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Brief Communications |
1 Division of Molecular Medicine, Wadsworth Center for Laboratories and Research, New York State Department of Health, Albany, NY; 2 Department of Biomedical Sciences, School of Public Health, University at Albany, Albany, NY;
aaddress correspondence to this author at: Division of Molecular Medicine, Wadsworth Center for Laboratories and Research, New York State Department of Health, Albany, NY 12201-0509. Fax 518-474-9185; e-mail tim{at}wadsworth.org.
Abstract
Background: Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hormone that exists in multiple forms. Immunoassays commonly used in clinical laboratories measure intact hCG, total β hCG (intact hCG + hCG free β-subunit), and/or hCG free β-subunit. Measurement of serum concentrations of hCG is useful for confirmation and monitoring of pregnancy, diagnosis of trophoblastic diseases and monitoring of the efficacy of treatment, and prenatal screening. Correctly reporting results for the various forms of hCG is clinically important.
Method: We prepared samples by addition of intact hCG and hCG free β-subunit to an essentially hCG-free human serum matrix. The samples were analyzed by participant laboratories using various immunoassay methods.
Results: We identified errors in participant reporting of intact hCG results as total β hCG (9.3%; 22 of 235 laboratories) and total β hCG as intact hCG (13.1%; 8 of 61 laboratories).
Conclusions: Many factors contribute to the erroneous reporting of hCG results, including (a) the complexity of hCG molecule and confusion of nomenclature on the various forms of hCG; (b) laboratory personnel’s lack of awareness of the distinctions of the forms of hCG and failure to recognize the specificity of assays for their measurement; (c) lack of clarity and uniformity in manufacturers’ reagent labeling; and (d) most product inserts’ lack of information on the specificity of each method to the various forms of hCG.
Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hormone composed of an 
-subunit (hCG) and a β-subunit (hCGβ). hCG is essentially identical to that of lutropin (LH), follitropin (FSH), and thyrotropin (TSH), whereas hCGβ is unique to hCG and confers its biological and immunological specificity(1). hCG is synthesized and secreted by trophoblastic tissue of the placenta in pregnancy, gestational trophoblastic diseases, and certain other pathological conditions(2)(3)(4)(5). At least 6 different forms of hCG have been identified in serum: nonnicked and nicked intact hCG (hCG and hCGn), nonnicked and nicked free β-subunit (hCGβ and hCGnβ), and regular and hyperglycosylated (or large) free hCG(4). In pregnancy, intact hCG predominates, whereas free hCGβ accounts for <1% of total β hCG (hCG + hCGβ) in serum(4)(6). However, the free hCGβ concentration in patients with trophoblastic diseases or testicular tumors is disproportionately increased, and the ratio of free hCGβ to intact hCG or to hCG + hCGβ is markedly increased(7)(8). Therefore, measurements of hCG + hCGβ and free hCGβ are important in diagnosis and management of trophoblastic diseases and testicular tumors(8)(9)(10)(11)(12). Free hCGβ and hCG + hCGβ have also been suggested to be better biomarkers than intact hCG in prenatal screening for Down syndrome(13).
There are >100 different commercial immunoassay test kits for measuring hCG. Most procedures detect intact hCG (nicked + nonnicked hCG) or hCG + hCGβ, and a few immunoassays detect free hCGβ alone or with other forms(4). In clinical laboratories in the US, fewer than 20 immunoassay methods are commonly used. These assays vary in immunospecificity toward different forms of hCG, and results of measurements may have distinct clinical applications. It is important that laboratorians, who perform the test and consult with physicians about test procedures and results, be aware of the heterogeneity of hCG and the specificity of the method in service.
The Wadsworth Center of the New York State Department of Health conducts CLIA’88-approved proficiency testing programs, and CLIA’88 regulations define acceptable performance criteria for qualitative and quantitative test results for hCG(14). Based on information from manufacturers’ product inserts for the packages of test kits commonly used by clinical laboratories in the US, the method principles, and the occurrence of test kit names in peer-reviewed articles, we request participant laboratories to report results as "Intact hCG" and/or "Total β hCG" for quantitative results of hCG as appropriate.
In the proficiency test event of January 2006, we prepared 1 of 5 specimens by addition of intact hCG and free hCGβ to final concentrations of 50 IU/L and 20 IU/L, respectively, as well as other non–hCG-related hormones, into human serum matrix that had essentially no endogenous hCG. Intact hCG, WHO standard (code 75/589), was purchased from the National Institute for Biological Standards and Control, and free hCGβ was from AspenBio Pharma. The addition of both intact hCG and free hCGβ was intended to mimic pathophysiological conditions and to challenge the specificity of various immunoassays. Along with 4 other proficiency test specimens prepared by the sole addition of intact hCG, the samples were delivered overnight on ice to participant laboratories. Results of total β hCG were reported by 266 laboratories. (Complete details on results from the proficiency test events discussed in this paper are available at http://www.wadsworth.org/chemheme/chem/endo/ptframes.htm.)
Total β hCG results for specimens with both intact hCG and hCGβ showed higher overall variability (16% CV) than the samples spiked with only intact hCG (9.6% CV). We also noticed a bimodal distribution of results reported by users of the Roche Elecsys system for total β hCG, with the specimen containing both forms of hCG: 2 medians were centered at approximately 47 IU/L and 67 IU/L (Fig. 1
). In contrast, we found normal distributions of results for the other 4 specimens with this method. Further investigation revealed that 2 different reagent systems are provided for the Roche Elecsys instrument, reagent "HCG STAT" for measuring intact hCG and reagent "HCG + β" for measuring total β hCG (hCG + hCGβ). Those laboratories that reported results of approximately 47 IU/L used the reagent HCG STAT, and those that reported values of approximately 67 IU/L used the reagent HCG + β. Unfortunately, many laboratories that used the HCG STAT procedure reported their results as total β hCG.
