We aimed to identify factors associated with zinc protoporphyrin (ZPP) in pregnant women, and to assess the diagnostic performance and utility of ZPP, either alone or in combination with hemoglobin, in detecting iron deficiency.
We studied 470 rural Kenyan women with singleton pregnancies, gestational age 13-23 weeks and hemoglobin concentration =90g/L. We used multiple linear regression analysis to identify factors associated with ZPP. In those without inflammation, Plasmodium infection or HIV infection, we analyzed Receiver Operating Characteristics curves with corresponding Area-Under-the-Curve (AUC) to assess the diagnostic performance of ZPP in detecting iron deficiency (plasma ferritin concentration <15µg/L).
Anemia, iron deficiency, plasma concentrations of soluble transferrin receptor and folate were independently associated with ZPP, whether measured in whole blood or erythrocytes. In addition, gravidity and gestational age were independently associated with whole blood ZPP, and plasma concentrations of transferrin, bilirubin (total) and LDH with erythrocyte ZPP. Whole blood ZPP and erythrocyte ZPP had limited accuracy in detecting iron deficiency (AUC, 95%CI: 0.66, 0.57-0.74; and 0.73, 0.65-0.80, respectively); addition of hemoglobin concentration had no additional diagnostic value. Conventional cut-off points for whole blood ZPP (>70µmol/mol haem) resulted in gross overestimates of the prevalence of iron deficiency.
In pregnant women with gestational age 13-23 weeks without inflammation, Plasmodium infection or HIV infection, whole blood ZPP or erythrocyte ZPP have limited diagnostic value in detecting iron deficiency. Based on these findings, guidelines on the use of ZPP to assess iron status in individuals or populations of pregnant women need review.