A coupled-enzyme equilibrium method for measuring urea in serum: optimization and evaluation of the AACC study group on urea candidate reference method

We describe a coupled-enzyme equilibrium method for measuring urea in serum, which is performed on supernates prepared by treating each specimen with Ba(OH)2 and ZnSO4 (Somogyi reagent). Analytical recovery of [14C]urea added to a variety of matrices was essentially complete (mean, 100.6%) for the supernates after precipitation. Nine variables were univariately examined in arriving at the reaction conditions for the method: glutamate dehydrogenase, urease, 2-oxoglutarate, ADP, Tris . HCI, NADH, EDTA, pH, and temperature. The reagent is stable for at least 48 days at--20 degrees C and for 23 days at 4 degrees C. Mean analytical recovery of urea (14 mmol/L) added to seven different specimens (three different matrices) was 100.8%. The analytical linear range of the method extends to 30 mmol of urea per liter. Of 22 potential interferents, only bilirubin at 1 mmol/L (580 mg/L), hemoglobin at 10 g/L, and hydroxyurea at 6 mmol/L showed more than 2% interference. We discuss precision and effects of specimen dilution, and compare results for 100 human serum specimens with those measured for the same specimens with four other urea methods. We examined the effects of measuring a blank, consisting of sample and reagent without urease, with each specimen.

Prevalence of C282Y and H63D mutations in the hemochromatosis (HFE) gene in the United States

CONTEXT

Population-based estimates of the prevalence of disease-associated mutations, such as hemochromatosis (HFE) gene mutations, are needed to determine the usefulness of genetic screening.

OBJECTIVE

To estimate the prevalence of the HFE mutations C282Y and H63D in the US population.

DESIGN

Cross-sectional population-based study of samples in the DNA bank from phase 2 of the Third National Health and Nutrition Examination Survey conducted from 1992 to 1994.

SETTING AND PARTICIPANTS

Genotyped samples of cells from a total of 5171 participants, cross-classified by sex, age, and race/ethnicity in the analysis.

MAIN OUTCOME MEASURES

Estimates of the prevalence of C282Y and H63D mutations.

RESULTS

The prevalence of C282Y homozygosity is estimated to be 0.26% (95% confidence interval [CI], 0.12%-0.49%); 1.89% (95% CI, 1.48%-2.43%) for H63D homozygosity; and 1.97% (95% CI, 1.54%-2.49%) for compound heterozygosity. The prevalence estimates for C282Y heterozygosity (C282Y/wild type) are 9.54% among non-Hispanic whites, 2.33% among non-Hispanic blacks, and 2.75% among Mexican-Americans. The prevalence estimates of the C282Y mutation in the US population are 5.4% (95% CI, 4.7%-6.2%) and 13.5% (95% CI, 12.5%-14.8%) for the H63D mutation.

CONCLUSIONS

Estimates of prevalence of HFE mutations are within the expected range for non-Hispanic whites and blacks but the estimated prevalence of the C282Y mutation among Mexican-Americans is less than expected. Mutation data now need to be linked to clinically relevant indices, such as transferrin saturation level.

Analysis of factors influencing the comparison of homocysteine values between the Third National Health and Nutrition Examination Survey (NHANES) and NHANES 1999+

Two important changes occurred in the time between the Third National Health and Nutrition Examination Survey (NHANES III) (1991-1994) and the later survey (NHANES 1999+) regarding total homocysteine (tHcy), i.e., a change in matrix from serum to plasma and a change in analytical methods. The goals of this study were to determine the magnitude of potential differences between plasma and serum with regard to tHcy concentrations, and between the two analytical methods used in these surveys. Optimally prepared plasma, serum allowed to clot for 30 and 60 min at room temperature and serum allowed to clot for 30 and 60 min and subjected to four freeze-thaw cycles, prepared from blood samples collected from 30 healthy people, were analyzed by both methods. Serum samples had significantly higher tHcy concentrations than plasma samples, and the difference increased with longer clotting time. Freeze-thaw cycles had little or no effect on the variability or bias in the serum sample results. The tHcy results produced by the two analytical methods were significantly different, but consistent across sample types. On average, the results of the method used in NHANES III were lower by 0.64 micromol/L; however, the relative bias varied with tHcy concentration. The tHcy results determined in surplus serum from NHANES III overestimated tHcy concentrations by approximately 10% compared with optimally prepared plasma. The average method bias was 6% between the two analytical methods. On the basis of changes in matrix and methodology, direct comparison of tHcy results between the two surveys is inappropriate.

A reference method laboratory network for cholesterol: a model for standardization and improvement of clinical laboratory measurements

BACKGROUND

Accurate and precise measurement of blood cholesterol plays a central role in the National Cholesterol Education Program's strategy to reduce the morbidity and mortality attributable to coronary heart disease. Matrix effects hamper the ability of manufacturers to adequately calibrate and validate traceability to the National Reference System for Cholesterol (NRS/CHOL). CDC created the Cholesterol Reference Method Laboratory Network (CRMLN) to improve cholesterol measurement by assisting manufacturers of in vitro diagnostic products with validation of the traceability of their assays to the NRS/CHOL.

METHODS

CRMLN laboratories established the CDC cholesterol reference method (modification of the Abell-Levy-Brodie-Kendall chemical method) and are standardized using CDC frozen serum reference materials. CRMLN laboratories use common quality-control materials and participate in monthly external performance evaluations conducted by CDC. The CRMLN performance criteria require member laboratories to agree with CDC within +/-1.0% and maintain a CV < or =2.0%.

RESULTS

From 1995 to 200 the CRMLN laboratories met the accuracy criterion 97% of the time and the precision criterion 99% of the time. During this time period, the CRMLN maintained an average bias to CDC of 0.01% and an average collective CV of 0.33%.

