Accuracy of Gram's stain in identifying pneumococci in sputum.

We prospectively examined the accuracy of Gram-stained sputum for identifying pneumococci in 42 patients with community-acquired pneumonia. We considered the Gram's stain positive if a preponderant flora or more than ten Gram-positive lancet-shaped diplococci were seen per oil immersion (x1,000) field. These criteria were met in 18 (62%) of 29 specimens that actually contained pneumococci as determined by a positive quellung reaction, or culture on plates or in mice. The Gram's stain was negative in 11 of 13 specimens that did not contain pneumococci. Of the 20 specimens yielding positive Gram's stains, 18 contained pneumococci. A positive Gram-stained smear of sputum strongly suggests the presence of pneumococci, but will miss 38% of specimens containing pneumococci. Deviation from these criteria substantially reduces the overall accuracy of the test.