▪ Objective: To assess the accuracy of Histoplasma capsulatum variety capsulatum polysaccharide antigen testing for the identification of histoplasmosis relapse in patients with the acquired immunodeficiency syndrome (AIDS).
▪ Design: A retrospective study using stored specimens.
▪ Setting: A referral center and several private hospitals.
▪ Patients: Twenty episodes of histoplasmosis relapse were evaluated in 17 patients with AIDS from November 1987 to August 1990. Controls included 30 patients with AIDS and histoplasmosis who did not have a relapse during maintenance therapy and who were initially tested during the same week as the patients with relapse. A second control group included seven patients with AIDS and histoplasmosis who were evaluated for relapse on 23 occasions; relapse, however, was excluded on each occasion.
▪ Measurements: To avoid interassay variability, specimens were tested for H.c. var. capsulatum polysaccharide antigen with the same radioimmunoassay.
▪ Main Outcome Measure: The change in the H.c. var. capsulatum polysaccharide antigen level during successful as opposed to unsuccessful maintenance therapy for the prevention of histoplasmosis relapse.
▪ Main Results: For the 20 episodes of relapse (17 patients), H.c. var. capsulatum antigen levels increased by at least 2 radioimmunoassay units in 12 of 14 serum specimens tested (85.7%; 95% Cl, 57.2% to 98.2%) and in 17 of 18 urine specimens tested (94.4%; Cl, 72.7% to 99.9%). Antigen levels increased in the urine or serum in 1 of 83 specimens (1.2%; Cl, 0.03% to 6.6%) obtained on 56 occasions (1.8%; Cl, 0.04% to 9.6%) from controls (specificity, 98.2%; Cl, 90.4% to 99.96%). In three cases of relapse, antigen levels increased before clinical relapse was suspected. Complement fixation titers increased by at least 2 dilutions in 4 of 11 cases (36.4%; Cl, 10.9% to 69.2%) but in 0 of 9 control patients (Cl, 0% to 28.3%).
▪ Conclusion: An increase in H.c. var. capsulatum polysaccharide antigen levels of 2 units or more strongly suggests histoplasmosis relapse. The presence of increasing titers of anti-H.c. var. capsulatum antibodies by complement fixation is less accurate for the diagnosis of relapse.