Detection of candida antigenemia in human invasive candidiasis by a new solid phase radioimmunoassay

There is a continuing need to develop reliable non-invasive methods to aid in the early diagnosis of disseminatyed candidiasis. We report the development of a solid phase "sandwich" radioimmunoassay (RIA) for the detection of soluble cytoplasmic protein antigens (SPA) of Candida albicans in patients with systemic candidiasis. SPA were prepared by ultrasonic disruption of blastospores of C. albicans. Anti-SPA IgG was covalently linked to a solid phase of polyacrylamide-like microspheres. Standard amounts of patient sample or SPA in pooled normal human serum were reacted with the conjugated microspheres, washed and reacted with anti-SPA IgG-125I. The amount of antigen was directly correlated with the amount of bound 125I-IgG anti-SPA. In a retrospective analysis, circulating SPA in the range of 0.5-1.6 microgram/ml was detected in the serum of 12 of 19 patients (63%) with documented systemic candidiasis. There was no detectable SPA in the serum of 20 hospitalized patients with bacteremia, two patients with aspergillosis, one with cryptococcosis, and three with invasive Candida tropicalis. We observed no crossreaction of the RIA with mannan from C. albicans, coccidioidin or culture filtrate antigen of Aspergillus fumigatus. There was significant inhibition of the RIA by serum samples from two patients with chronic candidiasis suggestive of anti-SPA antibody excess. The solid phase RIA to detect circulating SPA can provide a useful noninvasive method for the diagnosis of systemic C. albicans infection.