Spot ferritin assay for serum samples dried on filter paper

A spot method was developed for analyzing ferritin from 20-μL serum samples (n = 71) that were spotted and dried on filter paper and stored frozen (2 d). Spot samples were thawed, incubated in a buffer containing cellulase, and centrifuged and the supernate assayed for ferritin by a commercial radioimmunoassay. The geometric means (± 1 SD) for ferritin analyzed with the spot and traditional methods were 49.4 (range: 14.9-164.0) and 47.5 (range: 14.4-156.0) μg/L, respectively. The two methods correlated strongly (r = 0.98, P = 0.0001). Storage of spot samples (n = 31) under various conditions (at room temperature, refrigerated, or frozen for 2 wk, or at room temperature for 4 wk) in airtight bags before analysis yielded ferritin values that were not significantly different from those obtained by the traditional method. Ferritin values from spotted samples stored at room temperature for 4 wk before being analyzed were only 2.2 μg/L higher than those from samples analyzed by the traditional method. With iron depletion defined as a serum ferritin concentration < 15 μg/L, this method corresponded absolutely with the traditional method in classifying individuals as iron sufficient or deficient. Thus, the spot ferritin method (with samples stored at room temperature for 4 wk) offers a reliable, accurate, and practical tool for iron status assessment in field studies. Capillary blood can be sampled to avoid the costs and concerns associated with venipuncture and spotted serum samples can be stored at room temperature for ≤4 wk, eliminating the need for freezing during storage and transportation.