Adequate intake of vitamins is essential for normal growth and development and for the maintenance of health. Currently, the method of choice for assessing vitamin status in the laboratory is measuring their concentrations in plasma, whole blood, or erythrocytes (1-3). However, for most vitamins, objective quality specifications for such measurements have not been defined. Several approaches to setting desirable quality specifications for imprecision and bias have been suggested (4-6), including reference interval-based goals, clinically based goals for imprecision, quality specifications laid down by external quality assurance (EQA)(3) schemes, and biological variation-based goals. The European consensus is that quality specifications are best based on components of biological variation (7), but information on the biological variation of vitamins in blood is limited, apart from some studies on vitamins A and E and (3-carotene (8-12).
