Variability of post-methionine load plasma homocysteine assays.
BACKGROUND
Numerous variations of the methionine load test are frequently used as dynamic function tests to assess homocysteine metabolism. Lack of standardization impedes inter-laboratory comparisons. Criteria based on biological variation are suggested to standardize the methionine load test.


METHODS
Weekly methionine load tests (n=5) with blood sampling at 0, 4, 6 and 8 h were performed on 15 young men. For both basal and post-methionine load homocysteine measurements, total variance (sigma(S)(2)), within-subject variance (sigma(I)(2)), between-subject variance (sigma(G)(2)) and analytical variance (sigma(A)(2)) were calculated from an appropriate analysis of variance (ANOVA).


RESULTS
Plasma homocysteine concentrations measured 6 h after methionine loading had analytical, within-subject and between-subject coefficients of variation of 5.2%, 17.5% and 9.7%, respectively. Measurements at 4 h had a higher within-subject coefficient of variation. Adjustment of post-methionine load homocysteine concentrations for basal levels resulted in considerable increases of all the measures of variation.


CONCLUSIONS
Adjustment of post-methionine load plasma homocysteine concentrations for basal levels does not improve the interpretation of changes in serial results due to the higher analytical and biological variance of adjusted concentrations. It is suggested that the methionine load test is standardized to a single, unadjusted homocysteine measurement at 6 h.
Ubbink
Johan B
JB
Department of Chemical Pathology, University of Pretoria, P.O. Box 2034, South Africa. jubbink@medic.up.ac.za
Becker
Piet J
PJ
Delport
Rhena
R
Bester
Megan
M
Riezler
Reiner
R
Vermaak
W J Hayward
WJ
D000328:Adult; D019411:Clinical Laboratory Techniques / Q000592:standards; D005215:Fasting / Q000097:blood; D006710:Homocysteine / Q000097:blood*; D006801:Humans; D007753:Laboratories / Q000592:standards; D008297:Male; D008715:Methionine / Q000008:administration & dosage / Q000493:pharmacokinetics*; D012016:Reference Values; D013223:Statistics as Topic / Q000379:methods; D013997:Time Factors
D006710:Homocysteine; D008715:Methionine