AbstractThe accuracy and precision of measurements of iron status made on the capillary blood from a single 75 × 1‐mm microhematocrit tube were evaluated in 301 normal volunteers. After recording the hematocrit value, a hematofluorometer was used to assay erythrocyte protoporphyrin directly on the packed red cell fraction. Values in the low normal range were 50‐100% higher than assays by an extraction technique in simultaneously collected venous blood. However, measurements in the iron deficient range were only 12 % greater. Plasma ferritin concentration was determined by using an enzyme‐linked immunoassay on 10‐μl plasma retrieved after measuring the hematocrit. Capillary values were approximately 7% higher than venous concentrations, the proportional difference being consistent throughout the measured range. When the individual measurements were combined to calculate the iron storage status of each individual, the frequency distribution of iron stores in 141 female volunteers between 18 and 36 years of age was approximately Gaussian and paralleled that calculated for women aged 18‐44 years in NHANES II. These studies demonstrate that iron status can be r'etermined accurately from the measurement of hematocrit, erythrocyte protoporphyrin, and ferritin on a single microhematocrit tube of capillary blood.