[Interconversion of the results for erythrocyte zinc protoporphyrin expressed in different units]

As stated in a recent document of the National Committee for Clinical Laboratory Standards (NCCLS Document C42-P, April 1995), expression of the results of erythrocyte zinc protoporphyrin (ZPP) in different units is a main source of confusion which may hamper the wide use of this test. The possible inter-conversion of the ZPP values expressed as microgram/g Hb, microgram/dL blood and microgram/dL red cells was assessed. It was found that in all cases, despite a good correlation between these values (r > or = 0.939), the error of the estimation was higher than allowed by the American College of Pathologists criteria. Such findings show that ZPP values expressed in different units cannot be inter-converted as a function of the corresponding regression equation. Similarly, the use of hematofluorometers with an analogic-digital converter to express results as microgram ZPP/ dL blood might pose an additional source of error.