CONCLUSIONS

CDC established the CRMLN as the first international reference method laboratory network. The CRMLN assists manufacturers in the validation of the calibration of their diagnostic products so that clinical laboratories can measure blood cholesterol more reliably. The CRMLN can serve as a model for other clinical analytes where traceability to a hierarchy of methods is needed and matrix effects of the field methods with processed calibrators or reference materials are present.

Levels of seven urinary phthalate metabolites in a human reference population

Using a novel and highly selective technique, we measured monoester metabolites of seven commonly used phthalates in urine samples from a reference population of 289 adult humans. This analytical approach allowed us to directly measure the individual phthalate metabolites responsible for the animal reproductive and developmental toxicity while avoiding contamination from the ubiquitous parent compounds. The monoesters with the highest urinary levels found were monoethyl phthalate (95th percentile, 3,750 ppb, 2,610 microg/g creatinine), monobutyl phthalate (95th percentile, 294 ppb, 162 microg/g creatinine), and monobenzyl phthalate (95th percentile, 137 ppb, 92 microg/g creatinine), reflecting exposure to diethyl phthalate, dibutyl phthalate, and benzyl butyl phthalate. Women of reproductive age (20-40 years) were found to have significantly higher levels of monobutyl phthalate, a reproductive and developmental toxicant in rodents, than other age/gender groups (p < 0.005). Current scientific and regulatory attention on phthalates has focused almost exclusively on health risks from exposure to only two phthalates, di-(2-ethylhexyl) phthalate and di-isononyl phthalate. Our findings strongly suggest that health-risk assessments for phthalate exposure in humans should include diethyl, dibutyl, and benzyl butyl phthalates.

Serum dioxin levels in Seveso, Italy, population in 1976

On July 10, 1976, an explosion at a chemical plant near Seveso, Italy, released a mixture of chemicals, including 2,3,7,8-tetrachlorodibenzo-p-dioxin and 2,4,5-trichlorophenol. As a result, several thousand people in the Seveso area may have been exposed to those chemicals. At that time, human exposure assessment was based primarily on soil levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Medical examinations of this potentially exposed population and control subjects were begun in 1976 and in some cases continued until 1985. In 1988, we began assessing human exposure in this population by measuring 2,3,7,8-tetrachlorodibenzo-p-dioxin in small volumes of serum specimens remaining from the medical examinations. As expected, we found that the median serum dioxin levels were highest among people who lived closest to the explosion and were progressively lower among groups living farther away. These measurements have allowed us to assess exposure more accurately among individuals in this population and to relate exposure to various health effects. We found that some individuals in the exposed population had among the highest serum dioxin levels ever reported, yet chloracne was the only unequivocal effect found; cancer risks are still being investigated. We also found that other individuals with as high or higher serum dioxin levels did not develop chloracne. We also found that the serum half-life of dioxin in this population was 7-8 years, which agrees with other findings although we do report some differences in the serum half-life of TCDD for women and children. We also observed an increase in the percentage of female newborns to parents who resided in Zone A at the time of the explosion, and we also report on the 1976 serum dioxin levels in people who later developed cancer.

Paternal concentrations of dioxin and sex ratio of offspring

BACKGROUND

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD or dioxin), is commonly considered the most toxic man-made substance. We have previously shown that high serum concentrations of TCDD in parents from Seveso, Italy, were linked to their having a relative increase in the number of female births after the parents exposure to a release of dioxin in 1976. We have continued the study to determine whether the parents' sex and/or age at exposure affected the sex ratio of their children.

METHODS

We measured the TCDD concentrations in serum samples from potentially exposed parents collected in 1976 and 1977, and investigated the sex ratio of their offspring.

FINDINGS

Serum samples were collected from 239 men and 296 women. 346 girls and 328 boys were born to potentially exposed parents between 1977 and 1996, showing an increased probability of female births (lower sex ratio) with increasing TCDD concentrations in the serum samples from the fathers (p=0.008). This effect starts at concentrations less than 20 ng per kg bodyweight. Fathers exposed when they were younger than 19 years of age sired significantly more girls than boys (sex ratio 0.38 [95% CI 0.30-0.47]).

INTERPRETATION

Exposure of men to TCDD is linked to a lowered male/female sex ratio in their offspring, which may persist for years after exposure. The median concentration of dioxin in fathers in this study is similar to doses that induce epididymal impairments in rats and is about 20 times the estimated average concentration of TCDD currently found in human beings in industrialised countries. These observations could have important public-health implications.

Levels of methyleugenol in a subset of adults in the general U.S. population as determined by high resolution mass spectrometry

We developed a sensitive and accurate analytical method for quantifying methyleugenol (ME) in human serum. Our method uses a simple solid-phase extraction followed by a highly specific analysis using isotope dilution gas chromatography-high resolution mass spectrometry. Our method is very accurate; its limit of detection is 3.1 pg/g and its average coefficient of variation is 14% over a 200-pg/g range. We applied this method to measure serum ME concentrations in adults in the general U.S. population. ME was detected in 98% of our samples, with a mean ME concentration of 24 pg/g (range < 3.1-390 pg/g). Lipid adjustment of the data did not alter the distribution. Bivariate and multivariate analyses using selected demographic variables showed only marginal relationships between race/ethnicity and sex/fasting status with serum ME concentrations. Although no demographic variable was a good predictor of ME exposure or dose, our data indicate prevalent exposure of U.S. adults to ME. Detailed pharmacokinetic studies are required to determine the relationship between ME intake and human serum ME concentrations.

Exposure of the U.S. population aged 6 years and older to cadmium: 1988-1994

Cadmium was measured in urine specimens from 22,162 participants in the Third National Health and Nutrition Examination Survey (NHANES III 1988-1994). Urine cadmium, expressed either as uncorrected (microg/L) or creatinine corrected (microg/g creatinine) increased with age and with smoking. The arithmetic mean value for urine cadmium in the U.S. population was 0.57 microg/L or 0.48 microg/g creatinine. Based on our estimates, about 2.3% of the U.S. population have urine cadmium concentrations greater than 2 microg/g creatinine, and 0.2% have concentrations greater than 5 microg/g creatinine, the current World Health Organization health-based exposure limit.

