PREDICTIVE VALUE OF DETERMINATIONS OF ZINC PROTOPORPHYRIN FOR INCREASED BLOOD LEAD CONCENTRATIONS

Blood lead (PbB) and red cell zinc protoporphyrin (ZPP) concentrations are widely used biomarkers for lead toxicity. It is uncertain, howeve r, whether either or both are needed for monitoring lead exposure and how discordant PbB and ZPP values should be interpreted. We reviewed t he results of PbB and ZPP determinations in 94 workers in; a lead-batt ery plant over a 13-year period and retrieved all 807 sets of tests in which both PbB and ZPP were available, with a follow-up PbB value 6 m onths later. PbB exceeded 1.93 mu mol/L (40 mu g/dL) in 414 (51%), and 2.90 mu mol/L (60 mu g/dL) in 105 (14%) of the blood samples. We deri ved the test properties of various ZPP concentrations for concurrent ' 'toxic'' PbB concentrations, defined as greater than or equal to 1.93 and 2.90 mu mol/L (40 and 60 mu g/dL). The results indicated that, giv en a population of lead-exposed workers with a 10% prevalence of PbB o f greater than or equal to 2.90 mu mol/L, (60 mu g/dL:, a policy of te sting PbB only in those with ZPP > 0.71 mu mol/L (40 mu g/dL) would ob viate 42% of the PbB tests, but would miss about three cases with toxi c PbB concentrations in every 200 workers at risk. A finding of increa sed ZPP concentrations with a concurrent ''nontoxic'' PbB was associat ed with an increased risk of a toxic PbB concentration 6 months later. We conclude that (a) screening by testing only ZPP does not safeguard exposed persons against lead toxicity, and (b) the frequency of PbB m onitoring should be guided by estimates of the risk of future lead tox icity in individual workers.