The diagnostic accuracy of Gram stain on formalin-fixed paraffin-embedded sections of skin tissue in the diagnosis of bacterial skin infection.

GS-FFPE has poor sensitivity, and a negative result should not be used as evidence to exclude infection. In contrast, it has excellent specificity and, unless the pretest probability of infection is very low, a positive result would make infection much more likely. The value of the GS-FFPE lies in cases where sterile tissue was not submitted for microbiological studies, or sterile tissue culture was negative, and there is at least a low-to-moderate pretest probability of infection.