The Reproducibility of Sputum Gram Film Interpretation

Abstract

Objectives: To evaluate the reproducibility with which microbiologists interpret Gram-stained sputa and examine the effect of the smear preparation method.

Methods: Two hundred and ten coded slides prepared directly from a purulent portion of sputum (DS) and 140 slides prepared after homogenization of the same sputum (HS) were examined by three experienced microbiologists. Aproportion of the slides prepared by each method were recoded and represented to the raters. Intraobserver andinterobserver variation was assessed using the kappa statistic (κ).

Results: Evaluation of the smear as being infected and predicting Streptococcus pneumoniae as the infecting organism showed the greatest intraobserver agreement (κ=0.74–0.82) and interobserver agreement (κ=0.50–0.82). The agreement concerning the number of cells and infection with Haemophilus influenzae was only fair to moderate. Differences in the interpretation of smears prepared by the two methods could be explained by the intrinsic disagreement that occurs when the same smear is examined twice. The positive predictive value of a positive S. pneumoniae smear for a positive culture ([10⁶/ml) was 81% with the DS and 97% with the HS.

Conclusion: In this laboratory, a sputum Gram film interpretation suggesting infection with S. pneumoniae wasreproducible and predictive of a significant positive culture.