Clinical validation of a new dimeric inhibin-A assay suitable for second trimester Down's syndrome screening

Objective-To compare the Down's syndrome screening performance of a simplif ied dimeric inhibin-A assay (Diagnostic Systems Laboratories (DSL)) with an assay whose clinical utility has been established (Serotec). Setting-A case control set consisting of 51 Down's syndrome and 245 matched unaffected pregnancies collected as part of an earlier multicentre cohort study. Methods-Sera were assayed for dimeric inhibin-A using the DSL assay and Ser otec reference assay. Data analysis included a method comparison of mass va lues, fit of data to a logarithmic Gaussian distribution, and determination of detection and false positive rates. In addition, 234 fresh sera were as sayed using the simplified method. Results-The two assays showed a high correlation (r = 0.93) but average con centrations of the DSL assay were 48% higher. However, the differences were basically proportional over the range of values important for screening. T he detection rate was essentially equivalent for the DSL assay whether anal ysed univariately or in combination with other markers (for example, 79% v 75% at a 5% false positive rate for the DSL and Serotec assays for the comb ination of a fetoprotein, unconjugated oestriol, human chorionic gonadotrop hin, and dimeric inhibin-A, respectively). The 234 dimeric inhibin-A values measured on fresh sera fitted a logarithm Gaussian distribution for the DS L assay, as indicated by the fit to a probability plot. Conclusions-The Down's syndrome screening performance of a simplified dimer ic inhibin-A immunoassay was equivalent to a more labour intensive establis hed dimeric inhibin-A assay.