Chorionic gonadotropin

Context.-As part of a College of American Pathologists (CAP) proficiency testing survey, a comparison is made between human chorionic gonadotropin (hCG) results from an actual patient pool and a similarly targeted artificial sample. The goal is to gain insight into the possible source(s) of bias attributable to the proficiency testing material (PTM) with a view toward creating more appropriate survey materials.

Objective.-To compare hCG and related variants in a pool of fresh frozen sera (FFS) with that found in PTM.

Design.-The 2003 CAP K/KN-A Survey included a FFS specimen along with admixtures of PTM. The FFS (K-02) and 1 PTM admixture (K-01) had similar mean hCG values. Five hCG-related analytes were measured on these 2 samples by a reference laboratory.

Participants.-Approximately 1800 clinical laboratories and diagnostic test kit manufacturers participated in the K/ KN-A Survey.

Main Outcome Measures.-Method imprecision (coefficient of variation) and method bias (relative difference between peer group mean and all-method median) were computed for the 2 samples. Differences were evaluated with respect to hCG-related analytes levels.

Results.-All-method hCG results were 12.9 mIU/mL (12.9 IU/L) for the PTM material and 21.6 mIU/mL (21.6 IU/L) for the FFS material. Method biases for 14 manufacturers were greater for PTM than for FFS (-40% to +35% and -16% to +23%). Twelve of 14 methods had higher coefficients of variation on PTM. Total hCG and free β hCG measurements by the reference laboratory were 14.1 mIU/ mL (14.1 IU/L) for the PTM material and 18.5 mIU/mL (18.5 U/L) for the FFS material (FFS), and 2.4 (PTM) and 0.7 (FFS) mIU/mL (2.4 and 0.7 IU/L), respectively. On a molar basis, free β represented 17% and 4% of the total hCG, respectively. Levels of hyperglycosylated hCG, nicked hCG, and β core fragment were not measurable in either sample.

Conclusions.-It is unlikely that the hCG added to the PTM is the source of the increased bias and variability. The main difference is a 3-fold increase in free β found in the PTM, but methods previously found to strongly react with free β were not systematically elevated. The biases between manufacturers found for the FFS specimen are likely attributable to calibration differences.

(Arch Pathol Lab Med. 2005;129:328-330)

Specimens distributed by proficiency testing programs can yield systematically different (biased) results when compared with results obtained using actual patient specimens or pools. These differences can be the result of the artificial matrix used to manufacture proficiency testing specimens leading to unpredictable matrix effect biases. These biases can differ by analyte and by assay method. Immunoassays of hormones such as human chorionic gonadotropin (hCG) can experience another form of bias. Human chorionic gonadotropin found in body fluids exists in a variety of structural variants and charge isoforms. Assays differ in their ability to detect these variants1 because of the specificity of the antiserum used in the immunoassay, and because of the choice of standards used to calibrate the assay. The hCG added to the proficiency testing material (the spike) is purified from maternal urine, which contains hCG variants not found in maternal serum. Consequently, the spike may contain hCG variants or mixtures of variants not found in native serum, causing "spike effect" biases. The current study quantified hCG isoforms in 2 samples distributed to a large number of clinical laboratories (and manufacturers). One was a fresh frozen specimen (FFS) containing only hCG native to the sample, the other a manufactured proficiency test specimen (PTM). An analysis of the results for the 2 specimens may help determine whether differences reported by various hCG assays might be explained by the differences in isoforms, that is, the spike effect bias.

MATERIALS AND METHODS

Samples, Study Design, Data

Details of the collection and distribution of the FFS and PTM samples, study design, and data used in this analysis are described in an accompanying article.2 The 2 samples analyzed for this study were the PTM (K-01) and the fresh frozen material (K-02). Five hCG-related analytes were also measured on these 2 samples by an hCG reference laboratory (USA hCG Reference Service, Department of Obstetrics and Gynecology, University of New Mexico, Albuquerque). The analytes included total hCG and free β hCG (Immulite, Diagnostic Products Laboratories, Los Angeles, Calif), hyperglycosylated hCG (Nichols Advantage, Nichols Institute Diagnostics, San Clemente, Calif), nicked hCG (in-house enzyme immunoassay), and β core fragment (in-house enzyme immunoassay). Other products evaluated in this study included Abbott AxSYM and Abbott IMX B hCG (Abbott Diagnostics, Abbott Park, 111); Beckman Access/2 (Beckman Diagnostics, Fullerton, Calif); Bayer Advia Centaur, Bayer ACS:180, and Bayer Immuno-1 (Bayer Diagnostics, Cambridge, Mass); Dade Dimension HM (Dade Behring, Deerfield, Ill); Vitros ECI (Ortho-Clinical Diagnostics, Raritan, NJ); Roche Elecsys/E170, Roche Elecsys/ E170BETA, Roche Elecsys/E170STAT'(Roche Diagnostics, Indianapolis, Ind); DPC Immulite, DPC Immulite 2000 (Diagnostic Products Corporation, Los Angeles, Calif); Tosoh AIA-Pack BHCG (Diamond Diagnostics, Holliston, Mass).

