A 7-year-old girl is brought to the clinic due to fatigue and poor school performance. She lives in an old home undergoing renovations. Laboratory tests reveal microcytic, hypochromic anemia with basophilic stippling of red blood cells. A blood lead level of 35 µg/dL is confirmed. The physician orders a zinc protoporphyrin (ZPP) test as part of the evaluation. Which of the following best explains the role of elevated ZPP in diagnosing lead poisoning?
