Evaluation of a cold staining method for acid-fast bacilli in sputum.

Comparison between the Ziehl-Neelsen staining method for acid-fast bacilli, applied with and without heating, was carried out in a controlled investigation using smears prepared from 306 sputum samples collected prior to treatment from suspected cases of pulmonary tuberculosis. Smear and culture positivity were graded and the colour intensity of bacilli recorded. Results showed that the chance corrected agreement (Kappa) between Z-N and cold methods was only 78%. The sensitivity of the Z-N and cold methods were 84% and 77% respectively when compared with culture results. Assuming 10% smear positivity among symptomatics reporting to Peripheral Health Institutions (PHIs), the positive predictive value of the cold method was very low (53%). When compared to culture, the positive predictive value is 71% for the Z-N method and 57% for the cold method for a symptomatic population with 15% culture positivity. In the absence of heating, penetration of the stain was significantly reduced and consequently the number of bacilli detected was less. The inability to take the stain without heating was seen in smears from all grades of culture positive samples; thus even heavy positives were missed by the cold method. The evaluation of the cold method against the standard Z-N method highlights its limitations and demonstrates that it is not as reliable as the standard Z-N method.