Uranium and thorium in urine of United States residents: reference range concentrations

We measured uranium and thorium in urine of 500 U. S. residents to establish reference range concentrations using a magnetic-sector inductively coupled argon plasma mass spectrometer (ICP-MS). We found uranium at detectable concentrations in 96.6% of the urine specimens and thorium in 39.6% of the specimens. The 95th percentile concenetration for uranium was 34.5 ng/L (parts per trillion); concentrations ranged up to 4080 ng/L. Thorium had a 95th percentile concentration of 3.09 ng/L; concentrations ranged up to 7.7 ng/L.

Exposure assessment: serum levels of TCDD in Seveso, Italy

Accurate exposure assessment is an important step in both risk assessment and epidemiologic studies involving potential human exposure to environmental toxicants. Various methods have been used to assess human exposure. These methods include models based on one's temporal and spatial nearness to the source, environmental levels of toxicant, and biological measures. We believe that the latter measure is the "gold standard." In this article we present the serum 2,3,7,8-tetrachlorodibenzo-p-dioxin levels in residents of the contaminated zones in Seveso, Italy, in 1976, and delineate these data by age and gender. Some of these serum levels are among the highest ever reported and thus this population serves as a benchmark for comparison of human exposure and potential adverse health effects. One such potential population is that population consuming potentially contaminated fish.

Strategies for biological monitoring of exposure for contemporary-use pesticides

Pesticides are used on a massive scale in the United States. The widespread use of these pesticides has made it virtually impossible for the average person to avoid exposure at some level. Generally, it is believed that low-level exposure to these pesticides does not produce acute toxic effects; however, various cancers and other noncancer health endpoints have been associated with chronic exposure to several groups of pesticides. Therefore, it is imperative that well-designed studies investigate the potential relationship between contemporary pesticide exposure and health effects. For these studies to be accurate, reliable methods for determining individual exposure must be used. Biological monitoring is a useful tool for assessing exposure to some contemporary pesticides. As with any analytical method, biological monitoring entails many difficulties, but, in many instances, they can be overcome by the logical use of available information and information acquired in carefully designed studies. At the Centers for Disease Control and Prevention (CDC), we have acquired extensive experience in the development and application of specific techniques for biological monitoring of a variety of toxicants, including many of the contemporary-use pesticides. We have used these methods to measure the internal dose of pesticides received by people in acute and chronic incidents resulting from both environmental and industrial exposure. Additionally, we have established normative values, or reference ranges, of several pesticides based on measurements of their metabolites in the urine of randomly selected adults in the US population. These data have been successfully used to distinguish overt exposures from 'background' exposure. In this paper, we present several examples of the usefulness of biological monitoring in urine and blood and describe the difficulties involved with developing methods in these matrices. We also present a general strategy, considerations, and recommendations for developing biological monitoring techniques for measuring the internal dose of contemporary-use pesticides.

Current activities at the Centers for Disease Control and Prevention's National Diabetes Laboratory

In 1997, the Centers for Disease Control and Prevention established the National Diabetes Laboratory in order to help prevent and treat type 1 diabetes. This state-of-the-art laboratory collaborates with research scientists and key national and international organizations throughout the world to identify and study risk factors for type 1 diabetes by developing measurements for glycosylated proteins, developing and evaluating technology for measuring genetic risk factors for the disease, and working to standardize autoantibody measurements. Developing improved technologies for diagnosing and managing diabetes and developing reference materials for properly calibrating and standardizing blood glucose meters are also critical aspects of the laboratory's work. In addition, the laboratory provides quality storage for valuable collections of biologics and other materials and facilitates sharing of specimens, associated epidemiologic data, and test results. Working with our partners in diabetes research, we are improving the diagnosis, treatment, and prevention of type 1 diabetes.

Trace metals in urine of United States residents: reference range concentrations

We measured 13 metals in the urine of 496 United States residents to establish reference range concentrations using inductively coupled argon plasma mass spectrometry and Zeeman graphite furnace atomic absorption spectrometry. We frequently found 8 of these analytes at detectable concentrations in urine specimens: molybdenum (in 99.8%); lead (98.8%); tin (89%); thallium (77%); antimony (73.5%); manganese (73%); cesium (71%); tungsten (70%); and platinum (69.7%). The 95th percentile concentration for molybdenum was 168 micrograms/L; concentrations ranged up to 688 micrograms/L. Lead concentrations ranged up to 67 micrograms/L, and the 95th upper percentile was 6.4 micrograms/L. Tin had 95th upper percentile of 20.1 micrograms/L. Other analytes measured at detectable concentrations included barium (in 67% of the specimens); beryllium (67%); chromium (54%); thorium (44%); and cobalt (43%). In almost every case, the 95th upper percentiles of these analytes were less than 15 micrograms/L.

Development and validation of sensitive method for determination of serum cotinine in smokers and nonsmokers by liquid chromatography/atmospheric pressure ionization tandem mass spectrometry

We describe a sensitive and specific method for measuring cotinine in serum by HPLC coupled to an atmospheric pressure chemical ionization tandem mass spectrometer. This method can analyze 100 samples/day on a routine basis, and its limit of detection of 50 ng/L makes it applicable to the analysis of samples from nonsmokers potentially exposed to environmental tobacco smoke. Analytical accuracy has been demonstrated from the analysis of NIST cotinine standards and from comparative analyses by both the current method and gas chromatography/high-resolution mass spectrometry. Precision has been examined through the repetitive analysis of a series of bench and blind QC materials. This method has been applied to the analysis of cotinine in serum samples collected as part of the Third National Health and Nutrition Examination Survey (NHANES III).

