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Received on September 5, 2003
Accepted on February 20, 2004
Endocrinology and Metabolism |
1 USA hCG Reference Service, Department of Obstetrics and Gynecology, University of New Mexico, Albuquerque, NM 87131
2 Dynacare Kasper Medical Laboratories, Edmonton, Alberta, Canada
3 Department of Laboratory Medicine and Pathology, University of Alberta, Edmonton, Alberta, Canada
* To whom correspondence should be addressed. E-mail: larry{at}hcglab.com.
Background: Results on sera and calibrators vary 1.4- to 2.3-fold among commercial human chorionic gonadotropin (hCG) assays. The relative contributions of calibrators, standards, hCG charge isoforms, and major structural variants to this variation have not been quantified.
Methods: Purified hCG was separated by isoelectric focusing into four fractions with pI ranges of 3-4, 4-5, 5-6, and 6-7. These four fractions together with pure hCG, hyperglycosylated hCG, hCG 
-subunit (hCGb), nicked hCG, and hCGb core fragment (hCGbcf) were tested in nine commonly used commercial serum assays for hCG. The compositions of pure hCG preparations, standards, and commercial hCG preparations were determined by immunoassay.
Results: The three pure hCG preparations and the four hCG charge isoforms each showed parallel responses in the nine commercial hCG assays. Although wide variations were found in the detection of hCG structural variants by the nine assays (hyperglycosylated hCG, hCGb, nicked hCG, and hCGbcf), this did not correlate with the between-method variation observed in results for the three pure hCG preparations. Commercial preparations of hCG and calibrators showed great variation in their content of hCG structural variants.
Conclusions: Intermethod differences in hCG results were not explained by changes in responses attributable to hCG charge isoforms or to hCG structural variants, but wide variation was observed in concentrations of hCG structural variants in calibrators and in detection of these structural variants. Differences in assay specificity and in composition of the calibrators are the most likely sources of between-method variation.
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