Differences in recognition of the 1st WHO international reference reagents for hCG-related isoforms by diagnostic immunoassays for human chorionic gonadotropin.
BACKGROUND
The 1st WHO International Reference Reagents (IRRs) for 6 human chorionic gonadotropin (hCG)-related molecular variants, highly purified and calibrated in substance concentrations by the IFCC Working Group for hCG, permit experimental elucidation of what commercially available hCG methods measure in molar terms and enable assessment of their fitness for clinical purposes.


METHODS
Pools containing known amounts of the IRRs spiked into normal human serum were issued to participants through the UK National External Quality Assessment Service for hCG for a period of 7 years. Among 16 assays used, 4 recognized only hCG, whereas 6 recognized hCG and its free beta-subunit (hCGbeta), and 6 recognized hCG, hCGbeta, and the beta core fragment.


RESULTS
Differences in calibration of current hCG assays are moderate. Mean recovery of the current International Standard (IS), hCG IS 75/589, was 107% (range 93% to 126%), whereas that of the IRR 99/688 for hCG was 139% (range 109%-164%). Between-method variation for the latter (CV 12.3%) was also greater than for IS 75/589 (CV 8.8%). Recognition of hCGbeta varied markedly (CV 37%). Most assays overestimated it, but 2 RIAs produced results that were slight underestimations. Recognition of the beta core fragment was even more variable (CV 57%) and was closest to equimolarity for the RIAs.


CONCLUSIONS
Assays for hCG show considerable variation in their recognition of various forms of hCG, and this variability is the most important cause of method-related differences in hCG results in serum and an even more important cause of method-related differences in urine measurements. Equimolar recognition of the major hCG isoforms is essential if between-method comparability for hCG is to be improved.
Sturgeon
Catharine M
CM
Department of Clinical Biochemistry, Royal Infirmary, Edinburgh, UK. C.Sturgeon@ed.ac.uk
Berger
Peter
P
Bidart
Jean-Michel
JM
Birken
Steven
S
Burns
Chris
C
Norman
Robert J
RJ
Stenman
Ulf-Håkan
UH
D002138:Calibration; D006063:Chorionic Gonadotropin / Q000097:blood* / Q000652:urine*; D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000097:blood / Q000652:urine; D005260:Female; D015292:Glycoprotein Hormones, alpha Subunit / Q000097:blood / Q000652:urine; D006801:Humans; D007118:Immunoassay / Q000592:standards*; D010446:Peptide Fragments / Q000097:blood / Q000652:urine; D020033:Protein Isoforms / Q000097:blood / Q000652:urine; D012015:Reference Standards; D014944:World Health Organization
D006063:Chorionic Gonadotropin; D018997:Chorionic Gonadotropin, beta Subunit, Human; D015292:Glycoprotein Hormones, alpha Subunit; D010446:Peptide Fragments; D020033:Protein Isoforms; C063554:urinary gonadotropin fragment