Results of an international round robin for serum and whole-blood folate

Because of the increasing significance of folate nutriture to public health, a "round robin" interlaboratory comparison study was conducted to assess differences among methods. Twenty research laboratories participated in a 3-day analysis of six serum and six whole-blood pools. Overall means, SDs, and CVs derived from these results were compared within and across method types. Results reported for serum and whole-blood folate demonstrated overall CVs of 27.6% and 35.7%, respectively, across pools and two- to ninefold differences in concentrations between methods, with the greatest variation occurring at critical low folate concentrations. Although results for serum pools were less variable than those for whole-blood pools, substantial intermethod variation still occurred. The overall results underscore the urgent need for developing and validating reference methods for serum and whole-blood folate and for properly characterized reference materials. For evaluating study or clinical data, method-specific reference ranges (established with clinical confirmation of values for truly folate-deficient individuals) must be used.

Isotope dilution--mass spectrometric quantification of specific proteins: model application with apolipoprotein A-I

An enzymatic hydrolysis isotope dilution-mass spectrometric method was developed for reference quantification of specific proteins. The analytical procedure involved measuring a reproducibly hydrolyzed peptide (serving as the primary standard) unique to a specific protein. This new mass spectrometric method was evaluated by assessing the concentration of apolipoprotein (apo) A-I in the European Community Bureau of Reference (BCR) lyophilized Certified Reference Material (CRM 393). We used the method to make 96 measurements (4 replicate analyses of 4 enzymatic digests of 6 vials of BCR-CRM 393), which gave an average total protein mass of 1.048 mg (+/- 1.0% at 99% confidence limits). The total overall analytical CV was 3.95%. The results of this evaluation of our model approach to determine the concentration of a specific protein in a purified preparation demonstrated that our new mass spectrometric method can be used to measure apolipoproteins and other specific proteins without the use of epitopic immunoassay methods.

Results of an international round robin for serum and whole-blood folate

Because of the increasing significance of folate nutriture to public health, a "round robin" interlaboratory comparison study was conducted to assess differences among methods. Twenty research laboratories participated in a 3-day analysis of six serum and six whole-blood pools. Overall means, SDs, and CVs derived from these results were compared within and across method types. Results reported for serum and whole-blood folate demonstrated overall CVs of 27.6% and 35.7%, respectively, across pools and two- to ninefold differences in concentrations between methods, with the greatest variation occurring at critical low folate concentrations. Although results for serum pools were less variable than those for whole-blood pools, substantial intermethod variation still occurred. The overall results underscore the urgent need for developing and validating reference methods for serum and whole-blood folate and for properly characterized reference materials. For evaluating study or clinical data, method-specific reference ranges (established with clinical confirmation of values for truly folate-deficient individuals) must be used.

Isotope dilution--mass spectrometric quantification of specific proteins: model application with apolipoprotein A-I

An enzymatic hydrolysis isotope dilution-mass spectrometric method was developed for reference quantification of specific proteins. The analytical procedure involved measuring a reproducibly hydrolyzed peptide (serving as the primary standard) unique to a specific protein. This new mass spectrometric method was evaluated by assessing the concentration of apolipoprotein (apo) A-I in the European Community Bureau of Reference (BCR) lyophilized Certified Reference Material (CRM 393). We used the method to make 96 measurements (4 replicate analyses of 4 enzymatic digests of 6 vials of BCR-CRM 393), which gave an average total protein mass of 1.048 mg (+/- 1.0% at 99% confidence limits). The total overall analytical CV was 3.95%. The results of this evaluation of our model approach to determine the concentration of a specific protein in a purified preparation demonstrated that our new mass spectrometric method can be used to measure apolipoproteins and other specific proteins without the use of epitopic immunoassay methods.

Analysis of benzoylecgonine in dried blood spots by liquid chromatography--atmospheric pressure chemical ionization tandem mass spectrometry

Residual samples from blood spots (i.e., whole blood spotted onto filter paper) are a useful source for epidemiological screening studies involving newborns. However, the small volume of blood available from residual blood spots complicates the assay. A method for analyzing benzoylecgonine (BZE; the primary metabolite of cocaine) in blood spots, in which the blood spot is eluted with aqueous ammonium acetate-methanol containing N-methyl trideuterated-BZE as an internal standard, followed by high-performance liquid chromatography-atmospheric pressure chemical ionization tandem mass spectrometry using multiple reaction monitoring, has been developed. This approach provides a rapid, direct, sensitive (limit of detection, approximately 2 ng/mL, based on a 12-microL sample size), and highly specific means of determining BZE concentrations in blood spots. We have applied this method for confirmatory analyses in a large epidemiological study of the prevalence of cocaine use during late pregnancy.

Recommendations for the selection and use of protocols for assignment of values to reference materials

It is essential that testing of patient samples give values that are traceable to those in a recognized, authorizative reference material. In addition, samples used for laboratory proficiency testing must have values assigned from such a reference material if results are to be comparable among materials and laboratories. As a result, the assignment of values to secondary and tertiary reference materials, calibrants, controls, and proficiency samples should be performed as precisely as possible, within reasonable limits. The intent of this document is to give guidelines for assignment of values at three levels of transfer. 1) from primary to secondary reference, materials, such as international or national references; 2) from secondary to tertiary reference materials, such as manufacturers' in-house calibrants and controls; and 3) from tertiary reference materials, such as manufacturers' in-house calibrants and controls; and 3) from tertiary reference materials to working calibrants and controls. It is hoped that these guidelines will facilitate the selection and utilization of an appropriate value transfer protocol for each level of value assignment. Because of the wide variety and nature of analytes, however, the guidelines are intentionally broad and may require revision for specific analytes.

