NHANES III LABORATORY DATA FILE General Information Introduction The Laboratory Data File contains data from the urine collection and venipuncture components of the examination, including almost all laboratory test results (blood and urine) available to date. The exceptions are discussed elsewhere in this documentation. In addition, auxiliary information such as how long the examinee fasted, the time of day of the venipuncture, and the conditions precluding venipuncture has been included. This documentation presents information that should be reviewed before proceeding with data analysis. The documentation pertaining specifically to the Laboratory Data File is divided into four main sections. The first section, "General Information," provides information about the contents of the Laboratory Data File. The second section, "Data File Index," includes a brief description of all the variables on the data set and shows the standard name of each variable and its position in the data set. The third section, "Item Descriptions, Codes, Counts, and Notes" provides for each component a description, the standard variable name and a brief description of the values that variable can take on, a count of the frequency of occurrence of each value, notes by variable, and appendices as necessary. "References" are provided in the fourth section. Blood and urine specimens were collected on examinees aged one year and older at the mobile examination center (MEC). For those examinees aged one year and older who did not travel to the MEC, only blood specimens were collected during the Home Examination (HE). Hematologic profiles were completed for all examinees, and specified laboratory tests were performed upon each specimen based on the examinee's age and sex. Only a limited number of tests were performed on specimens collected during the Home Examination. Appendix 1 lists the laboratory tests by specimen type, age group, sex, and whether the specimen was collected in the Home Examination. The analysis of NHANES III laboratory data must be conducted with the key survey design and basic demographic variables. The NHANES III Household Youth Questionnaire Data File (ages two months to 16 years) and the NHANES III Household Adult Questionnaire Data File (ages 17 years and older) contain demographic data, health indicators, and other related information collected during household interviews. They also contain all survey design variables and sample weights for these age groups. These two household questionnaire files may be linked to the laboratory data file using the unique survey participant (sample person) identifier SEQN. Examinee Screening Prior to the phlebotomy (venipuncture), a questionnaire was administered to determine an examinee's eligibility for all phlebotomy procedures (including venipuncture and the oral glucose tolerance test). It included questions to determine if it was safe to perform the venipuncture, to document and determine fasting compliance, and to aid in analyzing the results of the laboratory tests performed. Examinees reporting hemophilia or recent cancer chemotherapy treatment were excluded from the venipuncture. For those examinees, the laboratory test results fields for all blood-based laboratory tests were left blank. Because examinees reporting current insulin therapy were excluded from receiving the oral glucose tolerance test (OGTT), the plasma glucose (G2P), serum insulin (I2P) and serum C-peptide (C2P) results from the second venipuncture were left blank as well. Although examinees aged 12 years and older were instructed to fast for 10-16 hours prior to the morning examination or for six hours before the afternoon or evening examination, the instructions were not followed uniformly. Laboratory test results and the duration of the fast have been included on the data file regardless of the examinee's fasting compliance. Analysts should consider whether fasting status is crucial before undertaking analyses. Examinees who reported insulin use during the household interview were not instructed to fast. Specimen Collection and Processing Procedures Detailed specimen collection and processing instructions are discussed in the Manual for Medical Technicians (U.S. DHHS, 1996). Vials were stored under appropriate refrigerated (4-8 degrees Centigrade) or frozen (-20 degrees Centigrade) conditions until they were shipped to analytical laboratories for testing. The analytical methods used by each of the participating laboratories are described in the Laboratory Procedures Used for NHANES III (U.S. DHHS, 1996). The manual contains quality control graphs and statistical summary information for each laboratory test at the end of the laboratory method description. Oral glucose tolerance testing: During NHANES III, the OGTT was conducted on MEC examinees aged 40-74 years. The protocol included two timed venipunctures and a glucose drink. Two glucose drinks were used to measure an examinee's ability to metabolize glucose -- Dextol(TM) and Trutol(TM). After the first venipuncture, the examinee drank the glucose drink, and a second venipuncture was performed approximately two hours later. Examiner Training and Quality Control The NHANES III laboratory staff consisted of medical technologists and phlebotomists. The medical technologists held baccalaureates in