High performance liquid chromatography method for the determination of pyridoxal-5-phosphate in human plasma:: How appropriate are cut-off values for vitamin B6 deficiency?

Objectives: Application of a HPLC (high performance liquid chromatography) method, using cyanide derivatisation, to the determination of plasma pyridoxal-5-phosphate (PLP) concentrations as an indicator of vitamin B-6 adequacy. Setting: The study was performed at the Institute of Food Research, Norwich, UK. Blood samples were taken at the Institute, at Health Centres, or in the volunteer's home. Subjects: 51 adolescent, 131 adult, 68 non-institutionalized elderly and 44 aged (>73 y) volunteers were recruited from local authority schools, local Health Centres and General Practitioners. Results: The mean PLP recovery was 92.8%. The intra- and inter-assay coefficients of variation were 2.8% and 5.2% respectively. Mean PLP concentrations for males and females, respectively, were: adolescents (13 - 14 y), 36.4 and 43.5 nM; adults (20-64 y), 39.2 and 40.0 nM; elderly (68-73 y), 34.8 and 35.3 nM; aged ( > 73 y), 57.8 and 49.0 nM. Percentages of subjects with PLP concentrations < 34.4 nM were over 26% in all population groups. Mean vitamin B-6 intakes (mu g/g protein intake), as assessed by weighed dietary records, were all above reference nutrient intakes (15 mu g/g protein). Conclusions: An HPLC method, using cyanide derivitisation, has been applied to the determination of plasma PLP. Comparisons of results for local population groups with current cut-off values for plasma PLP, show large numbers of volunteers at risk of vitamin B-6 deficiency although this is not reflected by vitamin B-6 intakes calculated from food tables. The 34.4 nM cut-off value for value for plasma PLP, indicating deficiency, is questioned.