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Clinical Chemistry 45: 670-675, 1999;
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(Clinical Chemistry. 1999;45:670-675.)
© 1999 American Association for Clinical Chemistry, Inc.

Articles

Comparison of Three Different Plasma Homocysteine Assays with Gas Chromatography–Mass Spectrometry

Johan B. Ubbink1,a, Rhena Delport1, Reiner Riezler2 and W.J. Hayward Vermaak1

1 Department of Chemical Pathology, University of Pretoria, P.O. Box 2034, Pretoria 0001, South Africa.

2 Severimed, Wiedaustrasse 202, 48163 Münster, Germany.

3 Total homocyst(e)ine refers to the sum of the concentrations of free homocysteine, protein-bound homocysteine, the disulfide homocystine, and the mixed disulfide homocysteine-cysteine.
a Author for correspondence. Fax 27-12-3283600; e-mail jubbink{at}medic.up.ac.za

Background: Various methods are available to measure plasma total homocyst(e)ine (tHcy) concentrations, but whether plasma tHcy assays may be used interchangeably is not known.

Methods: Results from three different methods [HPLC with fluorescence detection, enzyme immunoassay (EIA), and fluorescence polarization immunoassay (FPIA)] to determine fasting (n = 163) and post-methionine load (n = 80) plasma tHcy concentrations were compared with those obtained by gas chromatography–mass spectrometry (GC-MS). Difference plots on non-transformed and log-transformed data were used to assess the agreement between HPLC and GC-MS, EIA and GC-MS, and FPIA and GC-MS.

Results: The closest agreement between methods was observed between GC-MS and FPIA for fasting tHcy concentrations, with 95% of the FPIA values between 19% above and 24% below the corresponding GC-MS results. Post-methionine load tHcy concentrations measured by EIA showed the least agreement with GC-MS, with 95% of values measured by EIA ranging between 52% above and 16% below the GC-MS values. With respect to GC-MS, the above-mentioned methods showed a negative bias for fasting tHcy concentrations, but a positive bias for both immunoassays for post-methionine load tHcy concentrations.

Conclusions: The agreement among methods is insufficient to allow them to be used interchangeably. The intermethod differences emphasize the need for standardization of plasma tHcy assays.© 1999 American Association for Clinical Chemistry




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