TB Culture, Sputum

TB culture grows Mycobacterium tuberculosis from a sputum sample on either solid Lowenstein-Jensen (LJ) medium or in liquid Mycobacteria Growth Indicator Tube (MGIT) broth. Liquid MGIT is faster (1–3 weeks vs 4–8 weeks for LJ) and more sensitive, and is the WHO-preferred phenotypic method.

A positive culture confirms a viable mycobacterial isolate, which can be species-identified (MTB complex vs NTM) and tested for drug susceptibility — both essential for optimal treatment. Sputum culture is the most sensitive bacteriological test for pulmonary TB.

India's NTEP relies on culture to confirm diagnosis in difficult cases, perform drug susceptibility testing, and monitor treatment response. Specific scenarios where sputum culture is critical:

- AFB smear negative but TB still suspected clinically. - Suspected drug-resistant TB (need an isolate for DST). - Treatment failure or relapse investigation. - Treatment monitoring — sputum culture conversion (positive becoming negative) is a key milestone. - Pre-treatment baseline isolate for second-line DST.

Culture sensitivity in pulmonary TB is 85–95%, higher than smear and comparable to CBNAAT. It is, however, slower and requires biosafety level 3 lab facilities.

Three early-morning sputum samples on consecutive days give the highest yield. Sample must be deep sputum (not saliva) — rinse mouth with plain water, cough forcefully from the chest, and collect into the leak-proof container provided. Avoid eating, drinking or brushing teeth in the hour before collection. The sample should reach the lab within 24 hours, refrigerated.