Screening for iron deficiency with the erythrocyte protoporphyrin test.
Elevation of erythrocyte protoporphyrin (EP) level is one of the consequences of iron deficiency. As the EP test has been established to be a screening test for lead poisoning, the screening capability of the EP test for iron deficiency was investigated. A total of 4,160 children between ages 6 months to 12 years had EP determined together with serum ferritin and hematocrit. Comparing the relationship of EP to serum ferritin and using a serum ferritin value less than or equal to 15 micrograms/L as the criterion of iron deficiency, the optimal cutoff limit for the EP test appears to be 35 micrograms/dL of whole blood. At this level, 88% of the subjects with low levels of serum ferritin can be detected (sensitivity), in contrast to the 53% detected at a higher cutoff value (greater than or equal to 50 micrograms/dL) used to screen for lead toxicity, or to the 59% detected by age-related hematocrit value. At an EP screening level of 35 micrograms/dL of whole blood, 90% of the subjects with normal serum ferritin level are correctly determined to be screen negative (specificity). The predictive value of low levels of serum ferritin for all subjects above screening level is 38%. In general, an elevated EP level, by itself, represents inadequate iron supply for hematopoiesis and signals iron deficiency regardless of whether the serum ferritin value is below the diagnostic level or not. A trial course of orally administered iron is suggested for children who are found to have an elevated EP value, with an increase in hemoglobin or hematocrit value serving, retrospectively, as confirmation of prior iron deficiency.