Use of plasma methylmalonic acid in diagnosing vitamin B-12 deficiency in general practice.

Objectives - To examine the reasons why general practitioners (GPs) request plasma methylmalonic acid (MMA) tests and how they respond to a result above the reference interval. Design - Retrospective study of medical records. Setting - Primary health care, Aarhus County, Denmark. Subjects - 181 patients with increased concentrations of plasma MMA (> 0.28 mumol/l) attending 10 GPs. Main outcome measures - Reasons for requesting the analysis and reactions to a plasma MMA level above the reference interval. Results - A reason for requesting plasma MMA was stated in 129 (71%) of the 181 medical records, screening being the most frequent. A reaction to an increased concentration of plasma MMA was recorded in 128 (71%) patients, and vitamin B-12 treatment was initiated in 102 (80%) of them. Among the 105 patients with marginally elevated plasma MMA level (0.29-0.44 mumol/l), vitamin B-12 treatment was initiated in 38 (36%). Conclusion - Plasma MMA was frequently used as a screening test. Treatment with vitamin B-12 was initiated when MMA level was increased, and often when plasma MMA level was slightly above the reference interval. This strategy is likely to promote over-treatment with vitamin B-12.