New tool for use in addressing lead poisoning

Aug 6 2007

Lead in goat blood might not be on the top of your shopping list, but for U.S. medical personnel who each year perform more than 2 million human blood measurements, Standard Reference Material (SRM) 955c from the National Institute of Standards and Technology (NIST) can't be beat.

SRM 955c is an improved version of SRM 955b, a material clinicians already relied on heavily to provide quality assurance for lead blood measurements. Significant changes in material composition, lead levels and expanded uncertainties of the certified lead concentrations make SRM 955c an even more effective tool for use in addressing lead poisoning, a condition particularly harmful to the developing nervous systems of fetuses and young children, causing learning disabilities and behavior problems and, at high levels, seizures, coma and death.

Children can be exposed to lead from lead-based paint in older buildings, or from contaminated soil near highways where vehicles once used leaded gasoline. Lead levels in children have dropped since lead was banned from both paint and fuel, but they remain significant. In 1990 the U.S. Department of Health and Human Services (DHHS) established as a national goal reducing lead blood levels to no greater than 25 micrograms per deciliter (the equivalent of 250 parts per billion) by 2000 and no greater than 10 micrograms per deciliter (100 parts per billion) for 2010. The department's Centers for Disease Control and Prevention (CDC) currently estimates that 300,000 American children, aged one to five years, have lead blood levels greater than the 2010 objective. Research reports also provide evidence of adverse effects at an even lower lead blood level than that of the 2010 target among children younger than 72 months.

SRM 955c is packaged as four vials of frozen blood at four progressively elevated lead concentration levels. Unlike previous issues of SRM 955 that were based on hog or cattle blood, SRM 955c is based on blood obtained from goats. The red blood cell system of an adult goat is much closer to that of a human, making it a better model for assessing proficiency for erythrocyte protoporphyrin, a biomarker of lead exposure. NIST's partner in developing 955c, the New York State Department of Health's Wadsworth Center Lead Poisoning Laboratory, dosed adult goats with lead acetate to produce blood pools containing lead physiologically bound to red blood cells.* The new SRM provides a better match than its predecessor to the blood samples clinicians analyze. The lead concentrations were determined by NIST using highly specialized methodology that resulted in high accuracy and low measurement uncertainty.

The changing goal line for blood lead concentration makes SRM 955c especially useful to laboratories. The lowest lead concentration level in the previous standard was 4 micrograms per deciliter or 40 parts per billion. In contrast, the lowest lead concentration level of SRM 955c is 0.4 microgram per deciliter or four parts per billion, the level of lead in an undosed animal. It is intended to represent the natural level of lead in an unexposed human population (although it is not yet known if any lead is naturally present in human blood). The lowest concentration in the SRM is 25 times lower than the DHHS 2010 goal, and will enable the development of next generation clinical methods that will be needed to accurately measure blood lead levels in children as progress is made toward eliminating lead exposure.

In addition to lead, levels two through four of SRM 955c contain added amounts of arsenic, cadmium, mercury, methylmercury and ethylmercury to facilitate future efforts to develop clinical methods to measure these toxins in human blood. At present, NIST provides information values only for the concentrations of cadmium and total mercury (including methylmercury and ethylmercury.) As values become available for arsenic, methylmercury and ethylmercury, the certificate of analysis will be updated to reflect the new information. Information values are considered useful, but lack sufficient data for NIST to be able to assign an uncertainty to the measurement.