Comparison of three different plasma homocysteine assays with gas chromatography-mass spectrometry

Background: Various methods are available to measure plasma total homocyst( e)ine (tHcy) concentrations, but whether plasma tHcy assays may be used int erchangeably is not known. Methods: Results from three different methods [HPLC with fluorescence detec tion, enzyme immunoassay (EIA), and fluorescence polarization immunoassay ( FPIA)] to determine fasting (n = 163) and post-methionine load (n = 80) pla sma tHcy concentrations were compared with those obtained by gas chromatogr aphy-mass spectrometry (GC-MS). Difference plots on nontransformed 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 a nd FPIA for fasting tHcy concentrations, with 95% of the FPIA values betwee n 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% b elow the GC-MS values. With respect to GC-MS, the above-mentioned methods s howed 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 b e used interchangeably; The intermethod differences emphasize the need for standardization of plasma tHcy assays. (C) 1999 American Association for Cl inical Chemistry.