Statistical Analysis

Measurements of hCG results were available from 1895 participating laboratories using 14 methods. The PTM and the FFS had all-method values of 12.9 mlU/mL (12.9 IU/L) and 20.9 mIU/ mL (20.9 IU/L), respectively. Method imprecision (as measured by the coefficient of variation) and method bias (defined as the disparity between peer group mean and the median of all methods) were compared between the 2 samples for the 14 methods used by survey participants.

RESULTS

The Table shows the data for the PTM and FFS material for each of the 14 hCG methods that included at least 15 participating laboratories. The between-method biases were greater for the PTM specimen (range, -40% to +35%) versus the FFS specimen (range, -16 to +23%). Peer group coefficients of variation were only slightly, but systematically, higher for the PTM specimen (mean, 8.1%; range, 4.4%-13.6%) than for the FFS specimen (mean, 7.2%; range, 3.4%-12.3%). Nine of the 14 methods had lower coefficients of variation on the FFS specimen.

The total hCG values from the reference laboratory were 14.1 mlU/mL (14.1 IU/L) for the PTM specimen and 18.5 mlU/mL (18.5 IU/L) for the FFS specimen, similar to the all-method results from College of American Pathologists survey participants and close to others using the same method (Immulite). Free β hCG measurements were 2.4 mlU/mL (2.4 IU/L) for the PTM specimen and 0.7 mIU/ mL (0.7 IU/L) for the FFS specimen, representing 17% and 4% of the total hCG (on a molar basis), respectively. Levels of hyperglycosylated hCG, nicked hCG, and β core fragment were not measurable in either sample (

Column 3 of the Table also shows the efficiency in detecting an hCG free β hCG standard, calibrated in mIU/ mL as molar equivalents of hCG.1·"4 No correlation was found between the efficiency of the methods to detect the free β hCG, the mean of the PTM specimen, or the mean of the FFS specimen. Furthermore, the method that would be predicted to show the highest impact from the free β hCG concentration (Beckman Access/2) yielded the lowest PTM mean result. The method that was expected to experience the smallest impact from the free β hGC concentration (Tosoh AIA-pack hCGβ) gave mid-range results.

COMMENT

The PTM specimen was associated with a wider range of biases and slightly higher method-specific imprecision than the corresponding FFS specimen. The wider range of biases for the PTM specimen among participants might have resulted because of varying method-related specificities to hCG-related components present in the specimens. However, the only substantial difference detected by a reference laboratory was a 3-fold increase in the free β hCG in the PTM material. The previously demonstrated ability of methods to detect free β hCG did not correlate with levels measured in the 2 specimens. Furthermore, methods that exaggerated or underreported free β hCG concentrations were not associated with undue elevation or reduction in PTM of FFS values. We infer that the increased presence of free β hCG in the PTM specimen is not likely to be the cause of the wider range of biases. It is therefore speculated that the wider range of biases with the PTM specimen was likely due to a matrix problem. Manufacturing PTM that more closely approximates the matrix of normal human serum has the potential to improve the reliability of proficiency testing results. The remaining variability among methods results in the FFS specimen may be attributable to differences in calibration. However, these conclusions should be viewed with caution because the values of the FFS and PTM material are at the lower end of the hCG calibration curves. The differences might be less pronounced if the hCG values had been higher.

References

1. Cole LA. lmmunoassay of human chorionic gonadotrophin, its free subunits, and metabolites. CUn Chem. 1997:43:2233-2243.

2. Schreiber WE, Endres DB, McDowell CA, et al. Comparison of fresh frozen serum to proficiency testing material in College of American Pathologists surveys: a-fetoprotein, carcinoembryonic antigen, human chorionic gonadotropin, and prostate-specific antigen. Arch Pathol Lab /Kerf. 2005;129:331-337.

3. Cole LA, Button JM, Higgins TN, Cembrowski CS. Between-method variation in human chorionic gonadotropin test results. Clin Chem. 2004:50:874-882.

4. Cole LA, Shahabi S, Butler S, et al. Utility of commonly used commercial hCG immunoassays in the diagnosis and management of trophoblastic diseases. Clin Chem. 2001;47:308-315.

George I. Knight, PhD; Glenn E. Palomaki, BS; George G. Klee, MD, PhD; William E. Schreiber, MD; Lawrence A. Cole, PhD

Accepted for publication September 17, 2004.

From the Foundation for Blood Research, Scarborough, Me (Dr Knight and Mr Ralomaki); Mayo Clinic, Rochester, Minn (Dr Klee); Vancouver Hospital and Health Sciences Center, Vancouver, British Columbia (Dr Schreiber); and University of New Mexico, Albuquerque (Dr Cole).

Dr Klee declares that he has received research grants from Beckman Coulter and Biosite for work unrelated to the preparation of this manuscript. All other authors also have no relevant financial interest in the products or companies described in this article.

Reprints: George ). Knight, PhD, Foundation for Blood Research, 69 US Rte 1, PO Box 190, Scarborough, ME 04074 (e-mail: gknight@fbr. org).

Copyright College of American Pathologists Mar 2005
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