Pesticide residues in urine of adults living in the United States: reference range concentrations

We measured 12 analytes in urine of 1000 adults living in the United States to establish reference range concentrations for pesticide residues. We frequently found six of these analytes: 2,5-dichlorophenol (in 98% of adults); 2,4-dichlorophenol (in 64%); 1-naphthol (in 86%); 2-naphthol (in 81%); 3,5,6- trichloro-2-pyridinol (in 82%); and pentachlorophenol (in 64%). The 95th percentile concentration (95th PC) for 2,5-dichlorophenol (indicative of p-dichlorobenzene exposure) was 790 micrograms/liter; concentrations ranged up to 8700 micrograms/liter. 2,4-Dichlorophenol concentrations ranged up to 450 micrograms/ liter, and the 95thPC was 64 micrograms/liter. 1-Naphthol and 2-naphthol (indicative of naphthalene exposure) had 95thPCs of 43 and 30 micrograms/liter, respectively; concentrations of 1-naphthol ranged up to 2500 micrograms/liter. Chlorpyrifos exposure was indicated by 3,5,6-tricholoro-2-pyridinol concentrations of 13 (95thPC) and 77 micrograms/liter (maximum observed). Pentachlorophenol had a 95thPC of 8.2 micrograms/liter. Other analytes measured included 4-nitrophenol (in 41%); 2,4,5-trichlorophenol (in 20%); 2,4,6-trichlorophenol (in 9.5%); 2,4-dichlorophenoxyacetic acid (in 12%); 2-isopropoxyphenol (in 6.8%); and 7-carbofuranphenol (in 1.5%). The 95thPCs of these analytes were < 6 micrograms/liter. p-Dichlorobenzene exposure is ubiquitous; naphthalene and chlorpyrifos are also major sources of pesticide exposure. Exposure to chlorpyrifos appears to be increasing. Although pentachlorophenol exposure is frequent, exposure appears to be decreasing. These reference range concentrations provide information about pesticide exposure and serve as a basis against which to compare concentrations in subjects who may have been exposed to pesticides.

Relation of a seafood diet to mercury, selenium, arsenic, and polychlorinated biphenyl and other organochlorine concentrations in human milk

Human transition milk was sampled from 88 mothers at the Faroe Islands, where the seafood diet includes pilot whale meat and blubber. Milk mercury concentrations (median, 2.45 micrograms/liter) were significantly associated with mercury concentrations in cord blood and with the frequency of pilot whale dinners during pregnancy. Milk selenium concentrations (mean, 19.1 micrograms/liter) correlated significantly with concentrations in cord blood but not with seafood consumption. Arsenic concentrations were very low. Twenty-four of the milk samples were separated into four pools based on fish intake and milk mercury concentrations. The polychlorinated biphenyl (PCB) concentrations (1.8-3.5 micrograms/g lipid) were high and mainly due to congener numbers 153, 180, and 138. One pool contained a congener 77 concentration of 1380 ppt, which is the highest ever reported in a human specimen for a coplanar PCB. The highest PCB concentrations were seen in the pools from women who had eaten frequent whale dinners and whose milk contained high mercury concentrations. The concentrations of chlorinated dibenzo-p-dioxins and furans were not similarly elevated. Given the advantages associated with breast-feeding, advice to nursing mothers in this population should take into regard the possible risks associated with long-term exposure to milk contaminants.

Using biological monitoring to assess human exposure to priority toxicants

Scientifically valid exposure assessment is crucial to risk assessment, risk management, and prevention of environmental disease. Scientists have used three tools to assess exposure: exposure history/questionnaire, environmental monitoring (including personal monitoring), and biological monitoring. Combinations of these tools usually provide the exposure information needed to meet objectives of human studies evaluating the exposure-health effect relationship. Biological monitoring is a capable exposure assessment tool that has provided important information used in public health decisions. We briefly describe how risk assessment and risk management decisions for lead, dioxin, and volatile organic compounds have substantially benefited from exposure information obtained from biological monitoring.

The priority toxicant reference range study: interim report

The relationship between human exposure to environmental toxicants and health effects is of utmost interest to public health scientists. To define this relationship, these scientists need accurate and precise methods for assessing human exposure and effects. One of the most accurate and precise means of assessing exposure is to measure the level of the toxicant or its primary metabolite in a biologic specimen; this has been defined as measuring the internal dose. This measurement must be quantitative to best study the dose-response relationship. Pertinent questions asked during an exposure assessment include "How do the levels of a given toxicant in a particular population compare with the levels of that toxicant in other populations?" and "What is the prevalence of exposure to that toxicant in other populations?" To answer these questions for two chemical classes of environmental toxicants, we developed state-of-the-art analytic methods and then applied them to measure the levels of 44 environmental toxicants in biologic specimens from 1000 United States residents who participated in the Third National Health and Nutrition Examination Survey (NHANES III). These 1000 people are a cross-sectional subset of the NHANES III population and were selected from urban and rural communities in four regions of the United States; all were between 20 and 59 years of age. This subset is not a probability-based sample.(ABSTRACT TRUNCATED AT 250 WORDS)

Traditional lipoprotein profile: clinical utility, performance requirement, and standardization

The lipid and lipoprotein parameters which are predominantly measured and effectively comprise the traditional lipoprotein profile include total cholesterol, high density lipoprotein (HDL) cholesterol, low density lipoprotein (LDL) cholesterol, and triglyceride. Total cholesterol is accepted as the initial entry point in a case finding approach such as that recommended by the National Cholesterol Education Program (NCEP). HDL cholesterol, known to be a strong inverse predicator of risk, is an additional measurement to total cholesterol to improve risk assessments. The evidence for triglyceride association remains mixed: although strong associations are found in some studies, the evidence as an independent risk factor is still incomplete. Triglyceride is therefore measured primarily for LDL estimation. Final classification and potential intervention is ultimately based on the measurement of LDL cholesterol. Reliability in the measurement of total cholesterol, HDL, LDL, and triglyceride is especially important if the uniform decision points established by the NCEP are to be properly implemented. Attention must be placed on controlling preanalytical sources of variation, which can account for as much as 60% of the total measurement variability. The major analytical source of error comes from matrix effects, which results in problems of proper analytical calibration. Instrument system calibration should be verified by a comparison with an accuracy base using fresh patient specimens. CDC has established a network of reference method laboratories to provide access to these lipid and lipoprotein accuracy bases.

