Serum vitamin B12 concentrations within reference values do not exclude functional vitamin B12 deficiency in PKU patients of various ages.

Unlabelled: Homocysteine (Hcy) and in particular methylmalonic acid (MMA) are considered reliable parameters for vitamin B(12) status in healthy individuals. Phenylketonuria (PKU) patients are at risk for functional vitamin B(12) deficiency based on their diet.

Objective: The aim of this study was to investigate the prevalence of functional vitamin B(12) deficiency in continuously treated PKU patients and the association of parameters of vitamin B(12) and metabolic control.

Methods: In 75 continuously treated PKU patients of 1-37 years of age, serum vitamin B(12) concentrations, plasma Hcy, MMA, and phenylalanine concentrations were studied.

Results: Eight patients had vitamin B(12) concentrations below normal. Out of these eight patients, two had elevated MMA and/or Hcy concentrations. Ten other patients with normal vitamin B(12) concentrations had elevated concentrations of MMA and/or Hcy.

Conclusions: A vitamin B(12) concentration within the reference range does not automatically imply a sufficient vitamin B(12) status. We recommend measuring serum MMA, or alternatively plasma Hcy, yearly in all PKU patients to diagnose functional vitamin B(12) deficiency.