The change of validity of blood zinc protoporphyrin test by different cut-off level in level workers

Kim, Yong Bae; Ahn, Hyun Cheol; Hwangbo, Young; Lee, Gap Soo; Lee, Sung Soo; Ahn, Kyu Dong; Lee, Byung Kook

Abstract

Measurement of blood lead (PbB) and blood zinc protoporphyrin (ZPP) are most common biological indices to identify the individual at risk for excess or the health sequences by lead exposure. Because PbB is known most important and reliable index of lead exposure, PbB is often regarded as a gold standard to detect lead exposure. But in Korea PbB is a secondary test item of detailed health check-up with positive finding of screening test in most occasion. Our lead standard requires all lead workers to take annual heath-check twice a year for investigation of their health effect due to lead exposure. Blood ZPP is one of most important index to detect high lead absorption in lead workers as a screening test. Measurement of blood ZPP is known well to correlate with PbB in steady state of exposure in most lead workers and is often used as a primary screening test to detect high lead absorption of lead workers with the advantage of simplicity, easiness, portability and low cost. The current cut-off criteria of blood ZPP for further detailed health check-up is 100 ng/dl which is supposed to match the level of 40 ng/dl of PbB according to our standard. Authors tried to investigate the validity of current criteria of cut-off level(100 ng/dl) of blood ZPP and possible another better cut-off level of it to detect the lead workers whose PbB level over 40 ng/dl. The subjects in our study were 212 male workers in three small scale storage battery industries. Blood ZPP, PbB and hemoglobin(Hb) were selected as the indices of lead exposure. The results were as follows. 1. The mean of blood ZPP, PbB and Hb in lead workers were 79.5+/-46.7 ng/dl, 38.7+/-15.1 ng/dl, and 14.8+/-1.2 g/dl, respectively. There were significant differences in blood ZPP, PbB and Hb by industry(P<0.01). 2. The percents of lead workers whose blood ZPP were above 100 ng/dl in the group of work duration below 1, 1-4, 5-9 and above 10 years were 8.6%, 17.2%, 47.6%, and 50.0%, respectively. The percents of lead workers whose PbB were above 40 ng/dl in those were 31.4%, 40.4%, 71.4%, and 86.4%, respectively. 3. The percents of lead workers whose PbB were below 40 ng/dl, 40-59 ng/dl and above 60 ng/dl were 54.7%, 34.9% and 10.4%, respectively. Those of lead workers whose blood ZPP were below 100 ng/dl, 100-149 ng/dl and above 150 ng/dl were 79.2%, 13.7% and 7.1%, respectively. 4. Simple linear regression of PbB on blood ZPP was statistically significant(P<0.05) and as PbB was 40 ng/dl, blood ZPP was 82.1 ng/dl. 5. While the highest sensitivity and specificity of blood ZPP test to detect lead workers with PbB over 40 ng/dl were observed in the cut-off level of 50 ng/dl and 100 ng/dl of blood ZPP, respectively, the highest validity(sensitivity+specificity) of blood ZPP to detect lead workers with PbB over 40 ng/dl was observed in the cut-off level of around 70 ng/dl of blood ZPP. But even with optimal cut-off level of around 70 ng/dl of blood ZPP, still 25.0% of false negative and 20.7% false positive lead workers were found. As the result of this study, it was suggested that reconsideration of current blood ZPP cut-off of our lead standard from 100 ng/dl to somewhat lower level such as around 70 ng/dl and the inclusion of PbB measurement as a primary screening test for lead workers was highly recommended for the effective prevention of lead workers.