Serum Lactate Dehydrogenase/Pao₂ Ratio in the Diagnosis of Pneumocystis carinii Pneumonia

The diagnosis of Pneumocystis carinii pneumonia (PCP) requires the demonstration of organisms from lung-derived specimens. Adjunctive tests, including serum lactate dehydrogenase (LDH) and arterial blood gas analysis, lack adequate sensitivity and specificity when used alone. We attempted to improve the utility of these measurements in the diagnosis of PCP in patients with known or suspected human immunodeficiency virus (HIV) infection. We reveiwed the medical records of 173 consecutive inpatients in an urban teaching hospital who had an undiagnosed pulmonary disorder suggestive of PCP and who had fiberoptic bronchoscopy in 1993 and 1994. Serum LDH (international units), arterial Pao₂ (mmHg) on room air, and the ratio LDH/Pao₂ in patients with PCP was compared with patients who had another pulmonary disorder. We found that LDH/Pao₂ was greater than 4.5 in 84 patients, 80 patients with PCP, and 4 with other pulmonary disorders. Two of the false-positive cases had coexisting elevations in liver enzymes. LDH/Pao₂ was less than 4.5 in 89 patients, 43 with PCP, and 46 with other pulmonary disorders. For the diagnosis of PCP, LDH/Pao₂ greater than 4.5 had a sensitivity of 65% and a specificity of 92%. The positive predictive value was 95%, and the negative predictive value was 51%. In conclusion, LDH/Pao₂ greater than 4.5 while on room air was moderately sensitive but was highly specific and had a very high positive predictive value in the diagnosis of PCP in HIV-seropositive patients. This finding could be used to support the empiric administration of antipneumocystis therapy in HIV-infected patients suspected of having PCP.