AFB Stain, Sputum

AFB stain means acid fast stain. AFB stain is a differential stain used mainly to detect many acid-fast bacilli mainly Mycobacterium that may cause tuberculosis (Mycobacterium tuberculosis). This is recommended for the preliminary diagnosis of Mycobacterium tuberculosis infection by direct method. Mycobacter is a rod shaped bacteria, it is acid-fast bacilli, because during the staining procedure, it retains the primary color even after subsequent acid treatment. These bacteria are identified under microscope and graded according to the number of bacilli identified. AFB stain is a simple, inexpensive and rapid method for detection of mycobacteria. The sample is collected in a sterile container. It is properly mixed and a thin smear is made on a glass slide from the representative sample, stain Ziehl-Neelson stain (Acid Fast Stain) and examined under microscope for the detection of Mycobacterium. AFB Stain helps in diagnosing the infection caused by Mycobacterium, helps in the assessment of response to anti tuberculous treatment. Determining cure or treatment failure. Sensitivity of this test is 60-80%. Chances of detection of tuberculosis bacilli are increased, if sample is collected from the representative area or by examining multiple samples. Under microscope at least 100 fields in oil immersion are examined before reporting the smear as negative. A negative AFB result does not rule out the diagnosis of tuberculosis, since a few organisms may be present in the sample. AFB stain helps in giving a presumptive diagnosis within a few hours, before the culture results are known. But AFB stain is a less sensitive method compared to culture. AFB stain may be done along with culture, NAAT (Nucleic acid Amplification Test) etc. NAAT is a molecular test that detects the genetic component of Mycobacterium by amplifying the genetic material of the Mycobacterium into pieces. NAAT test is used for presumptive diagnosis of tuberculosis within short period of time (24 hours). NAAT is a more sensitive and specific method compared to AFB stain. According to CDC (Centres for disease control and prevention), it is recommend that persons with signs and symptoms suggestive or suspicious of tuberculosis should have at least one sample tested with AFB stain along with culture and NAAT. NAAT also gives information about the sensitivity or Resistance of bacteria to the anti tuberculous drug i.e Rifampicin. AFB culture test helps to differentiate Mycobacteium infections whether active M.tuberculosis infections or Non tuberculosiss Mycobacteria infection. AFB culture is used to monitor the effectiveness of the treatment to anti tuberculosis drugs. AFB culture test is more sensitive than AFB stain, takes longer time to get the results. Mycobacteria is a slow grower than other bacteria, positive identification take 2-3 weeks and Final negative results (No Mycobacteria grown) take 6 weeks. AFB Susceptibility testing is requested for positive Mycobacterium tuberculosis patients to know the Effective antibiotic to treat the infection. Mycobacterium tuberculosis may be resistant or sensitive to commonly used anti tuberculous drugs.

1. When is AFB test done?

When a person has symptoms such as chronic cough, weight loss, fever - evening rise of temperature, chills, loss of apetite, enlarged lymph nodes and weakness, when suspecting tuberculosis. When TB screening test is positive and to monitor effectiveness of treatment for Tuberculosis.

2. Can the tuberculosis infection be Asymptomatic?

Yes. Usually with active tuberculos infection will be symptomatic and manifest with symptoms like loss of weight, loss of apetite, fever, cough, chills etc. But when the tuberculosis infection is latent, persons can be asymptomatic.

3. What all organisms are detected by AFB stain apart from Mycobacterium tuberculosis?

Mycobacterium avium-intracellulare complex (MAC), Mycobacterium marinum, Mycobacterium fortuitum, Mycobacterium bovis, Nocardia species etc.

4. How is a AFB smear interpreted?

When the AFB smear is examined under microscope, if acid-fast bacilli (Mycobacterium) are found, it is reported as positive and graded according to the RNTCP guidelines. At least 100 oil immersion fields are examined before reporting the AFB stain as negative.