Zinc Protoporphyrin
Zinc protoporphyrin (ZPP) is a normal metabolite that accumulates in trace amounts in erythrocytes during hemoglobin synthesis. The final biochemical reaction in the heme pathway is the chelation of ferrous iron with protoporphyrin IX by the enzyme, ferrochelatase. During periods of iron deficiency or impaired iron utilization, zinc becomes an alternative metal substrate for ferrochelatase. Lead toxicity interferes with intracellular iron transfer, creating a condition similar to iron deficiency.
ZPP was widely used for lead screening until October 1991, when the Centers for Disease Control and Prevention (CDC) issued new guidelines for lead screening of children. These guidelines recommended that lead be detectable to 10 ug/dL in whole blood. Because the ZPP response to lead exposure is not dependable at lead levels below 25 ug/dL, the CDC stopped recommending this test for screening children.
For work related testing, OSHA requires measurements of ZPP in addition to blood lead. ZPP values are generally elevated in workers whose blood lead concentration exceeds 25 ug/dL. Most states require reporting lead levels of 25 ug/dL or greater to OSHA. ZPP levels rise more slowly than blood lead and remain elevated for longer periods. ZPP elevation, like most other biological markers, is not specific for lead exposure.
A hematofluorometer is used to directly measure ZPP fluorescence in whole blood without extraction. The instrument measures the ratio of ZPP fluorescence to heme absorption. Reference range is 30 - 80 umol ZPP/mol heme or 15 - 36 ug/dL.
Specimen requirement is one royal blue top (metal free, EDTA) tube for adults or one lavender top (metal free, EDTA) Microtainer for children. Hemolyzed or icteric specimens cannot be processed.
