Serum ferritin measurements differ according to assay used: implications for iron therapy in restless legs syndrome.

Abstract

Study objectives: Serum ferritin levels are used to determine the need for iron supplementation in patients with RLS. The purpose of the study was to determine whether immunoassay measurement of serum ferritin yields varying levels according to different manufacturer assays, with resultant variation in cut off values.

Methods: We compared serum ferritin levels using 116 clinical samples assessed by the Beckman and Roche methods.

Results: While there was a high correlation between results obtained from the 2 methods (R² = 0,99), the absolute values differed. The equivalent ferritin measures determined by Beckman, Roche were: 50 mcg/dL, 83 mcg/dL; 75 mcg/dL, 121 mcg/dL; 100 mcg/dL, 158 mcg/dL; and 300 mcg/dL, 457 mcg/dL.

Conclusions: It is uncertain which assays were used to measure serum ferritin in the seminal studies on which current guidelines for iron therapy for RLS are based. In view of this uncertainty, as well as the limited data on which current recommendations are based, clinicians should be flexible in using recommended serum ferritin cut off values, also utilizing percentage transferrin saturation. Assuming that Beckman or equivalent assays were used, centers using the Roche method need to adjust the cut offs for administration of oral iron and intravenous iron recommended by current practice guidelines to avoid withholding treatment for RLS patients who might benefit from iron supplementation.