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🔬 AFB Detection by ZN Stain – Body Fluids

Parameter	Details
Fasting Required?	❌ Not required
Sample Type	Body fluids: Pleural, Ascitic, Pericardial, Synovial, CSF, BAL, etc.
Primary Purpose	Detect Acid-Fast Bacilli (AFB) such as Mycobacterium tuberculosis
Target Organs	Depends on sample – lungs, meninges, pleura, peritoneum, joints, etc.

🧪 What is ZN Stain?

Ziehl–Neelsen (ZN) stain is a special staining technique that identifies acid-fast bacilli (AFB) under a microscope.
AFB resist decolorization due to high mycolic acid content in their cell walls.
Commonly detects Mycobacterium tuberculosis, but also NTM (non-tuberculous mycobacteria).

🔍 Procedure Summary

Body fluid is centrifuged.
A smear is prepared from the sediment.
The slide is stained with carbol fuchsin, heated, decolorized with acid-alcohol, and counterstained with methylene blue.
Viewed under oil immersion (1000x) microscope.

📈 Interpretation of Results

Smear Result	Interpretation
AFB Negative	No AFB seen; does not rule out TB, especially in low bacillary load
AFB 1+ to 3+	AFB seen; TB highly likely, especially in clinical context
Scanty	1–9 bacilli/100 fields – considered positive

⚠️ Sensitivity of ZN stain in body fluids is low (~10–40%) compared to sputum because of:

🧠 Common Body Fluids Tested

Fluid Type	Target Disease	Additional Notes
Pleural Fluid	Tuberculous pleuritis	Lymphocyte-rich; ADA also useful
Ascitic Fluid	TB peritonitis	Look for exudative features + ADA
CSF	TB meningitis	Very low yield, but useful when positive
Pericardial Fluid	TB pericarditis	Combined with ADA and GeneXpert
Synovial Fluid	TB arthritis	AFB detection rare; culture better
BAL (Bronchoalveolar Lavage)	Pulmonary/extrapulmonary TB	Higher yield in deep pulmonary lesions

📋 Recommended Conjunction Tests

Test	Purpose
GeneXpert/CBNAAT	Rapid, sensitive molecular detection of TB + rifampicin resistance
AFB Culture (MGIT/LJ)	Gold standard for TB; detects live organisms
ADA (Pleural/Ascitic/CSF)	Supports TB diagnosis in lymphocyte-rich fluids
Cell Count & Biochemistry	Helps differentiate TB from other causes (malignancy, pyogenic)
PCR-based TB Tests	Especially useful in smear-negative cases

⚠️ Limitations of ZN Stain in Body Fluids

Limitation	Explanation
Low Sensitivity	Cannot detect low bacillary load effectively
Operator Dependent	Accuracy depends on proper staining and microscope skill
Time-consuming	Microscopy requires viewing hundreds of fields
Not Drug-Sensitivity Testing	Cannot determine resistance pattern

✅ Summary Table

Test Name	AFB Detection – ZN Stain (Body Fluids)
Main Use	Identify acid-fast bacilli (AFB) such as Mycobacterium tuberculosis
Sensitivity	Low (~10–40%); higher in concentrated/deeper samples
Result Turnaround	Same day (few hours)
Sample Types	Pleural, Ascitic, CSF, BAL, Pericardial, Synovial
Paired Tests Recommended	GeneXpert, AFB culture, ADA, cell count, imaging

AFB-detection by smear examination ZN Stain Body fluids