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Clinical Chemistry 44: 1417-1422, 1998;
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(Clinical Chemistry. 1998;44:1417-1422.)
© 1998 American Association for Clinical Chemistry, Inc.

Enzymes and Protein Markers

Comparison of seven immunoassays for the quantification of CA 125 antigen in serum

Elvira M. Davelaar1, Gerard J. van Kamp2, Rob A. Verstraeten1, and Peter Kenemans1,a

Departments of
1 Obstetrics and Gynaecology, and
2 Clinical Chemistry, Academic Hospital Vrije Universiteit, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands.
a Author for correspondence. Fax 31-20-4444811; e-mail kenemans{at}azvu.nl.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Seven CA 125 immunoassays were compared for their clinical performance. CA 125 concentrations were determined in 289 serum samples obtained from women with benign pelvic tumors (samples from 98 patients) and patients with various cancers (samples from 111 patients). In the range of 0–1000 kilounits/L, all assays tested were linearly correlated, with correlation coefficients 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 identical 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.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
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 glycoprotein; 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 antibody. 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 detection. 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 reported to be more in agreement (6)(7)(8)(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
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
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 endometrium (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 assays (Centocor), the Boehringer Mannheim (BM) Enzymun® CA 125 II assay (Boehringer Mannheim), the BYK Liamat CA 125 II assay (Byk-Sangtec), the Mochida CA 602 assay (Mochida Pharmaceutical Co.), the CanAg OV 185 assay (CanAg Diagnostics), and the Abbott IMx® CA 125 (recalibrated) assay (Abbott Diagnostics Division). The characteristics of each assay are given in Table 1 .


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  		Table 1. Assay description.

statistical methods
The results of the CA 125 determinations with the seven assays were compared by least-squares linear regression analysis for all patient serum samples, for serum samples of the different patient groups, and for different test result ranges. The relative difference (RD) between the Centocor CA 125 assays (a) and the other assays (b), was calculated according to the following formula: RD = ((a - b)/a) x 100, for test results obtained in pretreatment serum samples from ovarian cancer patients. In addition, Receiver Operating Characteristic (ROC) curves for the discrimination of ovarian cancer from benign pelvic tumors were established for each assay tested, and sensitivity, specificity, positive and negative predictive values, and overall test accuracy were calculated for both the cutoff at optimal accuracy and at the generally accepted cutoff value of 35 kilounits/L.


   Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
comparison of ca 125 assay results
CA 125 assay results, median values, and the ranges found in the different study groups are given in Table 2 . The median CA 125 value found in the total study group (n = 289), varies from 20 (with CanAg OV 185) to 57 (with Mochida CA 602) kilounits/L, the median CA 125 value being 29 kilounits/L in the original Centocor CA 125 assay.


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  		Table 2. Comparison of CA 125 results (kilounits/L) in different study groups.

linear regression analysis
Regression analysis of CA 125 values in all patient serum samples obtained with the seven methods and separately for all serum samples with CA 125 values in the ranges of 0–35, 0–1000, and >1000 kilounits/L gave correlation coefficients ranging from 0.61 to 0.99 (only data for the range 0–1000 kilounits/L are shown in Table 3 ). For patients with benign pelvic tumors, linear regression analysis of CA 125 values in the ranges 0–35 and 0–1000 kilounits/L gave correlation coefficients ranging from 0.59 to 0.99 for all assays (data not shown); no CA 125 values >1000 kilounits/L were found for patients with benign pelvic tumors. Linear regression analysis of CA 125 values for patients with ovarian cancer, the total group, and in the CA 125 ranges 0–35, 0–1000, and >1000 kilounits/L gave correlation coefficients ranging from 0.68 to 0.99 for all assays (data not shown). For patients with adenocarcinoma of the endometrium and colon, linear regression analysis of CA 125 values in the ranges 0–35 and 0–1000 kilounits/L gave correlation coefficients ranging from 0.64 to 1.0 for all assays (data not shown). None of the patients in this group had CA 125 values >1000 kilounits/L.


