Start with the sputum
To prove active lung TB, we try to find the bacteria in sputum — the phlegm coughed up from deep in the airways (not saliva from the mouth). A good early-morning sample, coughed up from deep down, is the single most valuable specimen in TB diagnosis. From one cup of sputum, the lab can run all three pillars below.
Pillar 1 — the smear: fast and cheap
Sputum smear microscopy is the oldest and cheapest test. A drop of sputum is smeared on a slide and stained; because of its waxy wall, M. tuberculosis holds the stain as an acid-fast bacillus (AFB) and glows against the background. A technician counts the bugs under a microscope. Results come back the same day.
The smear has two big limits. It is not very sensitive — it needs thousands of bugs per millilitre to turn positive, so it can miss milder disease. And it cannot tell TB apart from its non-tuberculous cousins, since both look acid-fast. Still, a strongly positive smear flags the most infectious patients, which makes it priceless for public health even today.
Pillar 2 — culture: slow but the gold standard
Sputum culture grows the bacteria on special media. It is the most sensitive test — it can detect as few as a handful of live bugs — and it remains the gold standard. Critically, once the bug grows, the lab can test which drugs kill it (drug-susceptibility testing), the only way to map resistance fully.
The price of all this is time. Because the bug grows so slowly, cultures can take 2 to 6 weeks to turn positive. That is far too long to wait before starting a sick, infectious patient on treatment — which is exactly the gap the third pillar fills.
Pillar 3 — molecular: fast and smart
Modern molecular tests, the best known being GeneXpert, have transformed TB diagnosis. They search the sputum for the bug's DNA. In under two hours, a cartridge-based machine can confirm that the germ is truly M. tuberculosis (not a cousin) and — remarkably — simultaneously detect the genetic signature of resistance to rifampicin, a key first-line drug.
This means a clinic can confirm TB and get an early warning of multidrug-resistant TB in a single morning, instead of waiting weeks. Molecular tests do not fully replace culture — culture is still needed for complete drug-susceptibility testing — but they let treatment begin promptly and correctly.
Choosing the right TB test — what each pillar answers Test Speed Names the bug? Checks resistance? Sensitivity ---------- --------- -------------- ------------------ ----------- Smear (AFB) same day no (AFB only) no low (needs many bugs) GeneXpert ~2 hours yes (TB DNA) yes (rifampicin) moderate-high Culture 2-6 weeks yes yes (full panel) highest (gold standard) Typical real-world flow: 1. Cough productive >2-3 weeks -> collect deep sputum sample 2. Run GeneXpert -> confirms TB + flags rifampicin resistance in <2h 3. Send culture in parallel -> full drug-susceptibility in weeks 4. Smear used to grade infectiousness and follow response on treatment