False negative biopsy in Pneumocystis carinii pneumonia

Unlike most pneumonias, the diagnosis of Pneumocystis carinii pneumonia is based solely on identifying organisms by stain, usually with methenamine-silver. Because of technical problems involved with adequate staining, control samples usually are done concurrent with tissue specimens to be examined. Lung containing fungi often is used as a control. We recently observed false-negative biopsy specimens in a case of P carinii pneumonia where the Pneumocystis organism failed to stain with methenamine-silver on several occasions, although fungal controls were positive. This report emphasizes the importance of using P carinii as a control whenever attempting to diagnose P carinii pneumonia.