Inter-Method Variability of Ferritin and Transferrin Saturation Measurement Methods in Patients on Hemodialysis

Serum ferritin level and transferrin saturation (TSAT) are widely used to evaluate iron status in patients with chronic kidney disease, and are also important variables for performing statistical analyses. Many guidelines have set control targets or upper limits for these markers. Inter-method variability is an important consideration in iron control and statistical analysis. We used 10 ferritin assay kits and five iron/unsaturated iron-binding capacity/total iron-binding capacity assay kits to determine ferritin levels and TSAT in 114 patients on maintenance dialysis, and evaluated measurement bias using Passing-Bablok regression analyses. The variance of distributions categorized by differences in assay kits was examined using Fisher's exact test. Slopes ranged from 1.00 to 1.63 (1.00 to 0.61) for ferritin and 1.00 to 1.10 (1.00 to 0.91) for TSAT. The distribution according to the 2015 JSDT Guideline for Renal Anemia in Chronic Kidney Disease significantly changed (P = 0.01). TSAT thus provides more precise control than ferritin in multi-center comparisons where no particular assay is specified. Developers must reduce variability in serum ferritin assay kits. Researchers must analyze measured values by taking into account the propagation of errors, and clinicians must evaluate laboratory data carefully.