Technical and scientific developments in exposure marker methodology

Recent advances in techniques to measure markers of exposure to environmental toxicants in humans are changing the ways in which environmental scientists, epidemiologists, and policymakers characterize and interpret such exposure. In this article we review some major technical and scientific developments in exposure marker methodology for estimating internal dose, with special reference to studies conducted at the US Centers for Disease Control and Prevention. We consider important characteristics of laboratory methods, advances in laboratory technology, analytical standards, and quality assurance of laboratory measurements; comparisons with indirect methods for estimating exposures, such as exposure indices and questionnaires; human pharmacokinetic data; sampling problems; surveillance of human exposures to toxicants; and interpretation of measurements. With a view to increasing the reliability of exposure assessment, we make recommendations for obtaining more data on human exposure to toxicants.

Estimating and minimizing effects of biologic sources of variation by relative range when measuring the mean of serum lipids and lipoproteins

Biologic intraindividual variation (CVb) is a major source of inaccuracy in current lipid and lipoprotein measurements. Metaanalysis has been used to estimate the average CVb of serum total cholesterol (TC), high-density lipoprotein cholesterol (HDLC), low-density lipoprotein cholesterol (LDLC), and triglyceride (TG). These CVb values are larger than the National Cholesterol Education Program-accepted and -proposed analytic (CVa) goals. Measuring serial specimens reduces the error in determination of the mean concentration used in classification of the patient by cutoff points. We show (a) a convenient technique, based on the relative range, to qualitatively estimate and interpret biologic variation of TC, HDLC, LDLC, and TG, and (b) the number of serial specimens required to meet a total variation goal for measurements of mean lipid and lipoprotein values. A total variation goal has been selected that can be met by two serial specimens for a majority of individuals.

Estimating and minimizing effects of biologic sources of variation by relative range when measuring the mean of serum lipids and lipoproteins

Biologic intraindividual variation (CVb) is a major source of inaccuracy in current lipid and lipoprotein measurements. Metaanalysis has been used to estimate the average CVb of serum total cholesterol (TC), high-density lipoprotein cholesterol (HDLC), low-density lipoprotein cholesterol (LDLC), and triglyceride (TG). These CVb values are larger than the National Cholesterol Education Program-accepted and -proposed analytic (CVa) goals. Measuring serial specimens reduces the error in determination of the mean concentration used in classification of the patient by cutoff points. We show (a) a convenient technique, based on the relative range, to qualitatively estimate and interpret biologic variation of TC, HDLC, LDLC, and TG, and (b) the number of serial specimens required to meet a total variation goal for measurements of mean lipid and lipoprotein values. A total variation goal has been selected that can be met by two serial specimens for a majority of individuals.

Tryptophan contaminants associated with eosinophilia-myalgia syndrome. The Eosinophilia-Myalgia Studies of Oregon, New York and New Mexico

Eosinophilia-myalgia syndrome (EMS) has been linked to ingestion of tryptophan contaminated with 1,1'-ethylidene-bis[L-tryptophan] (EBT), but other contaminants have received little study. The authors identified 101 lots of L-tryptophan that had been consumed either by persons with EMS or by asymptomatic tryptophan users and quantified the amounts of EBT and five other contaminants in each lot. After stratification of case and noncase lots by time of manufacture to adjust for the strong sequential pattern over time among case and noncase lots, higher EBT levels were still associated with a lot's case status, but the association lacked statistical significance (p = 0.120, odds ratio = 1.56, 95% confidence interval 0.758-3.23). While these findings do not rule out the possibility that EBT is the etiologic agent in EMS, they raise the possibility that other chemical contaminants in manufactured tryptophan modify the effects of EBT or that the causal agent of EMS is an entirely distinct compound.

Contaminants in L-tryptophan associated with eosinophilia myalgia syndrome

In late 1989, an epidemic of eosinophilia-myalgia syndrome (EMS) that resulted in several thousand cases of the syndrome and 36 deaths was recognized in the United States. Physicians in New Mexico linked the epidemic to the ingestion of L-tryptophan (LT). Results of studies indicated that one or more trace contaminants in LT were likely causes of the EMS epidemic. Investigators traced the LT that was taken by most patients with EMS to a single manufacturer, Showa Denko K.K. of Japan. We now report results of high performance liquid chromatographic analysis of LT samples from this manufacturer. Three sets of blind-coded samples were analyzed: the priority case lot set, which included 54 case-associated LT lots and 50 noncase-associated LT lots that were taken by case and control subjects who used only one brand of LT; the single lot case set, which included 73 case-associated LT lots and 25 noncase associated LT lots taken by case and control subjects who used only a single lot of LT; and the South Carolina tablet set, which included LT tablets taken by case subjects (n = 26) and by control subjects (n = 52). We statistically compared the concentration of each contaminant in case-associated, noncase-associated, and control samples of each sample set. The analyses showed that there were more than 60 minor contaminants in the LT from Showa Denko K.K., and that six of these contaminants were associated with EMS. The structures of three contaminants are known, but the identities of the other three contaminants are currently unknown.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological variability in concentrations of serum lipids: sources of variation among results from published studies and composite predicted values

To obtain the best estimates of the average intraindividual biological variability (CVb) in the concentrations of total cholesterol (TC), low-density lipoprotein cholesterol (LDLC), high-density lipoprotein cholesterol (HDLC), and triglyceride serum lipids in a person's blood, we evaluated results from 30 studies published from 1970 to 1992. The usually more applicable random-effects model estimated an average CVb of 6.1% for TC, 7.4% for HDLC, 9.5% for LDLC, and 22.6% for triglyceride. Composite estimates of the average CVb from all evaluated published studies by different models of estimation ranged from 6.0% to 6.4% for TC, from 6.2% to 7.5% for HDLC, from 7.0% to 9.6% for LDLC, and from 22.4% to 22.9% for triglyceride. Two important factors influenced the reported biological variation of the study subjects: (a) the magnitude of the variability of the analytical method used and (b) the design characteristics of the study--primarily the number of subjects, the sampling interval, and the number of measurements per subject. For TC, we found a statistically significant positive correlation between the reported mean CVb and both the number of study subjects and the analytical variation. For TC and LDLC we estimate CVb as a function of the study design features. The number of patient specimens required to obtain reliable estimates for serum lipid concentrations are determined from the CVb and the current analytical variation.

