Equivalence of acid alone or acid-alcohol as decolourizing agent in Ziehl - Neelsen method.

ABSTRACT

Background: Microscopists opine that acid-alcohol decolourized slides may enhance acid-fast bacilli (AFB) smear positivity, and published documents on equivalence of acid and acid-alcohol in ZN staining method are not easily accessible. Setting: National Institute for Research in Tuberculosis, Chennai, India. Objective: To document the equivalence of 25% sulphuric acid (ZN-acid method) and 3% hydrochloric acid-alcohol (ZNalcohol method) as decolourizing agents in ZN method for detection of acid-fast bacilli. Methods: Two smears from each of 253 sputum samples from pulmonary tuberculosis patients, prepared and allocated, one to ZN-acid method and another to ZN-alcohol method were read blind. All the specimens were cultured for Mycobacterium tuberculosis by modified Petroff’s method. Culture of M. tuberculosis was gold standard. Results: The concordance between the methods was 85% (kappa 0.68), and the sensitivity (79%) and specificity (89%) were same for both the methods. Conclusion: In conclusion, the common belief that acid-alcohol decolourized slides give enhanced smear positivity stands void.