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  		Table 3. CA 125 immunoassay comparison study for all patient samples in the range 0–1000 kilounits/L.

The assays tested in this study showed a linear relation with each other in all 21 comparisons done for all patient serum samples, for serum samples of the different patient groups, and for the different test result ranges (data not shown).

When the generally accepted cutoff value of 35 kilounits/L was used in the independent assay, the calculated values for the dependent assays ranged from as low as 12 kilounits/L to as high as 86 kilounits/L (Table 3Up ).

relative difference
Relative differences between the CA 125 values measured with the Centocor CA 125 II assay and the comparison assays are shown in Fig. 1 , A-F. A tendency to lower absolute CA 125 values in the range 0–100 kilounits/L is found for all assays, except for the Mochida CA 602 assay.



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  		Figure 1. Scatter diagram depicting the relative difference between assays of pretreatment serum samples from ovarian cancer patients (n = 46).

(A) Centocor CA 125 II and Centocor CA 125; (B) Centocor CA 125 II and BM Enzymun CA 125 II; (C) Centocor CA 125 II and BYK Liamat CA 125 II; (D) Centocor CA 125 II and Mochida CA 602; (E) Centocor CA 125 II and CanAg OV 185; and (F) Centocor CA 125 II and Abbott IMx CA 125.

In relation to the original Centocor CA 125 assay, we found that all assays showed a tendency towards higher absolute values in the lower CA 125 value range and lower absolute values in the higher range, except for the CanAg OV 185 assay, which measures lower values over the total range (data not shown).

roc curves and different diagnostic assay characteristics
ROC curves for ovarian cancer (n = 46) vs benign pelvic tumors (n = 98) for pretreatment serum samples are given in Fig. 2 . The greatest area under the ROC curve was found with the Mochida CA 602 assay (0.896); all other assays had similar areas (Fig. 2 ), not markedly different from each other and from the Mochida CA 602 area. Test accuracy in discriminating pretreatment ovarian cancer patients (n = 46) from patients with benign pelvic tumors (n = 98) is ~83%, with an optimal accuracy at a cutoff ranging from 50 to 95 kilounits/L (Table 4 A). Assay characteristics at the widely used cutoff of 35 kilounits/L are shown in Table 4B .



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  		Figure 2. Actual ROC curves for CA 125 assays [pretreatment ovarian cancer (n = 46) vs benign pelvic tumor (n = 98) samples].

Area under the curve ± SE: Centocor CA 125 (0.865 ± 0.0036), Centocor CA 125 II (0.874 ± 0.0034), BM Enzymun CA 125 II (0.872 ± 0.0034), Mochida CA 602 (0.896 ± 0.028), CanAg OV 185 (0.871 ± 0.034), Abbott Imx CA 125 (0.880 ± 0.033), and BYK Liamat CA 125 II (0.884 ± 0.032).


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  		Table 4. Assay characteristics in pretreatment ovarian cancer (n = 46) vs benign ovarian tumor samples (n = 98).

ovarian cancer follow-up
Serial serum samples were from 22 ovarian cancer patients monitored for the course of the disease during treatment and follow-up. All assays showed similar patterns in all patients studied; an example is given in Fig. 3 .



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  		Figure 3. Serial measurement in an ovarian cancer patient monitored for course of disease during treatment and follow-up.

Ca, cancer.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
This study shows that the second generation CA 125 assays and assays applying Moabs of the M11 group are highly concordant with each other and with the original Centocor CA 125 assay. This is important because, at this moment, the original CA 125 homologous double-determinant assay has been replaced by one of the second generation heterologous double-determinant CA 125 II assays, which is well established and commonly used today in gynecologic practice. CA 125 has an important diagnostic value in discriminating ovarian cancer from benign pelvic tumors when used in combination with ultrasonography (11) or CA 15.3 (12). It is also useful in discriminating ovarian cancer from colorectal malignancies (13). Furthermore, CA 125 is of value in tumor status determination after completion of first line chemotherapy and initial debulking, monitoring of disease in ovarian cancer patients, and early detection of recurrence (2), in the early prediction of outcome of response to first line chemotherapy (3), and also when paclitaxel is given in relapsed ovarian cancer patients (14). CA 125 also proved to be useful in diagnosing advanced endometriosis when measured during the midfollicular phase (15) and in monitoring endometriosis during gonadotropin-releasing hormone agonist treatment (16).

