External Quality Assessment of Methylmalonic Acid and Total Homocysteine

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Abstract
Background: The use of analyses for methylmalonic acid (MMA) and total homocysteine (HCY) in plasma has become widespread. Realizing the need for external quality assessment for these measurements, we started a program in 1997. The results for 1998 are reviewed in this report. Methods: Fourteen laboratories participated with 15 sets of results for MMA, and 28 laboratories participated with 34 sets of results for HCY. Results for four identical samples, made up from the same unmodified serum (MMA) or EDTA-plasma (HCY) pool, sent out under different identifications, were used for assessing the imprecision. Samples made up from the same pools supplemented with MMA or l-HCY to three concentrations were used for assessing the recovery. By using literature data for the biological variation, quality goals for both analytes were calculated. Results: The overall within-laboratory CV was 12% for MMA and 7.5% for HCY. Gas chromatography–mass spectrometric HCY results had lower imprecision than the HPLC or immunoassay results. For MMA, no significant between-laboratory component of variance was found. Only results for HCY obtained with HPLC methods showed significant between-laboratory variance. Conclusions: Eight of the 15 participants achieved the minimum imprecision goal for MMA vs 9 of the 34 participants for HCY. The minimum quality goals for bias as approximated by the recovery were achieved by 13 participants for MMA and 26 for HCY.