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🔬 AFB Detection – ZN Stain (Sputum, 1 Sample)

Parameter	Details
Fasting Required?	❌ Not required
Sample Type	Sputum (expectorated) – not saliva; must come from deep coughing
Primary Purpose	Detect Acid-Fast Bacilli (AFB) such as Mycobacterium tuberculosis
Target Organ	Lungs
Primary Disease Screened	Pulmonary Tuberculosis (PTB)

🧪 What is the ZN Stain?

The Ziehl-Neelsen stain uses carbol fuchsin, acid-alcohol, and methylene blue to stain AFB.
AFB retain the red dye even after acid decolorization — hence “acid-fast.”
Viewed under oil immersion (1000x), AFB appear as bright red rods against a blue background.

📋 Procedure (1 Sample Sputum)

Patient expectorates sputum into a sterile container (preferably early morning).
Smear prepared from a portion of thick, yellowish sputum (not saliva).
Stained with ZN method and examined under a microscope.
Reported as Negative, Scanty, 1+, 2+, or 3+ depending on bacillary count.

📈 Result Interpretation

Smear Result	Bacilli Seen (per 100 fields)	Interpretation
Negative	0	No AFB seen, but TB cannot be ruled out
Scanty	1–9	Positive, low bacillary load
1+	10–99	Moderate TB load
2+	1–10 per field	Significant TB load
3+	>10 per field	High infectivity, advanced pulmonary TB

🧠 When is ZN Stain Used on Sputum?

📉 Sensitivity and Limitations

Factor	Details
Sensitivity	~50–60% for smear-positive TB (depends on sample quality and bacterial load)
Limitations	May be negative in early TB, HIV+ patients, children, and low-load cases
Specificity	High for TB (if AFB seen, almost always mycobacteria)

A single sample is less sensitive than 2–3 serial samples. WHO still recommends at least 2 samples in many settings.

🔗 Recommended Further Diagnostics

Test	Purpose
GeneXpert (CBNAAT)	Molecular test to detect TB + Rifampicin resistance
AFB Culture (MGIT/LJ)	Confirms presence of live TB bacteria; allows drug testing
Chest X-ray	Imaging for cavities, infiltrates, consolidation
Line Probe Assay (LPA)	Detects resistance to first-line TB drugs
HIV Test	Strong association between TB and HIV co-infection

✅ Summary Table

Test Name	AFB Smear – ZN Stain (Sputum)
Main Use	Identify pulmonary TB
Sample Required	1 sputum sample, early morning preferred
Result Timing	Same day (few hours)
Infectious Risk	High if AFB 2+ or 3+
Limitations	Low sensitivity in early disease or immunosuppressed
Follow-up Required	Molecular & culture tests to confirm & assess resistance

AFB-detection by smear examination ZN Stain Sputum, 1 samples