Acid-Fast Bacilli Staining
- Sample Type
- Sputum / Urine / Tissue
- Fasting
- No
- Unit
- Grades (1+ to 3+)
Function
Acid-Fast Bacilli (AFB) staining, also known as the Ziehl-Neelsen stain, is a laboratory technique used to identify bacteria that have a high lipid (mycolic acid) content in their cell walls. These 'acid-fast' organisms resist decolorization by acids during the staining process, appearing bright red against a blue background under a microscope.
Why it is Ordered
The primary reason for ordering an AFB stain is to screen for Mycobacterium tuberculosis, the causative agent of TB. It is also used to detect other mycobacteria (NTM) and Nocardia. It is ordered for patients with a persistent cough, weight loss, night sweats, or abnormal chest X-rays. Because TB is highly contagious, rapid identification is vital for public health.
Associated Conditions
- Pulmonary Tuberculosis: The most common cause of a positive sputum AFB.
- Leprosy: Caused by Mycobacterium leprae.
- MAI/MAC Complex: Opportunistic infections often seen in HIV/AIDS patients.
- Nocardiosis: A rare bacterial infection that can mimic TB.
Why Context Matters
A single negative AFB smear does not rule out Tuberculosis because the bacterial load must be high (5,000-10,000 organisms per mL) to be seen. Sputum quality is critical; if the patient provides saliva instead of deep phlegm, the test will be falsely negative. Conversely, non-tuberculous mycobacteria (NTM) can look identical to TB on a smear, necessitating a culture or PCR for confirmation.
Lab ranges are statistical averages, not biological laws. "Normal" for a 20-year-old male isn't normal for a 60-year-old female.
Clinical References
Source-of-truth databases and clinical guidelines for Acid-Fast Bacilli Staining:
Official Sources
Research & Guidelines
Related Indicators
Medically Reviewed by Dr. Binoy Babu, MBBS
Board Certified Doctor • 10+ Years Clinical Experience
Dr. Babu is a practicing physician dedicated to empowering patients with clear, actionable medical information. He founded 2opi to bridge the gap between complex lab reports and patient understanding, ensuring everyone has access to a reliable second opinion.
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