Day-to-day variation of transferrin receptor and ferritin in healthy men and women.

To determine the total day-to-day intraindividual variability of ferritin and transferrin receptor (TfR), we collected venous serum and plasma and capillary serum and plasma samples during 10 nonconsecutive days over a 4-wk period from a group of healthy men (n = 10) and women (n = 11) aged 19-46 y. On the basis of a method previously established in younger and older adults, biological (sigma 2 day) and analytical (sigma 2 rep) variance components were computed and summed to obtain the total day-to-day intraindividual variability (sigma 2 tot). The total day-to-day intraindividual CV (CVtot) was lower for TfR than for ferritin measurements. One to two discrete blood samples are required to accurately determine capillary and venous TfR, whereas capillary and venous ferritin, which have a higher CVtot, require three to six samples. Results from the current study showing the low biological and analytical variability of TfR support the use of this new test for assessing a person's iron status.