initial test.
8. Biopsy: Tissue (lymph node, pleura, bone) for AFB, culture, histology (caseating granulomas).

Diagnostic Algorithm

Presumptive TB → Xpert MTB/RIF (if available) + culture/DST. LTBI → IGRA or TST. Clinical correlation essential throughout.