What this test measures
Sputum smear stained with fluorescent dye (auramine-O ± rhodamine) and examined under a fluorescence microscope (or LED-FM in resource-limited settings). Acid-fast bacilli (mycobacteria) fluoresce bright yellow against a dark background. Faster screening than ZN microscopy (1 slide in 2 minutes vs 10 minutes) and ~10% more sensitive. WHO endorses LED-FM as an upgrade from conventional ZN in TB diagnostic centres.
Why it matters
India has the highest TB burden globally (~26% of world cases). National Tuberculosis Elimination Programme (NTEP) algorithms have shifted from sputum smear microscopy alone toward CBNAAT (Xpert MTB/RIF) for primary diagnosis, but smear microscopy remains widely used for screening, treatment monitoring, and where CBNAAT isn't available. Fluorescent microscopy can process more slides per technician per day, important for high-throughput centres.
How to prepare
Early-morning expectorated sputum (best). Two samples (spot + early-morning) for diagnosis; one for monitoring. For sputum-shy patients: induced sputum, gastric lavage (children), or bronchoscopy.
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 Fluorescence (Grade 0 to 3+)[1][2] | Negative (0) | Negative — no AFB seen. Doesn't rule out TB (smear-negative TB common in HIV / paediatric); proceed to CBNAAT, culture, or imaging. | 1+ (10-99 AFB / 100 fields), 2+ (1-10 AFB / field), 3+ (> 10 AFB / field). Positive smear confirms mycobacterial disease but doesn't distinguish M. tuberculosis from non-TB mycobacteria (NTM). Need CBNAAT, culture, or LPA for confirmation + drug-resistance assessment. |
TB diagnostic test comparison
| Test | Sensitivity | Specificity | Drug resistance info |
|---|---|---|---|
| AFB smear (ZN or fluorescent) | 40-60% | 95-100% (for AFB) | None |
| CBNAAT (Xpert MTB/RIF) | 85-95% | 99% | Rifampicin only |
| Xpert Ultra | 90-95% | 95% | Rifampicin |
| LPA (line probe assay) | Variable | High | Rifampicin + Isoniazid |
| MGIT culture | 95-99% | ~100% | All first/second-line via DST |
| Solid culture (LJ) | 70-90% | ~100% | All via DST (slower) |
Frequently asked questions
Should I get fluorescent or ZN microscopy?
Either is acceptable but fluorescent (auramine-O) is faster and slightly more sensitive. WHO recommends LED-FM as an upgrade from conventional ZN.
Can fluorescent smear distinguish TB from NTM?
No — both are acid-fast. CBNAAT, culture, or NTM-specific tests are needed for species identification.
Why is my smear negative if I have TB?
Smear sensitivity is 40-60% for pulmonary TB and lower in HIV-positive or paediatric cases (paucibacillary disease). Negative smear doesn't rule out — pursue CBNAAT, culture, and imaging.
Can it monitor TB treatment?
Yes — sputum smear at end of intensive phase (month 2) and at end of treatment is standard NTEP monitoring. Persistence of AFB at month 5 indicates treatment failure or drug resistance.
Is fluorescent microscopy widely available in India?
LED-FM is increasingly available in NTEP-designated microscopy centres. Conventional ZN remains the most widely available technique.
Related Histopathology / Cytology tests
Tests commonly ordered alongside FLUORESCENT SMEAR, or that help interpret an unexpected result.
Sources & references
- WHO — TB Diagnostic Methods · accessed 2026-05-30T00:00:00.000Z
- NTEP India — TB Diagnostic Algorithm · accessed 2026-05-30T00:00:00.000Z
- CDC — TB Laboratory Methods · accessed 2026-05-30T00:00:00.000Z
- IUATLD — Sputum Microscopy · accessed 2026-05-30T00:00:00.000Z
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