What this test measures
The AFB (acid-fast bacilli) smear examines a stained smear of sputum or other body fluid under a microscope for bacilli that retain dye despite acid-alcohol decolorisation — a defining feature of mycobacteria. Two staining methods are used: Ziehl-Neelsen (ZN, conventional carbol fuchsin) and Auramine-O fluorescence. NTEP uses ZN at peripheral labs and fluorescence at higher-volume centres.
The result is graded by the number of bacilli per high-power field (Scanty / 1+ / 2+ / 3+), which corresponds roughly to bacterial load and infectiousness.
Why it matters
AFB smear is the oldest and simplest TB test, still widely used because it identifies the most infectious patients quickly and cheaply. A smear-positive patient is significantly more contagious than a smear-negative one, and same-day smear results guide immediate isolation and treatment decisions.
Limitations matter, though: smear sensitivity is only 50–60% for pulmonary TB (lower in HIV co-infection and paediatric TB), it cannot identify the species (M. tuberculosis vs NTM), and it does not detect drug resistance. NTEP has progressively moved CBNAAT to the front line as the initial test, with smear retained for treatment monitoring (sputum conversion) and as a backup.
How to prepare
Three early-morning sputum samples on consecutive days are traditionally recommended; NTEP now accepts two samples (one spot, one early morning). The sample should be deep, thick sputum (not saliva) — rinse mouth with plain water and cough deeply from the chest. Avoid eating or brushing teeth in the hour before collection.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| AFB Smear Grade (—)[1][2] | No AFB seen | — | AFB seen — supports TB diagnosis. Grade indicates bacterial load: Scanty (1–9 AFB per 100 fields) < 1+ (10–99 AFB per 100 fields) < 2+ (1–10 AFB per field) < 3+ (>10 AFB per field). Higher grades = more infectious. Confirm with CBNAAT and culture; cannot distinguish MTB from NTM by smear alone. |
AFB smear vs other front-line TB tests
| Test | Sensitivity (pulmonary TB) | Turnaround | Detects resistance? |
|---|---|---|---|
| AFB smear (this test) | 50–60% | 24 h | No |
| CBNAAT (Xpert MTB/RIF Ultra) | 85–90% | 2 h | Yes — rifampicin |
| MGIT culture | 85–95% | 1–3 weeks | Yes (via DST) |
| LPA | 95% (smear-positive) | 5 days | Yes — INH + RIF |
Frequently asked questions
How sensitive is the AFB smear?
50–60% for pulmonary TB in adults; lower in HIV-positive patients and children. A negative smear does not exclude TB — CBNAAT and culture are needed in clinical TB suspects.
How many samples should I give?
NTEP recommends 2 sputum samples (one spot, one early morning). Some labs and clinical contexts still use 3 samples on consecutive days.
Why is sputum collection important?
Sputum (deep chest secretion) contains the bacteria. Saliva (mouth fluid) does not. A "salivary" sample is often inadequate and lowers detection. Rinse mouth and cough deeply from the chest.
Does a positive smear mean I am contagious?
Yes — smear-positive pulmonary TB is contagious through coughing. Infectiousness drops sharply within 2–3 weeks of starting effective treatment. Mask use and isolation are recommended during this window.
Can the smear tell what species the bacteria are?
No. AFB seen on smear could be M. tuberculosis or a non-tuberculous mycobacterium. CBNAAT or culture is needed for species identification.
How is the smear used during treatment?
For treatment monitoring — sputum smear conversion (smear-positive becoming smear-negative) is a key milestone usually checked at 2 months and at end of treatment.
Is fluorescence microscopy better than ZN?
Yes — fluorescence (auramine) is faster and slightly more sensitive than conventional ZN, so high-volume centres prefer it. Both are equally specific.
Related Tuberculosis / Mycobacterial tests
Tests commonly ordered alongside SMEAR AFB, or that help interpret an unexpected result.
Sources & references
- WHO — Sputum smear microscopy · accessed 2026-05-30T00:00:00.000Z
- NTEP Sputum Smear Microscopy Manual · accessed 2026-05-30T00:00:00.000Z
- CDC — TB Laboratory Diagnostics · accessed 2026-05-30T00:00:00.000Z
- India TB Report 2024 · accessed 2026-05-30T00:00:00.000Z
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