Microbiology: Interpreting the Sputum Gram Stain Report

A retrospective review of patient records was performed to determine the statistical relationship between sputum Gram stain and culture results in adult men at a Veterans Affairs Medical Center in the Midwest. Gram stain results from 1996 were statistically compared with culture results for 2,105 sputum specimens. Positive predictive values for common Gram stain morphotypes and their corresponding organisms varied from 7.1% to 90.6%. In some instances, the absence of a particular Gram stain morphotype (eg, gram-negative diplococci) was more predictive of the absence of an organism in culture (eg, Moraxella catarrhalisj than its presence. Also, Gram stain reports noting the presence of gram-negative bacilli were not predictive of cultures with gram-negative bacilli potential respiratory pathogens (positive predictive values 32.4%-54.9%). In conclusion, Gram stain results often were not accurate predictors of sputum culture results. One way to improve agreement between Gram stain and culture results is to develop Gram stain interpretation guidelines based on statistical relationships between stain and cultu