The marker of cobalamin deficiency, plasma methylmalonic acid, correlates to plasma creatinine.

OBJECTIVE To examine the relationship between the two diagnostic tests, plasma methylmalonic acid and plasma cobalamins, and their association with plasma creatinine, age and sex. DESIGN Cross-sectional study of simultaneous laboratory measurements. SETTING County of Aarhus, Denmark. SUBJECTS Records on 1689 patients who had their first plasma methylmalonic acid measurement during 1995 and 1996, and who had a simultaneous measurement of plasma cobalamins. Plasma creatinine values measured within a week of measurements of plasma methylmalonic acid and plasma cobalamins were available for 1255 of the patients. MAIN OUTCOME MEASURES Predictors of variation in plasma methylmalonic acid; plasma cobalamins, plasma creatinine, age and sex. RESULTS Plasma methylmalonic acid was positively correlated with plasma creatinine, even for plasma creatinine within the normal range. These associations remained in a multiple regression analysis. For plasma cobalamins below 200 pmol L-1, there was a strong negative correlation between plasma methylmalonic acid and plasma cobalamins, whilst the association was weak for higher plasma cobalamin levels. Plasma methylmalonic acid increased and plasma cobalamins decreased with age. CONCLUSIONS The strong correlation between plasma methylmalonic acid and plasma creatinine suggests that plasma creatinine - also within the normal range - must be taken into consideration when interpreting plasma methylmalonic acid.