Biological variability in concentrations of serum lipids: sources of variation among results from published studies and composite predicted values

To obtain the best estimates of the average intraindividual biological variability (CVb) in the concentrations of total cholesterol (TC), low-density lipoprotein cholesterol (LDLC), high-density lipoprotein cholesterol (HDLC), and triglyceride serum lipids in a person's blood, we evaluated results from 30 studies published from 1970 to 1992. The usually more applicable random-effects model estimated an average CVb of 6.1% for TC, 7.4% for HDLC, 9.5% for LDLC, and 22.6% for triglyceride. Composite estimates of the average CVb from all evaluated published studies by different models of estimation ranged from 6.0% to 6.4% for TC, from 6.2% to 7.5% for HDLC, from 7.0% to 9.6% for LDLC, and from 22.4% to 22.9% for triglyceride. Two important factors influenced the reported biological variation of the study subjects: (a) the magnitude of the variability of the analytical method used and (b) the design characteristics of the study--primarily the number of subjects, the sampling interval, and the number of measurements per subject. For TC, we found a statistically significant positive correlation between the reported mean CVb and both the number of study subjects and the analytical variation. For TC and LDLC we estimate CVb as a function of the study design features. The number of patient specimens required to obtain reliable estimates for serum lipid concentrations are determined from the CVb and the current analytical variation.

Factors influencing the accuracy of the national reference system total cholesterol reference method

Previous comparisons between the Reference and Definitive Methods for measuring serum cholesterol have demonstrated a small but persistent positive bias in the Reference Method, averaging about +1.6%. Here we describe the results of further investigations designed to better characterize the nature of this bias. Analysis of a well-characterized model serum sample (SRM 909) suggests that more than half of the difference in cholesterol values determined by the two methods is the result of small contributions from cholesterol precursor sterols and phytosterols, which are also measured for the Reference Method. An additional significant contribution may be from cholesterol oxidation products, particularly 7-hydroxycholesterol isomers, which are active in the Liebermann-Burchard reaction. The 7-hydroxycholesterol in SRM 909, most of which appeared to be already present in the serum rather than formed during saponification, may account for as much as 20% of the observed difference between the methods. Contributions from other possible sources, including impurities in the cholesterol standard and incomplete saponification of cholesteryl esters, are very small. Because the observed bias is both quite small and consistent among samples, the cholesterol Reference Method continues to meet all of the requirements generally expected for a dependable and effective Reference Method.

2,3,7,8-Tetrachlorodibenzo-p-dioxin levels in adipose tissue of exposed and control persons in Missouri. An interim report

The compound 2,3,7,8-tetrachlorodibenzo-p-dioxin, commonly known as dioxin, was measured in the adipose tissue of 39 persons with a history of residential, recreational, or occupational exposure in Missouri and in 57 persons in a control group. All participants had detectable levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin in their adipose tissue, but the exposed group had significantly higher levels. Levels of six of the exposed persons were more than five times greater than the level of the highest control. Measuring 2,3,7,8-tetrachlorodibenzo-p-dioxin in adipose tissue provides a much improved index of exposure, an important advance for research studies evaluating the possible health effects of this compound.

Multivariate examination of response surfaces around the reaction conditions for the Scandinavian Society's recommended method for creatine kinase determinations

We examined the reaction surfaces around five variables (imidazole, ADP, creatine phosphate, magnesium, and pH) in the Scandinavian method for determining creatine kinase, using factorial experimentation (five level, five factor) at reaction temperatures of 30 and 37 degrees C. Theoretical response surfaces were computed by fitting a quadratic polynomial equation to the experimental data by least-squares regression. Essentially no differences were apparent in the theoretical curves among the five specimens we analyzed, or between reaction temperatures. Our response-surface data showed the following: for pH and imidazole, activity optima in the region of the Scandinavian conditions; for creatine phosphate, a broad plateau over the concentration range investigated (10 to 50 mmol/L); and for magnesium and ADP, gently increasing contours with maximal creatine kinase activity at concentrations greater than those investigated in our study (magnesium 15 mmol/L, ADP 3.5 mmol/L).

Multivariate examination of response surfaces around the reaction conditions for the Scandinavian Society's recommended method for creatine kinase determinations

We examined the reaction surfaces around five variables (imidazole, ADP, creatine phosphate, magnesium, and pH) in the Scandinavian method for determining creatine kinase, using factorial experimentation (five level, five factor) at reaction temperatures of 30 and 37 degrees C. Theoretical response surfaces were computed by fitting a quadratic polynomial equation to the experimental data by least-squares regression. Essentially no differences were apparent in the theoretical curves among the five specimens we analyzed, or between reaction temperatures. Our response-surface data showed the following: for pH and imidazole, activity optima in the region of the Scandinavian conditions; for creatine phosphate, a broad plateau over the concentration range investigated (10 to 50 mmol/L); and for magnesium and ADP, gently increasing contours with maximal creatine kinase activity at concentrations greater than those investigated in our study (magnesium 15 mmol/L, ADP 3.5 mmol/L).