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We carried out further studies in the proficiency test event of May 2007. We prepared sample E03 by the sole addition of intact hCG at approximately 45 IU/L; sample E04 by the sole addition of free hCGβ at approximately 30 IU/L; and sample E05 by spiking with both intact hCG (approximately 40 IU/L) and free hCGβ (approximately 20 IU/L). These preparations, plus other non-hCG related analytes, were added to an essentially hCG-free human serum matrix. In sample E04, the intact hCG values should be undetectable, since no intact hCG was added, and total β hCG values should be close to the calculated concentration of 30 IU/L. Results for total β hCG were reported by 235 participant laboratories using 12 FDA-approved immunoassay methods and ranged from <1 to 49.6 IU/L for sample E04 (Table 1
). In contrast, results ranged from 33.3 to 53.9 IU/L for sample E03 and 40 to 83 IU/L for sample E05. Results for intact hCG were reported by 61 laboratories using 7 immunoassay instruments and ranged from 1 to 50 IU/L for sample E04, in comparison to results ranging from 34 to 53.3 IU/L for sample E03 and 35 to 77.2 IU/L for sample E05 (Table 1
). Investigation of the sources of the discrepancy revealed that the Dade Behring Dimension Flex reagent cartridge hCG detects only the intact hCG molecule, and therefore the hCG values measured on this instrument were close to zero in sample E04, close to 45 IU/L in sample E03, and about 40 IU/L in sample E05. However, 20 of 65 users of this method reported results as total β hCG, rather than the correct intact hCG. Errors were observed with the following 3 automated immunoassay instruments, which detect both intact hCG and free hCGβ (total β hCG or hCG + hCGβ): Bayer Advia Centaur (Total hCG), Vitros ECi/ECiQ (Total β-hCG), and Siemens Immulite series (Immulite 1000 HCG and Immulite 2000 HCG). For these instruments, however, a total of 8 laboratories reported results as intact hCG. Two Elecsys HCG STAT users also reported intact hCG results as total β hCG (Table 1
).
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In addition, the recovery of total β hCG differed among these immunoassay methods (Table 1
). The variation is likely due to the difference in immunoreactivity of these immunoassays toward hCGβ and/or differences in calibration.
CLIA requires that proficiency test specimens be treated in the same manner as patient specimens(14). Erroneous reporting of proficiency test results will thus reflect errors in reporting of patient results that could cause misdiagnosis and mismanagement of trophoblastic diseases, although such errors may not affect the diagnosis of normal pregnancy, in which both intact hCG and total β hCG increase dramatically(4)(5). Given the overall reporting error rate of 13.1% (8 of 61) for intact hCG and 9.3% (22 of 235) for total β hCG, the occurrence of such errors is likely to be a significant problem worldwide. Such errors also occur in the literature; for instance, publications have reported "total hCG" determined on the Dade Dimension instrument and "intact hCG" measured on the Immulite system(15)(16).
The utility of monitoring hCG and its various forms reaches beyond the detection of pregnancy to various disease conditions, e.g., ectopic pregnancy, choriocarcinoma, testicular cancer, trophoblastic diseases, and nontrophoblastic tumors(2)(3)(5)(7)(9)(10)(11)(12)(17). National Academy of Clinical Biochemistry (NACB) and European Group on Tumor Markers (EGTM) guidelines emphasize that both intact hCG and free hCGβ be detected by assay kits for tumor markers(18)(19).
To provide reliable services, laboratorians and clinicians must know which forms of hCG the method in service detects. We conclude that at least 4 factors have contributed to the incorrect reporting of hCG results by laboratories: (a) the complexity of the hCG molecule and confusion of nomenclature on the various forms of hCG; (b) laboratory personnel’s lack of awareness of the distinctions of the forms of hCG and failure to recognize the specificity of assays for their measurement; (c) manufacturers’ lack of clarity and uniformity in reagent labeling; and (d) most product inserts’ lack of information on the specificity of each method to the various forms of hCG.
Solving this problem will require involvement of multiple parties in reaching a consensus on nomenclature and clarity of information provided in package inserts. The availability of new WHO reference materials for hCG and its 5 other forms makes this task feasible(20). An urgent need is for manufacturers to clearly state what forms of hCG are detected by each assay. Appropriate labeling and information on assay specificity should then be provided to the user, since current labeling is both inadequate and confusing(4). Furthermore, it has been noted that when hCGβ is measured by assays detecting hCG and hCGβ together, the results are erroneous as they are based on the IU for hCG; it has been recommended that assay procedures report results in molar concentrations(5).
Acknowledgments
Grant/Funding Support: None declared.
Financial Disclosures: None declared.
References
- and β-subunits, and their ratios. Eur J Endocrinol 2005;153:565-575.The following articles in journals at HighWire Press have cited this article:
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  C. M. Sturgeon, P. Berger, J.-M. Bidart, S. Birken, C. Burns, R. J. Norman, U.-H. Stenman, and on behalf of the IFCC Working Group on hCG Differences in Recognition of the 1st WHO International Reference Reagents for hCG-Related Isoforms by Diagnostic Immunoassays for Human Chorionic Gonadotropin Clin. Chem., August 1, 2009; 55(8): 1484 - 1491. [Abstract] [Full Text] [PDF]
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A. M. Gronowski and D. G. Grenache Characterization of the hCG Variants Recognized by Different hCG Immunoassays: An Important Step Toward Standardization of hCG Measurements Clin. Chem., August 1, 2009; 55(8): 1447 - 1449. [Full Text] [PDF]
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