The CA 125 antigen carries two major antigenic domains, and monoclonal antibodies against CA 125 can be classified as either OC125-like or M11-like (10). With the introduction of second generation CA 125 assays, it was assumed that the use of two different antibodies would improve the quality of the assay. However, when the assay test results obtained in pretreatment serum samples of ovarian cancer patients vs patients with benign ovarian tumors are assessed (Table 4AUp ), we find sensitivity, specificity, positive and negative predictive values, and overall test accuracy to be highly similar. This is also reflected by nearly identical ROC curves (Fig. 2Up ). When the generally accepted cutoff of 35 kilounits/L is applied, the second generation assays seem to improve slightly in sensitivity but not in specificity (Table 4BUp ).

Median CA 125 values obtained for all serum samples measured with the Mochida 602 and CanAg OV 185 assays are higher and lower, respectively, compared with the original Centocor CA 125 and the Centocor CA 125 II assays (Table 2Up ). This may be due to differences in specificity and affinity of the Moabs used in these assays. The assays tested in this study showed a linear relationship with each other in all 21 comparisons done for all patient serum samples, for serum samples of the different patient groups, and for the different test result ranges. The CanAg OV 185 assay has the tendency to yield lower values than the other assays, which is reflected by lower slopes (Table 3Up ) and in a higher relative difference over the whole range when compared with both the original Centocor CA 125 (data not shown) and the Centocor CA 125 II assay (Fig. 1EUp ). One explanation could be that the CanAg OV 185 assay is a homologous double-determinant assay, using the same M11-like antibody both as capture- and as tracer antibody. The M11-like group of antibodies is more homogeneous than the OC125-like group, showing a strong cross-inhibition with most other antibodies from the same group, and in general, they do not form good immunoassay pairs with other members of their group (10).

The other assays, when compared to the original Centocor CA 125 assay, tend to measure higher absolute values in the lower range (0–100 kilounits/L) and lower absolute values in the higher range, except for the CanAg OV 185 assay. In clinical practice, systematically lower values in the higher range do not pose a problem, but higher values in the lower range may have important clinical consequences, especially when the generally accepted cutoff value of 35 kilounits/L is used. In a comparison of the other assays with the new standard assay, the Centocor CA 125 II (Fig. 1Up , A-F), an overall tendency was found to lower absolute values in the lower CA 125 value range and higher absolute values in the higher CA 125 range for the BM Enzymun and BYK CA 125 II assays. For the Mochida CA 602, on the contrary, higher values were found over the whole range. With the CanAg OV 185 and the Abbott IMx CA 125 assays, lower values were measured over the whole range. This might be explained by the fact that M11-like antibodies are incorporated in the Mochida CA 602 and the CanAg OV 185 assays, whereas the Abbott IMx CA 125 assay uses polyclonal antibodies for catching, with the OC 125 Moab as detector. Moreover, differences in standardization are of influence. In those assays incorporating OC 125 as the detector antibody, one sample was found to measure higher than the original Centocor CA 125 II assay. An explanation for this could not be found in the clinical data.

