Aim Ferritin is an iron-storage protein with plasma levels used as an indicator of iron deficiency. We undertook parallel testing of ferritin levels in serum and plasma to measure the range of variation and we present data for specimens from a panel of otherwise healthy volunteers. Methods We utilised a commercially-available ELISA for parallel quantification of ferritin levels in serum and plasma samples. In a second experiment, ferritin levels were measured over a post-collection time course; day of collection, and one day and seven days post-collection. For the one and seven day measurements levels were determined before and after a second centrifugation. Results For volunteers who had normal/high serum ferritin (females: 15–200 ng/mL; males: 30–300 ng/mL), on average the plasma ferritin value was 51%, (range 36–69%) of the serum level. The largest variation was a low serum ferritin (10 ng/mL) with a plasma ferritin 2.3 times greater. The trend in plasma and serum ferritin levels for the individuals studied was consistent over the time course. Discussion In this study, the discrepancy between plasma and serum ferritin levels was rarely as low as the widely-quoted 10% minimum variation and care should be taken when establishing normal ranges for ferritin testing.