Determination of steroid hormones in a human-serum reference material by isotope dilution--mass spectrometry: a candidate definitive method for cortisol

We report a method, based on isotope dilution--mass spectrometry, for determining cortisol in a pooled specimen of human serum. Isotopically labeled cortisol is added to 5.0 mL of serum so that the molar concentrations of labeled cortisol and unlabeled cortisol are approximately equal. The specimen and two calibration standards are extracted with dichloromethane, and the extracted cortisol is converted to the methoxime-trimethylsilyl ether derivative. Samples and standards are analyzed by gas chromatography--mass spectrometry by monitoring the peak areas for m/z 605 and 608. The cortisol concentration is calculated by linear interpolation between the two bracketing standards. Variances of data collected during six weeks showed that the overall coefficient of variation (CV) was 0.69% (n = 32); the within-vial CV, 0.63%; the among-vial CV, 0.22%; and the among-day CV, 0.15% (means = 3.973 nmol/vial). Method specificity was demonstrated by liquid chromatographic as well as C8 mini-column cleanup of samples before derivation, by alternative ion monitoring at m/z 636 and 639, and by negative-ion chemical ionization at m/z 459 and 462. Derivatives of all observed degradation products of cortisol under basic, neutral, and acidic conditions did not interfere.

Determination of steroid hormones in a human-serum reference material by isotope dilution--mass spectrometry: a candidate definitive method for cortisol

We report a method, based on isotope dilution--mass spectrometry, for determining cortisol in a pooled specimen of human serum. Isotopically labeled cortisol is added to 5.0 mL of serum so that the molar concentrations of labeled cortisol and unlabeled cortisol are approximately equal. The specimen and two calibration standards are extracted with dichloromethane, and the extracted cortisol is converted to the methoxime-trimethylsilyl ether derivative. Samples and standards are analyzed by gas chromatography--mass spectrometry by monitoring the peak areas for m/z 605 and 608. The cortisol concentration is calculated by linear interpolation between the two bracketing standards. Variances of data collected during six weeks showed that the overall coefficient of variation (CV) was 0.69% (n = 32); the within-vial CV, 0.63%; the among-vial CV, 0.22%; and the among-day CV, 0.15% (means = 3.973 nmol/vial). Method specificity was demonstrated by liquid chromatographic as well as C8 mini-column cleanup of samples before derivation, by alternative ion monitoring at m/z 636 and 639, and by negative-ion chemical ionization at m/z 459 and 462. Derivatives of all observed degradation products of cortisol under basic, neutral, and acidic conditions did not interfere.

Creatine kinase response surfaces explored by use of factorial experiments and simplex maximization

We conducted a five-component, five-level response-surface experiment to optimize the pH and the concentrations of magnesium, creatine phosphate, adenosine diphosphate, and buffer in an assay for creatine kinase. Under optimal conditions, creatine kinase activity was about 5% greater than that obtained with a previously reported assay (Clin Chem 23: 1569, 1977). We also applied a simplex maximization algorithm to the response-surface equation to locate areas of maximum sensitivity. Reaction conditions for two such areas were found, each yielding approximately 11% more activity than with the previously reported method.

Creatine kinase response surfaces explored by use of factorial experiments and simplex maximization

We conducted a five-component, five-level response-surface experiment to optimize the pH and the concentrations of magnesium, creatine phosphate, adenosine diphosphate, and buffer in an assay for creatine kinase. Under optimal conditions, creatine kinase activity was about 5% greater than that obtained with a previously reported assay (Clin Chem 23: 1569, 1977). We also applied a simplex maximization algorithm to the response-surface equation to locate areas of maximum sensitivity. Reaction conditions for two such areas were found, each yielding approximately 11% more activity than with the previously reported method.

The development and evaluation of a homogeneous immunoassay for the isoenzymes of aspartate aminotransferase

Purified isoenzymes of aspartate aminotransferase (AST) from human liver (mitochondrial) and erythrocytes (cytoplasmic) were used to elicit antisera in rabbits. Each antiserum was characterized for titer and specificity. Complexes formed upon addition of each isoenzyme to its specific antiserum were demonstrated to be catalytically inactive. Results obtained when either filtration or centrifugation was used for separating the complexes (heterogeneous assay) were comparable to those obtained when the complexes were not separated (homogeneous assay) from the mixture before assay. A quality control system was designed to monitor specificity in addition to the usual parameters. The precision for the inhibition of cytoplasmic (CV 4.8%) and mitochondrial (CV 3.5%) isoenzymes was within that of the enzymatic assay. Several parametric conditions of the enzyme-antibody reactions were examined, and the assay was adapted for semi-automation. The homogeneous assay was evaluated with a series of pseudo specimens containing known mixtures of pure isoenzymes to determine the extent of recovery (99.8%) for a particular isoenzyme in the presence of varying concentrations of the other isoenzyme. In addition, sera from patients having elevated AST concentrations were examined for isoenzyme contributions to total AST activity. A mean recovery of 96% was obtained for these specimens.

Multivariate and univariate optimization studies of liquid-chromatographic separation of steroid mixtures

We developed a high-performance liquid-chromatographic separation of five steroids (estriol, estradiol, cortisol, progesterone, and testosterone), eluting with a water-acetonitrile gradient from a reversed-phase (C18) column. By applying a simplex search algorithm to maximize a chromatographic-response function, we sought to optimize the original conditions of the chromatographic analysis, which did not separate two pairs of overlapping peaks. Our chromatographic-response function incorporated both peak separation and total time of analysis. Three factors were varied simultaneously to maximize this function: flow rate, column temperature, and gradient shape. From the simplex optimization, we selected a flow rate of 1.50 mL/min, a temperature of 52 degrees C, and a linear gradient for our analysis. Subsequent univariate studies of the initial mobile phase composition showed that acetonitrile/water (20/80 by vol) gave an adequate separation.