Ninety-five percent of tHcy determinations by HPLC-ED and by EIA were 0.20 and 0.76 /,mol/L lower to 1.26 and 1.79 [micro]mol/L higher than concentrations determined by GC/MS.
When we grouped laboratories by method type and performed a two-way ANOVA with laboratory and method as variables to compare methods individually, we found no significant differences among the methods except that HPLC-FD trialkylphosphine/ABD-F (laboratory 6) gave significantly lower results than HPLC-FD NaBH4/ MBrB (laboratories 10 and 11) and HPLC-ED (laboratories 1-3).
Our analysis for methodological differences in mean concentrations showed significant differences for two method comparisons: HPLC-FD trialkylphosphine/ ABD-F compared with HPLC-FD NaB[H.sub.4]/MBrB and compared with HPLC-ED. Although we could not confirm that these differences were method specific, Dias et al.
 Nonstandard abbreviations: tHcy, total homocysteine; GC/MS, gas chromatography/mass spectrometry; TCEP, tris(2-carbo)cyethyl)phosphine; FPIA, fluorescence polarization immunoassay; EIA, enzyme immunoassay; HPLCFD, HPLC with fluorometric detection; ABD-F, 4-(aminosulfonyl)-7-fluoro2,1,3-benzoxadiazole; HPLC-ED, HPLC with electrochemical detection; QC, quality control; TBP, tributylphosphine; SBD-F, ammonium 7-fluoro-2,1,3-benzoxadiazole-4-sulfonate; NaB[H.sub.4], sodium borohydride; and MBrB, monobromobimane.
We have shown that the resulting assay for specific detection of this urinary metabolite correlates well with measurements made by HPLC-ED, which is unquestionably the most widely used method for assay of catecholamine metabolites.
Furthermore, nonisotopic immunoassays are now commonly used by clinical biology laboratories lacking access to techniques like HPLC-ED. Development of assays like ours therefore provides readily accessible techniques for accurate measurement of substances that are indirect markers of certain tumors.