['To determine the day-to-day variation in biochemical measures of iron status in a group of elderly women with rheumatoid arthritis compared with a group of healthy elderly women.', 'Venous blood samples were collected from each subject on 3 nonconsecutive days during a 2-week study period; subjects had fasted overnight. Variability in hemoglobin level, hematocrit value, serum iron concentration, total iron-binding capacity, transferrin saturation, serum ferritin concentration, and plasma transferrin receptor level was determined.', 'Two groups of women, one with rheumatoid arthritis (n=10) and another that was apparently healthy (n=10).', 'Variance component analysis was used to estimate the biological variation (sigma square day) and analytic variation (sigma square rep) for each iron index. The coefficient of variation (CV) for each variance component was calculated: coefficient of biological variation = CV day, coefficient of analytic variation = CV rep, and coefficient of a single future determination = CV fd.', 'The CV rep for all iron indexes was smaller than the CV day in both groups. The CV day was considerably higher for serum iron concentration and for transferrin saturation than for the other indexes in both groups (16.6% and 16.6% in healthy subjects and 33.6% and 28.2%, respectively, in subjects with rheumatoid arthritis). The higher CV day for serum iron concentration and transferrin saturation translated into a higher CV fd for these indexes. Because of the higher variance for these two indexes, more sampling days were required for reliable estimates. CV day and CV fd for plasma transferrin receptor level were relatively low.'] Is day-to-day variation in iron status indexes similar for most measures in elderly women with and without rheumatoid arthritis?