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🧪 Acid-Fast Bacilli (AFB) Smear

Aspect	Details
Fasting Required?	❌ Not required
Sample Type	Usually sputum; may also be urine, pus, CSF, tissue biopsy, etc.
Main Organ Systems	Lungs (primarily), lymph nodes, gastrointestinal tract, genitourinary system
Test Purpose	To detect Mycobacterium tuberculosis (MTB) or other acid-fast mycobacteria

🔬 What Are Acid-Fast Bacilli (AFB)?

AFB are rod-shaped bacteria that retain dye after acid-alcohol decolorization (a property due to mycolic acid in their cell wall).
Most common AFB:
- Mycobacterium tuberculosis (causes TB)
- Non-tuberculous mycobacteria (NTM) like M. avium, M. kansasii

🧫 How the Test Works

A clinical specimen (typically sputum) is smeared on a slide.
The slide is stained using Ziehl-Neelsen stain (or auramine-rhodamine for fluorescent microscopy).
Under the microscope, AFB appear as bright red rods against a blue or green background.

📈 AFB Smear Grading (Based on WHO/IUATLD Guidelines)

Grade	Number of AFB/Fields	Interpretation
Negative	No AFB in 100 fields	No TB bacilli seen
Scanty	1–9 AFB in 100 fields	Very low load
1+	10–99 AFB in 100 fields	Low bacillary load
2+	1–10 AFB per field (in 50 fields)	Moderate bacillary load
3+	>10 AFB per field (in 20 fields)	High bacillary load (very infectious)

🧪 Clinical Use Cases

Scenario	Use of AFB Smear
Pulmonary TB symptoms	Detects M. tuberculosis in sputum
Suspected extra-pulmonary TB	Used on pleural fluid, CSF, lymph node aspirate, urine
TB treatment monitoring	Check for bacilli clearance over time
Suspected leprosy or NTM	Detects acid-fast organisms in skin or biopsy samples

⚠️ Limitations of AFB Smear

Limitation	Explanation
❌ Not confirmatory	Cannot differentiate TB from non-TB mycobacteria
⬇️ Low sensitivity	May miss infection in early or paucibacillary cases
🧪 Culture/NAAT required	Confirm diagnosis using culture or molecular tests (e.g., CBNAAT/GeneXpert)

🧪 Recommended Additional Tests

Test	Purpose
GeneXpert (CBNAAT)	Detects TB DNA and rifampicin resistance
AFB Culture (LJ or MGIT)	Confirms TB, allows drug sensitivity testing
Chest X-ray / CT Chest	Imaging support for pulmonary TB
ESR, CRP	General inflammatory markers
Tuberculin Skin Test (TST)	Screening test, especially for latent TB
Interferon Gamma Release Assays (IGRAs)	Especially useful for latent TB in BCG-vaccinated individuals

✅ Summary Table

Test Name	Acid-Fast Bacilli (AFB) Smear
Primary Use	Rapid screening for TB and mycobacteria
Sample Types	Sputum, urine, CSF, biopsy, gastric lavage, etc.
Turnaround Time	Same day (few hours)
Sensitivity	Moderate (higher in high bacterial load cases)
Confirmation Required?	✅ Yes – with culture or molecular test

Acid Fast Bacilli Smear (AFB)