Laboratory diagnosis of Pneumocystis carinii pneumonia

The role of the laboratory in the diagnosis of any infection depends on the clinical circumstances. Pneumocystis carinii pneumonia (PCP) is an excellent example of this. When patients with human immunodeficiency virus (HIV) infection present late in their illness with pneumonia, the clinical presentation alone can be diagnostic and laboratory diagnosis of PCP may be unnecessary. However, in patients with early disease, including those who may be receiving prophylactic chemotherapy, or those who are not obviously immunocompromised, the laboratory diagnosis of PCP is critical for correct management. PCP can be confirmed only by laboratory tests and current dogma is that these require respiratory samples obtained by invasive procedures. The fact that such procedures may not be possible in very ill patients has stimulated interest in a plethora of non-invasive methods, including various exercise-induced hypoxaemia tests, pulmonary function tests, gallium scanning and radiolabelled technetium diethylenetriamine penta-acetate (DTPA) clearance tests. To complicate matters, some workers have postulated a scoring system to diagnose PCP, and others have suggested a diagnostic algorithm. There is a danger that investigation may become over-complicated, taking diagnosis in the wrong direction. All non-invasive procedures are non-specific and there is a good case for developing definitive laboratory tests that require less invasive specimens.