Pregnancy testing. 1. Immunoassays and radioimmunoassays
Horwitz, C.A.; Lee, C.Y.
Postgraduate Medicine 63(5): 193-196
1978
ISSN/ISBN: 0032-5481 PMID: 643765 Document Number: 129378
Sensitivity and accuracy of pregnancy tests are reviewed. The standard immunoassay, based on classic antigen-antibody reaction, have virtually replaced bioassays as the principle methods for detection of human chorionic gonadotropin hormone (HCG). The urinary concentrations detected by commercial tests vary considerably and influence accuracy at different stages of pregnancy. A table is provided discriminating these factors for many commercially available tests. 2 types of immunoassays are available: rapid slide tests, which are very insensitive (2.3 IU/ml), reasonably accurate ( 90%) only in specimens collected at least 45 days after the last menstrual period, and subject to false-positive reports in the presence of proteinaceous urine; and standard tube tests, which because of their greater specificity are the test of choice when 1) the specimen is received within 2 weeks of possible implantation, 2) diagnostic curettage or hysterectomy is planned, or 3) elective radiologic procedures are planned. Problems with interpretation of immunoassays are discussed. The beta subunit HCG radioimmunoassay is the most sensitive test available. It uses antisera against the isolated polypeptide chains of HCG to obtain maximum specificity. Other pituitary hormones do not interfere with this assay. Because of its specificity and great sensitivity (5 mU/ml), the beta subunit assay is ideal for diagnosing early and ectopic pregnancy as well as for managing all phases of trophoblastic disease.