Smear for AFB

Function

The Acid-Fast Bacilli (AFB) smear is a rapid microscopic examination used to detect bacteria that have a 'waxy' cell wall containing mycolic acid. These bacteria do not stain well with standard Gram stains but retain red dye even after being washed with acid (hence 'acid-fast'). The most clinically relevant AFB is Mycobacterium tuberculosis.

Why it is Ordered

The test is ordered for patients presenting with symptoms of pulmonary tuberculosis (TB), such as a chronic cough (lasting more than 3 weeks), coughing up blood, night sweats, unexplained weight loss, and fever. It is a critical public health tool because a 'smear-positive' patient is generally highly infectious to others.

Associated Conditions

• Tuberculosis (TB): The primary condition diagnosed via AFB smear.
• Nontuberculous Mycobacteria (NTM): Other species like M. avium complex can cause lung disease, particularly in immunocompromised individuals (e.g., those with HIV/AIDS).
• Leprosy: Caused by Mycobacterium leprae, though this usually requires a skin biopsy smear rather than sputum.
• Treatment Monitoring: The test is repeated during TB treatment to ensure the bacterial load is decreasing and the patient is becoming non-infectious.