Utility of lactate dehydrogenase vs radiographic severity in the differential diagnosis of Pneumocystis carinii pneumonia

Study objectives: In patients with HIV infection, serum lactate dehydrogenase (LDH) level is commonly stated to be more elevated in Pneumocystis carinii pneumonia (PCP) than in non-PCP, We hypothesized that LDH level reflects radiographic extent and severity of pneumonia rather than P carinii infection specifically and therefore is not useful in the differential diagnosis of lung infections in AIDS. Design: We compared radiographic features and LDH values in 93 sequential patients with HIV infection and a new hospital admission for pneumonia (53 PCP and 40 non-PCP) after excluding all patients with other potential causes for elevated LDH levels, The chest radiograph was graded using a quantitative scale (0 to 24) to assess radiographic extent and severity of pneumonia by two independent observers in blinded fashion, The relationship between radiographic score and hospital admission LDH level was analyzed by linear regression and Bayesian analysis was applied to different LDH ranges to calculate the clinical value of LDH measurements. Setting: Tertiary care teaching hospital and regional AIDS referral center. Results: Mean LDH level was higher in the PCP group (1,217+/-88 U/L compared with 776+/-55 U/L; p <0.001), as was mean radiographic score (12.4+/-0.6 for PCP compared with 6.3+/-0.5 for non-PCP; p <0.001). For the whole sample of 93, LDH level was significantly related to chest radiographic score (r=0.43, p <0.0001). Significant overlap occurred between the two groups at all levels of LDH such that no cutoff level could be established that impacted significantly on posttest probability of PCP, whereas a radiographic score of >12 yielded a 96% posttest probability of PCP. Conclusions: Serum LDH level reflects the degree of radiographic abnormality and is elevated in both PCP and non-PCP pneumonia to arm extent that limits its utility in differentiating the two processes in hospitalized patients. The extent of radiographic involvement more clearly distinguishes the two conditions.