Comparison between the Ziehl-Neelscn 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 positively 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
