Interlaboratory Variation of Plasma Total Homocysteine Measurements: Results of Three Successive Homocysteine Proficiency Testing Surveys (General Clinical Chemistry‪)‬

In recent years, the measurement of plasma total homocysteine (tHcy) [1] and its association with cardiovascular and thromboembolic diseases have received considerable attention. In the last decade, numerous epidemiologic studies have demonstrated that moderately increased tHcy, whether measured after fasting or 2-6 h after a methionine load, is associated with an increased risk for coronary artery disease (CAD)(1-5). It has also been indicated that plasma tHcy concentrations are a strong predictor of mortality in patients with angiographically confirmed CAD (6, 7). In addition, high plasma tHcy concentrations have been reported to be a risk factor for deep-vein thrombosis in the general population (8-10). As a result of these studies, the volume of tHcy testing in clinical laboratories has grown steadily during the last decade. Initially, tHcy was measured with amino acid analyzers, using the ninhydrin reaction as the method of detection. These methods were generally insensitive, and HPLC with fluorometric detection (HPLC-FD) became the method of choice (11). Such methods are based on the reduction of protein-bound homocysteine, homocystine, and mixed disulfides to reduced homocysteine, followed by derivatization of the reduced homocysteine with thiol-specific fluorogenic reagents. HPLC with electrochemical detection (HPLC-ED) is also used and has the advantage that no derivatization of the sample is required before detection (11). More recently, commercially available automated immunoassays have been introduced. They include the microtiter plate enzyme immunoassay (12) and the fluorescence polarization immunoassay (FPIA) on the Abbott IMx[R] analyzer (13). Both methods are based on enzymatic conversion of homocysteine to S-adenosyl-L-homocysteine, which is subsequently detected by a competitive immunoassay. Gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-electrospray tandem mass spectrometry (LC-MS/MS) methods have also recently been reported (11,14,15). These methods are used for the routine quantification of tHcy in some laboratories and may be useful in the standardization of plasma tHcy assays. In particular, the LC-MS/MS method has been suggested as a reference method (11).