Weevaluated theHybritech Tandem-E procedure forquanti- fyingchoriogonadotropin

(hCG)in human serum. In this “sandwich”-typeassay,two monoclonalantibodiesdirected againstdifferent regionsofthehCGmolecule areused,one coatedona plasticbead,thesecondconjugatedto alkaline phosphatase.The assaycan detectaslittle as1.0mt.unit of thehormone per liter,showsa linearresponseup to at least 200 i. units/L,andhasgoodprecision. By prolonging the incubations forformation of the sandwich andforsubstrate hydrolysis, onecanachieve higher sensitivityat theexpense ofa narrower linearrange.Correlation witha conventional radioimmunoassay forthe f3subunitof hCGwas generally excellent,but in one instancethe Tandem-E gave an apparentlyfalse positiveresult. Additional Keyphrasea: monoclonalantibodies . pregnancy . radloimmunoassay compared . p-subunit Sensitive, highly specific immunoassays for human thoriogonadotropin (hCG) have gained an important role in clinical practice, particularly for the early diagnosis of pregnancy and the detection of ectopic pregnancy.’ Until recently, the only widely used assays were radioimmunoassays, with polyclonal antibodies directed toward the p subunit of hCG. Besides requiring radioisotopes, these assays may involve long incubations, a centrifugation step, and multiple-point standardization; they are particularly cumbersome for urgent (“stat”) testing of a single specimen. More recently, immunoenzymetric testing for serum hCG has been introduced (1-4), and several commercial kits are now available. The “Tandem-E hCG” assay (Hybritech, San Diego, CA 92121) is a solid-phase immunoenzymometric assay (IEr) in which two mouse monoclonal antibodies recognize different regions of the hCG molecule. The first

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