To compare vitamin B6 status indices with each other and with potential confounding factors, in the datasets from two national British surveys and associated studies.

Vitamin B6 status was measured by plasma pyridoxal phosphate (PLP) and plasma pyridoxic acid (PA) in both surveys, and also by erythrocyte aspartate aminotransferase activation coefficient (EAATAC) in one of the surveys. Plasma α1-antichymotrypsin was measured as an index of acute phase status; plasma creatinine was measured as an index of renal function; and plasma total alkaline phosphatase activity was measured as a proxy for PLP hydrolase activity.

The survey of people aged 65 years and over was carried out in 80 postcode sectors across mainland Britain during 1994–95 and the survey of young people was carried out in 132 postcode sectors across mainland Britain during 1997.

Blood samples from c. 1000 subjects of both sexes in each survey permitted measurements of plasma PLP and PA. There were also measurements of EAATAC in the young people's survey.

According to published limits of normality, only 5% or less of the young people had unacceptable vitamin B6 status as measured by plasma PLP. About half had apparently unacceptable status by EAATAC, but this observation is difficult to interpret. The young people had considerably higher plasma concentrations of PLP and lower concentrations of PA than the older people. In both surveys, plasma PLP was strongly correlated with plasma PA and in the young persons’ survey it was also correlated, although much less strongly, with the basal activity and activation coefficient of aspartate aminotransferase. Both plasma PLP and EAATAC (but not PA nor basal aspartate aminotransferase activity) were influenced by acute phase status in young people, as indicated by significant correlations with α1-antichymotrypsin. In people aged 65 years and over, PA (but not PLP) was correlated with renal function, as indicated by its relation with plasma creatinine; however PLP (but not PA) was correlated with plasma alkaline phosphatase activity.

Several potential confounders — acute phase reaction, kidney malfunction and hydrolase activity — may influence vitamin B6 status indices, although differently for different indices and different age groups. Since older people have relatively poor vitamin B6 status, which may have important health implications for them, more reliable vitamin B6 status indices are needed.