Foamy alveolar casts: Diagnostic specificity for Pneumocystis carinii pneumonia in bronchoalveolar lavage fluid cytology

Pneumocystis carinii pneumonia (PCP) is a major infectious complication of immunodeficiency states, including the acquired immunodeficiency syndrome (AIDS). Bronchoalveolar lavage (BAL) is a safe and effective procedure for making this diagnosis. In addition to the characteristic organisms, both histologic and cytologic material often reveals exudate in the form of foamy alveolar casts (FACs). To test the diagnostic utility of FACs in BAL fluids, we compared 20 PCP‐positive and 28 PCP‐negative fluids as assessed by silver stains. All PCP‐positive fluids contained FACs on Papanicolaou‐stained material. Only one PCP‐negative lavage contained FACs, and transbronchial biopsy in this case revealed PCP. We suggest that FACs in BAL fluids are highly sensitive and specific for the diagnosis of PCP.