Performing and Interpreting Acid-Fast Bacilli (AFB) Sputum Testing

Introduction

Acid-Fast Bacilli (AFB) sputum testing is a crucial diagnostic procedure for identifying Mycobacterium tuberculosis, the causative agent of tuberculosis (TB). This test employs specialized staining methods, such as Ziehl-Neelsen and Fluorescent staining, to detect acid-fast bacteria in patient samples.

Ziehl-Neelsen Staining Method

Procedure

• Prepare a smear of sputum on a clean glass slide.
• Heat-fix the smear to adhere the sample.
• Flood the smear with carbol fuchsin and heat gently.
• Wash with water and decolorize using acid-alcohol.
• Counterstain with methylene blue.
• Examine under a microscope for red-colored AFB against a blue background.

Fluorescent Staining Method

Procedure

• Prepare and heat-fix the smear.
• Stain with auramine-rhodamine.
• Decolorize using acid-alcohol.
• Counterstain with potassium permanganate.
• View under a fluorescence microscope for bright fluorescent bacteria.

Interpretation of Results

The presence of acid-fast bacilli indicates a possible tuberculosis infection, requiring further confirmatory tests such as culture and molecular assays. Negative results do not completely rule out TB, as the bacterial load may be insufficient for detection.

Conclusion

AFB sputum testing remains a vital tool in early tuberculosis diagnosis and treatment initiation. Choosing between Ziehl-Neelsen and Fluorescent staining depends on laboratory capabilities and desired sensitivity.