Comparison of seven immunoassays for the quantification of CA 125 antigen in serum Elvira M. Davelaar, 1 Gerard J. van Kamp, 2 Rob A. Verstraeten, 1 and Peter Kenemans 1* Seven CA 125 immunoassays were compared for their clinical performance. CA 125 concentrations were deter- mined in 289 serum samples obtained from women with benign pelvic tumors (samples from 98 patients) and patients with various cancers (samples from 111 pa- tients). In the range of 0 –1000 kilounits/L, all assays tested were linearly correlated, with correlation coeffi- cients ranging from 0.89 to 0.99. In relation to the original Centocor CA 125 assay, there was an overall tendency to measure higher absolute values in the lower CA 125 value range. This was not seen in relation to the Centocor CA 125 II assay. ROC curves (benign vs pretreatment ovarian cancer patients) were nearly iden- tical for all assays, and the areas under the ROC curves were not markedly different. We conclude that the CA 125 assays tested are strongly related to each other and are clinically reliable for the quantification of serum CA 125 and that none of the assays offers higher diagnostic accuracy or better discrimination between patient groups, especially not in the lower ranges. Highly specific double-determinant monoclonal antibody- based immunoassays have been developed during the last decade for the quantification of tumor markers in serum. The OC 125 monoclonal antibody (Moab) was generated by immunization of BALB/c mice with the OVCA 433 cell line isolated from ascitic fluid of a patient with a serous papillary cystadenocarcinoma of the ovary (1). This OC 125 Moab was incorporated into an assay detecting a mucin-like glycopro- tein; therefore named CA 125. The CA 125 antigen became an established marker and at present is commonly used in gynecologic practice for patient management in ovarian cancer (2, 3). In the original Centocor CA 125 assay, a homologous double-determinant assay, the OC 125 Moab was used both as a catcher antibody and as a tracer anti- body. Repetition of the antibody-defined epitope on the CA 125 antigen is mandatory for binding and detection. The second generation CA 125 assays are of the heterologous double-determinant assay type, where the M11 murine Moab is used as the capture antibody, replacing the OC 125 Moab on the solid phase. Co-expression of both epitopes on the same antigen molecule is needed for binding and detec- tion. It should be realized, however, that marker values obtained with the first generation commercial CA 125 kits can give discordant (4), and even discrepant, results (5), whereas in the second generation assays, results are re- ported to be more in agreement (6 –9). Evaluations of second generation CA 125 assays have shown excellent analytical performance in combination with high quality and good quantitative relations with the original CA 125 assays (8, 9). More Moabs, reactive with other epitopes on the CA 125 antigen, were generated and classified (10), and some were incorporated into CA 125 assays. The aim of this study was to compare results obtained with first and second generation CA 125 assays and with assays applying other Moabs of the M11 category. Materials and Methods patients and sera CA 125 concentrations were quantified in 289 serum samples obtained from 98 female patients with benign pelvic tumors [i.e., uterine fibroids (n = 15), endometriotic lesions (n = 26), and benign ovarian tumors (n = 57)] and from 89 patients with adenocarcinoma of the endome- trium (n = 23), colon (n = 20), and ovary (n = 46). In addition, 102 serum samples obtained serially from 22 ovarian cancer patients with active disease (during and after chemotherapy) were included in the evaluation. All blood samples were collected by venipuncture before surgery or serially during follow-up; sera were kept frozen at -70 °C until assayed for CA 125. ca 125 assays CA 125 values were measured using the following assays: the original Centocor CA 125 and the Centocor CA 125 II Departments of 1 Obstetrics and Gynaecology, and 2 Clinical Chemistry, Academic Hospital Vrije Universiteit, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands. *Author for correspondence. Fax 31-20-4444811; e-mail kenemans@azvu.nl. Received September 17, 1997; revision accepted March 30, 1998. Clinical Chemistry 44:7 1417–1422 (1998) Enzymes and Protein Markers 1417