The analytical specificity of human chorionic gonadotropin assays determined using WHO International Reference Reagents

Background: Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hormone with considerable molecular heterogeneity. There is uncertainty regarding which hCG variants are detected by different hCG assays. The analytical specificity of 8 hCG assays was investigated. Methods: WHO International Reference Reagents for hCG, nicked hCG (hCGn), beta subunit (hCGβ), nicked beta subunit (hCGβn), and beta core fragment (hCGβcf) were individually added to hCG-free human serum. Specimens were analyzed with 8 commercially available hCG assays. Equimolar detection of hCG variants was defined as a recovery of 90-110%. Results: All assays detected hCG and hCGn with mean recoveries of 98.3 and 94.6%, respectively. Seven assays detected hCGβ (mean recovery 103.8%) but with high variation, and equimolar detection was observed only in four. The mean recovery of hCGβn was 85.5% but was highly variable with only two assays showing equimolar detection. With a mean recovery of 53.4%, two assays detected hCGβcf and both underestimated it considerably. Information provided by the assay manufacturer regarding hCG variant analytical specificity was inadequate or unclear in 75% of the assays. Conclusions: hCG assays vary considerably in their ability to detect different hCG variants. Manufacturers of hCG assays should clearly indicate the hCG variant specificity of their reagent systems.