medical technology. Both they and the phlebotomists were certified by the American Society for Clinical Pathologists or by a similar organization. All laboratory staff completed comprehensive training in standardized laboratory procedures before they began working in the MEC. The MEC phlebotomists completed comprehensive training in pediatric phlebotomy techniques, including instruction by a pediatric nurse practitioner. Laboratory team performance was monitored using several techniques. NCHS and contract consultants used a structured quality assurance evaluation during unscheduled visits to evaluate both the quality of the laboratory work and the quality-control procedures. Each laboratory staff person was observed for equipment operation, specimen collection and preparation, and testing procedures, and constructive feedback was given to each team. Formal retraining sessions were conducted annually to ensure that required skill levels were maintained. Laboratory Protocol Changes from 1988 to 1994 Most laboratory tests were performed for the entire six years of NHANES III. Exceptions are detailed below. Apolipoprotein AI and B tests were included during 1988-1991 only. Lipoprotein(a), Vitamin B12, and antibody tests for immunoglobulin E, rubella, varicella, and toxoplasmosis were conducted during 1991-1994. For the 1991-1994 period, the OGTT procedure was modified to add tests for C-peptide and insulin on specimens from the second venipuncture. For statistical analyses of these laboratory test results, the appropriate Phase 1 or Phase 2 sample weight should be used. Incomplete Data Release At the time of this data release, some laboratory test results were not available. Tests for which results were unavailable included vitamin D, immunoglobulin E, diphtheria antitoxin, measles antibody, homocysteine, periodontal pathogens, thyroxine, thyroid stimulating hormone, antithyroglobulin antibody, antimicrosomal antibody, and methylmalonic acid. Cotinine test results for 1988-1991 have been included in this laboratory data file. Cotinine testing is still being carried out for 1991-1994, and the laboratory test results will be released at a future date. Results from urine pregnancy tests are included in the NHANES III Examination Data File, rather than in the Laboratory Data File. Serologic testing for human immunodeficiency virus (HIV) antibody and urine testing for drugs of abuse were performed anonymously. The drugs of abuse for which examinees were tested were cocaine, marijuana, opiates, phencyclidine, and amphetamines. To maintain anonymity, the examinee's serum and urine were labeled with a random identifying number, and limited demographic data were linked to that number. The new identifier was not linked to the original sample identifier. Therefore, these data cannot be linked to other NHANES III data. The HIV test was performed from 1988 through 1994; the urine drug testing was done from 1991 through 1994. Because of the limited analytic potential of the HIV and drug data, this file is not included in this data release. Data Preparation and Processing For laboratory tests with a lower detection limit, results below the lower detection limit were replaced with a value equal to the detection limit divided by the square root of two. This value was created to help the user distinguish a nondetectable laboratory test result from a measured laboratory test result. Appendix 2 documents the detection limit for each laboratory test. The SI unit (le Systeme International d Unites) is an outgrowth of the metric system that has been used throughout most of the world. In addition to providing a uniform international system of units of measurement, a uniform style is prescribed. Laboratory test results not originally reported in SI units were converted to SI units if applicable. Conversion factors, the format of the NHANES and SI results, and NHANES and SI units of measure are in Appendix 3. In converting NHANES III data to SI units, the goal was to preserve the level of detail reported by the laboratories in the original laboratory test result. Therefore, the number of significant digits in the laboratory test results data may be different from that in published references. The Laboratory Data File contains laboratory test results for glucose (G1P), triglycerides (TGP), cholesterol (TCP), and iron (FEP) measured by contract laboratories using reference analytic methods. For these methods, consult the Laboratory Procedures Used for NHANES III (U.S. DHHS, 1996). However, the biochemistry profile also included measurements of these analytes. In general, for most analyses, the appropriate variables to use are G1P, TGP, TCP and FEP. The values from the biochemistry profile (SGP, CHP, TRP, SFP) should not be used routinely. The definition of a reference method by the National Committee for Clinical Laboratory Standards (NCCLS) is "a thoroughly investigated method in which exact and clear descriptions of the necessary conditions and procedures are given for the accurate determination of one or more property values; the documented accuracy and precision of the method are commensurate with the method's use for assessing the accuracy of other methods for measuring the same property values or for assigning reference method values to reference materials" (NCCLS, 1991).