In suspected pulmonary TB, which diagnostic test is usually ordered first?

• A. Sputum smear for AFB
• B. Chest X-Ray
• C. NAAT
• D. Sputum culture for AFB

Answer: (C)

When evaluating suspected pulmonary TB, the test that provides rapid, specific evidence of disease and guides urgent management is a nucleic acid amplification test on sputum. This molecular assay detects the Mycobacterium tuberculosis complex directly from the sputum and can also reveal rifampin resistance, which helps decide on treatment and isolation early.

It’s favored over other options because it works faster than culture—giving results in hours rather than weeks—and has higher sensitivity than a smear, especially in cases where smear-negative disease is common (such as in HIV co-infection). A smear is quick but misses many cases and cannot show drug resistance. A chest X-ray is useful for supporting suspicion and assessing extent, but it cannot confirm TB or provide drug-susceptibility information. Sputum culture remains the gold standard for confirmation and full drug susceptibility testing, but its slow turnaround makes it unsuitable as the initial diagnostic test.

So, the initial diagnostic choice is the NAAT on sputum to rapidly confirm TB and guide early, appropriate management.