The reproducibility of sputum gram film interpretation

Objectives: To evaluate the reproducibility with which microbiologists inte rpret Gram-stained sputa and examine the effect of the smear preparation me thod. 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. A pro portion of the slides prepared by each method were recoded and represented to the raters. Intraobserver and interobserver variation was assessed using the kappa statistic (kappa). Results: Evaluation of the smear as being infected and predicting Streptoco ccus pneumoniae as the infecting organism showed the greatest intraobserver agreement (kappa=0.74-0.82) and interobserver agreement (kappa=0.50-0.82). The agreement concerning the number of cells and infection with Haemophilu s influenzae was only fair to moderate. Differences in the interpretation o f smears prepared by the two methods could be explained by the intrinsic di sagreement that occurs when the same smear is examined twice. The positive predictive value of a positive S. pneumoniae smear for a positive culture ( >10(6)/ml) was 81% with the DS and 97% with the HS. Conclusion: In this laboratory, a sputum Gram film interpretation suggestin g infection with S. pneumoniae was reproducible and predictive of a signifi cant positive culture. (C) 2000 The British Infection Society.