Homocysteine measurement by Vitros® Microtip homocysteine assay

Background: Elevated circulating total homocysteine is an independent vascular risk factor. Enzymatic homocysteine measurements represent an alternative to HPLC- or immunochemistry-based assays, suitable for automation. Here, we report on analytical performance of a commercial cystathionine beta-synthasebased assay, for use on Vitros (R) automated analyzers. Methods: Linear range, limit of detection and analytical sensitivity were inferred from duplicate measurements of homocystine standard solutions (1-65 mu mol/L). Imprecision was assessed using commercial controls according to NCCLS EP5-A2 and accuracy using NIST-SRM1955 reference material. Agreement with a clinically validated HPLC method was examined on 207 patient samples. Results: The enzymatic assay was linear from 1 to 90 mu mol/L homocysteine. Total (within-day) imprecision ranged from 4.5 (3.9)% to 2.8 (1.6)% at homocysteine 9.7-43.2 mu mol/L. Accuracy was acceptable at 8.9 and 17.7 mu mol/L homocysteine, with +6.4% and -1.2% bias, respectively, but showed substantial negative bias (-20.1%) at 4.0 mu mol/L. High triglycerides (19.8 mu mol/L) negatively interfered. The enzymatic method was slightly less sensitive than the HPLC method (limit of detection 0.7 and 0.2 mu mol/L, respectively) but correlated well with the latter (r(2) =0.9997, slope=1.04, intercept=-0.66 mu mol/L) and was more precise (p<0.05). Conclusions: The Vitros (R) homocysteine assay met the CLIA Desirable Analytical Quality Specifications at homocysteine >= 9 mu mol/L. Its analytical performance and suitability for automation make the Vitros (R) assay an analytically acceptable alternative to HPLC-based methods.