It can be concluded that the new CA 125 assays are strongly related to each other and are clinically reliable for the quantification of serum CA 125, but they do not offer higher diagnostic accuracy or better discrimination between patient groups, especially not in the lower ranges. However, one should not interchange results from different methods during the course of monitoring disease progression in the same patient.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Bast RC, Jr, Feeney M, Lazarus H, Nadler LM, Colvin RB, Knapp RC. Reactivity of a monoclonal antibody with human ovarian carcinoma. J Clin Invest 1981;68:1331-1337.
  2. Kenemans P, Yedema CA, Bon GG, von Mensdorff-Pouilly S. CA 125 in gynecological pathology–a review. Eur J Obstet Gynecol Reprod Biol 1993;49:115-124. [ISI][Medline] [Order article via Infotrieve]
  3. Yedema CA, Kenemans P, Voorhorst FG, Bon GG, Schijf CPT, Beex LVAM, et al. CA 125 half-life in ovarian cancer: a multivariate survival analysis. Br J Cancer 1993;67:1361-1367. [ISI][Medline] [Order article via Infotrieve]
  4. van Kamp GJ, Verstraeten AA, Kenemans P. Discordant serum CA 125 values in commercial immunoassays. Eur J Obstet Gynecol and Reprod Biol 1993;49:99-103. [ISI][Medline] [Order article via Infotrieve]
  5. Kenemans P, Bon GG, Kessler AC, Verstraeten AA, van Kamp GJ. Multicenter technical and clinical evaluation of a fully automated enzyme immunoassay for CA 125. Clin Chem 1992;38:1466-1471. [Abstract/Free Full Text]
  6. Kenemans P, van Kamp GJ, Oehr P, Verstraeten AA. Heterologous double-determinant immunoradiometric assay CA 125 II: reliable second-generation immunoassay for determining CA 125 in serum. Clin Chem 1993;39:2509-2513. [Abstract]
  7. Uhl W, Denk B. Improved CA 125 determinations using two different monoclonal antibodies. Klapdor R eds. Current tumor diagnosis: applications, clinical relevance, research, trends 1994:384-388 Zuckschwerdt Verlag Munich. .
  8. Kenemans P, Verstraeten AA, van Kamp GJ, von Mensdorff-Pouilly S. The second generation CA 125 assays. Ann Med 1995;27:107-113. [ISI][Medline] [Order article via Infotrieve]
  9. Bonfrèr JMG, Baan AW, Jansen E, Lentfer D, Kenemans P. Technical evaluation of three second generation CA 125 assays. Eur J Clin Chem Clin Biochem 1994;32:201-207. [ISI][Medline] [Order article via Infotrieve]
  10. Nustad K, Bast RC, Jr, O'Brien TJ, Nilsson O, Seguin P, Suresh MR, et al. Specificity and affinity of 26 monoclonal antibodies against the CA 125 antigen: first report from the ISOBM TD-1 workshop. Tumor Biol 1996;17:196-219.
  11. Schutter EMJ, Kenemans P, Sohn C, Kristen P, Crombach G, Westermann R, et al. Diagnostic value of pelvic mass examination, ultrasound, and serum CA 125 in postmenopausal women with a pelvic mass. Cancer 1994;74:1398-1406. [ISI][Medline] [Order article via Infotrieve]
  12. Yedema CA, Massuger L, Hilgers J, Servaas J, Poels L, Thomas CMG, et al. Pre-operative discrimination between benign and malignant ovarian tumors using a combination of CA 125 and CA 15.3 serum assays. Int J Cancer 1988;3(Suppl):61-67.
  13. Yedema CA, Kenemans P, Wobbes T, Thomas CMG, Bon GG, Mulder C, et al. Use of serum tumor markers in the differential diagnosis between ovarian and colorectal adenocarcinomas. Tumor Biol 1992;13:18-26.
  14. Davelaar EM, Bonfrèr JMG, Verstraeten AA, ten Bokkel Huinink WW, Kenemans P. CA 125: a valid marker in ovarian carcinoma patients treated with paclitaxel?. Cancer 1996;78(1):118-127. [ISI][Medline] [Order article via Infotrieve]
  15. Hompes PGA, Koninckx PR, Kennedy S, van Kamp GJ, Verstraeten AA, Cornillie F. Serum CA 125 concentrations during midfollicular phase, a clinically useful and reproducible marker in diagnosis of advanced endometriosis. Clin Chem 1996;42:1871-1874. [Free Full Text]
  16. Özaksit G, Çaglar T, Çiçek N, Kusçu E, Batioglu S, Gökmen O.. Serum CA 125 levels before, during and after treatment for endometriosis. Int J Gynecol Obstet 1995;50:269-273. [Medline] [Order article via Infotrieve]



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