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

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.National Institute for Research in Tuberculosis, Chennai, India.To document the equivalence of 25% sulphuric acid (ZN-acid method) and 3% hydrochloric acid-alcohol (ZN-alcohol method) as decolourizing agents in ZN method for detection of acid-fast bacilli.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.The concordance between the methods was 85% (kappa 0.68), and the sensitivity (79%) and specificity (89%) were same for both the methods.In conclusion, the common belief that acid-alcohol decolourized slides give enhanced smear positivity stands void.