Pneumocystis carinii in sputum. Comparable efficacy of screening stains and determination of cyst density.

To determine whether accurate quantification of Pneumocystis carinii cysts in sputum is possible using currently available means and to determine the optimal stain for screening sputum for P carinii, we undertook a comparative evaluation of readily available stains: (1) Diff-Quik, a stain that provides tinctorial detail comparable with Giemsa, (2) toluidine blue, and (3) silver methenamine on simultaneously thawed sputum samples containing P carinii. We also studied the Diff-Quik stain in combination with (4) toluidine blue and (5) silver methenamine. The three stains were comparable for screening purposes. Each disclosed similar numbers of P carinii aggregates on similarly prepared slides. Cysts were difficult to recognize on Diff-Quik due to the negatively staining character of the cyst wall with this stain. The cyst-wall stains, toluidine blue and silver methenamine, clearly stained the cyst wall. The combination of a cyst-wall stain with Diff-Quik allowed visualization of cysts, highlighted by the cyst-wall stain, and aggregate area, delineated by the Diff-Quik, allowing expression of number of cysts per aggregate area, a measure termed cyst density.