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Technical Briefs |
1
Division of General Pediatrics, North Shore University Hospital, 865 Northern Boulevard, Great Neck, NY 11021;
2
Becton Dickinson Vacutainer Tube Systems, 1 Becton Drive, Franklin Lakes, NJ 07417;
3
Wadsworth Center, New York State Department of Health, P.O. Box 509, Albany, NY 12210-0509;
4
Use of product or trade names is for informational purposes only and does not imply an endorsement by the New York State Department of Health.
a address correspondence to this author at: Division of General Pediatrics, Suite 108, 410 Lakeville Road, New Hyde Park, NY 11042, fax 516-465-5399, e-mail DJPJBJMJ{at}aol.com
Lead poisoning is an important preventable environmental health problem. Blood lead concentrations as low as 100 µg/L (0.48 µmol/L) in whole blood have been shown to affect children's neuropsychologic or cognitive performance adversely (1)(2)(3). In view of this, in 1991 the Centers for Disease Control and Prevention (CDC) lowered the concentration of lead in blood considered safe from 250 to 100 µg/L (from 1.21 to 0.48 µmol/L) (4). This lowered threshold has been reaffirmed by the CDC in their 1997 document on screening children for lead exposure (5). Before 1991, erythrocyte protoporphyrin (EP) was the standard test for screening children for lead poisoning; however, it was subsequently found to be an insensitive predictor of blood lead concentrations above the lower threshold (6). However, serial EP measurements, paired with matching blood lead measurements, are still useful in managing children with lead poisoning. In the absence of additional exposure of the child to lead, the EP concentration will continue to decrease at a faster rate than is reflected by the blood lead concentration as lead reequilibrates among blood, soft tissue, and bone stores.
Another recommendation in the 1991 CDC statement was to screen all children 6 months to 6 years of age for lead poisoning, using a direct blood lead measurement. Venous blood is preferred for blood lead measurement because capillary measurements may be falsely increased by skin contamination with lead. The issue of lead contamination of capillary blood specimens obtained by fingerstick was reported earlier (7). For diagnostic purposes, EDTA-containing evacuated tubes are traditionally used in collecting blood for lead and/or EP determinations. Each manufactured lot of tubes should be certified as free of analytically significant lead contamination by the blood lead testing laboratory before use. Such a practice is proposed in a recent NCCLS document on blood lead testing (8). A significant lead contamination is defined as one that would increase the blood lead concentration by >5 µg/L. Alternatively, the testing laboratory may recommend the use of trace element tubes (stoppers of tubes used in the US are royal blue) containing Na2EDTA or heparin.
The aim of this study was to evaluate a recently introduced 3-mL draw
volume, plastic Vacutainer Tube (VACUTAINER
PLUS4 Low Lead; Becton Dickinson Vacutainer
Tube Systems) containing powdered K2EDTA, designed
specifically for pediatric lead testing. Each manufactured lot of tubes
is pretested and contains 
2.5 µg/L of lead per tube before market.
Patients were recruited into the study from the Division of General Pediatrics, North Shore University Hospital, Manhasset, New York. Two tubes of venous blood were drawn from each participant for lead testing. One tube was a standard lavender-stoppered K2EDTA Vacutainer Tube (control), which had already been independently certified by the testing laboratory for use in blood lead testing; the other tube was the VACUTAINER PLUS Low Lead tube (evaluation). The study included samples with lead concentrations within (Pb <100 µg/L, EP <350 µg/L) and above the health-related reference limits. Informed consent was obtained from all parents/guardians of participants before blood collection. The study protocol and the informed consent forms were reviewed and jointly approved by the Human Subjects Institutional Review Boards of North Shore University Hospital and the New York State Department of Health.
The control tubes were provided precertified by the Lead Poisoning/Trace Elements Laboratory, Wadsworth Center, New York State Department of Health, Albany, NY. The laboratory randomly prechecks each manufacturer's lot of tubes, as well as the needles used to collect blood for lead testing, by an well-established protocol (8). The laboratory serves as a referee laboratory in several national proficiency testing programs for blood lead and free EP, and participates in several international interlaboratory programs for blood lead. The VACUTAINER PLUS Low Lead tubes (evaluation) were provided by Becton Dickinson.
Blood specimens were protected from exposure to light by wrapping the tubes with aluminum foil to prevent photodegradation of EP. Control and evaluation specimens were shipped at ambient temperature to the Wadsworth Center for analysis within 5 business days of collection. The time between receipt and analysis in the laboratory varied from 1 to 3 days, during which time the specimens were stored refrigerated at 4 °C. Blood lead was determined by graphite furnace atomic absorption spectrometry with Zeeman background correction on a Perkin-Elmer 4100 ZL instrument (9). All specimens were analyzed for lead in duplicate on 2 different days, using two replicate injections per analysis for a total of four measurements. Three 4100ZL graphite furnace instruments were available for use in this project, and duplicate analyses were not always conducted using the same instrument, although the matched pairs (control and evaluation tubes) were included in the same analytical batch. Analyses for EP were performed in duplicate by the ethyl acetate-acetic acid extraction method with use of a Perkin-Elmer LS 50B fluorometer (10).
We obtained 62 paired samples. All specimens were satisfactory for lead and EP determinations. Whole blood lead ranged from <10 to 265 µg/L. Differences in blood lead between the control and evaluation tubes ranged from <10 to 32 µg/L. EP values ranged from 150 to 1490 µg/L, with differences between the control and evaluation tubes ranging from -60 to 85 µg/L.
Difference plots (Fig.1) were used to analyze the data (11). The graphs were generated by plotting the difference between each experimental and control tube value vs the mean of the respective control and evaluation values. As shown, the evaluation tube values fell within ± 2 SD of the control tube ranges. The SD limits represent the allowable variation as calculated from the control tube. The values suggest a slight upward trend in abnormal lead (>100 µg/L) and EP (>350 µg/L). The slopes of the upward trends, for lead and EP, were significantly different from zero at P <0.001.
The mean (± 2 SD) of the control tube for blood lead was 67 ± 9.7 µg/L compared with 69 ± 9.7 µg/L for the evaluation tube. The average difference (evaluation minus control) was 7 ± 1.8 µg/L for lead. For EP, the mean (± 2 SD) was 360 ± 23 µg/L for the control tube and 370 ± 23 µg/L for the evaluation tube, with an average difference between tube types of 35 ± 4.3 µg/L. The test results for blood lead and EP on samples collected in the evaluation tubes were clearly equivalent to those collected in the independently certified, lead-free control tubes. The availability of pediatric pretested lead-free collection tubes will contribute to accurate and reliable diagnosis of lead poisoning, a preventable environmental disease.
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Acknowledgments
This study was carried out with the cooperation of Becton Dickinson Vacutainer Tube Systems, Franklin, NJ, who provided the evaluation tubes and covered the costs of laboratory tests.
References
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