Iron deficiency (ID), a common nutrient deficiency, impairs brain development. Newborn screening for ID has advantages, as earlier recognition of at-risk children could prevent long-term neurocognitive morbitity. ZnPP/H is an available, cost effective and sensitive biomarker of incomplete iron incorporation into erythrocytes. ZnPP/H, a potential candidate for newborn screening for iron deficiency, is measurable on whole cord blood or filter paper spots. Previous studies rinsed cord blood samples to remove pigments including bilirubin, that interfere with readings. Filter paper newborn screen samples cannot be rinsed and are obtained at 24-48 hrs after birth, when bilirubin levels may be elevated. Our aims were first, to examine the reproducibility of filter paper ZnPP/H measurements and second, to examine methods to limit bilirubin interference with ZnPP/H. Methods: We measured reproducibility of cord blood ZnPP/H eluted as hemolyzed blood from filter paper blood spots and whether eluted values correlated to the same whole blood samples. We measured the degree of bilirubin interference with ZnPP/H readings. We evaluated potential candidates to remove bilirubin interference with ZnPP/H, including detergents, albumin, and bilirubin oxidase. Results: Filter paper ZnPP/H was reproducible. Filter ZnPP/H correlated to whole or rinsed cord blood, P<0.0001, but lines of identity did not pass through zero. Although 5 mg/dL levels of bilirubin were similar to ZnPP/H without bilirubin, 10, 15 and 20 mg/dL increased the levels of ZnPP/H in a dose response fashion (P<0.0001). Tween 20 and Triton X-100 increased ZnPP/H baseline variability and did not remove bilirubin interference. Albumin decreased baseline ZnPP/H values to those of rinsed whole blood, but did not remove bilirubin interference. Bilirubin oxidase reagent is inexpensive and stable when refrigerated or frozen in water. Although not returned to baseline, bilirubin oxidase removes bilirubin interference by 70% (p<0.0001). Conclusion: Filter paper ZnPP/H correlated to intact whole and rinsed ZnPP/H, but bilirubin levels of 10 mg/dL interfere with readings. Bilirubin oxidase removes most bilirubin interference, improving the accuracy of filter paper ZnPP/H. Further studies are needed to validate the use of ZnPP/H from blood on filter paper. (poster)