Component Description
Trace metals have been associated with adverse health effects in occupational studies or laboratory studies, but have not been monitored in general population groups.
This method is used to achieve rapid and accurate quantifications of multiple elements of toxicological and nutritional interest. The method is sensitive and rapid enough to screen urine specimens from subjects suspected to be exposed to a number of important toxic elements, or to evaluate environmental or other nonoccupationally exposure to these same elements.
Eligible Sample
Participants aged 6 years and older, who met the subsample requirements, were eligible.
Description of Laboratory Methodology
Urine specimens are processed, stored, and shipped to the Division of Environmental Health Laboratory Sciences, National Center for Environmental Health, Centers for Disease Control and Prevention for analysis. Detailed specimen collection and processing instructions are discussed in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM). Read the General Documentation on Laboratory Data file for detailed data processing and editing protocols.
Caffeine and 14 of its metabolites are quantified in urine by use of high performance liquid chromatography-electrospray ionization-tandem quadrupole mass spectrometry (HPLC-ESI-MS/MS) with stable isotope labeled internal standards. A 50-µL aliquot of urine is first diluted with 450 µL of water. 100 µL of the diluted urine is then combined with 120 µL of a 0.2 N NaOH solution containing stable isotope labeled internal standards. The mixture is allowed to incubate for at least 30 min at room temperature, facilitating the conversion of an unstable uracil metabolite into a more stable form. Samples are then acidified 30 µL of 2.0 N HCl and 250 µL of a 1:9 methanol/water solution containing 0.1% formic acid such that the matrix of the sample is similar to the starting mobile phase composition of the analysis step. Samples are then filtered and analyzed by HPLC-ESI-MS/MS in both positive and negative ionization modes. Quantitation is based on peak area ratios interpolated against an 11-point calibration curve derived from calibrators in synthetic urine
This is the first two year cycle that caffeine and caffeine metabolites were measured. Detailed instructions on specimen collection and processing can be found in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM).
Data Processing and Editing
Read the General Documentation on Laboratory Data file for detailed data processing and editing protocols. The analytical methods are described in the Description of Laboratory Methodology section above.
Laboratory Quality Assurance and Monitoring
The NHANES quality control and quality assurance protocols (QA/QC) meet the 1988 Clinical Laboratory Improvement Amendments mandates. Detailed QA/QC instructions are discussed in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM). Read the General Documentation on Laboratory Data file for detailed QA/QC protocols.
Analytic Notes
NHANES Survey Design:
The analysis of NHANES laboratory data must be conducted with the key survey design and basic demographic variables. The Demographic file contains: Status Variables providing core information on the survey participant including examination status, Recoded Demographic Variables including age, gender, race etc., and Interview and Examination Sample Weight Variables and Survey Design Variables. The Questionnaire Data Files contain socio-economic data, health indicators, and other related information collected during household interviews. The Phlebotomy Examination file includes auxiliary information on duration of fasting, the time of day of the venipuncture, and the conditions precluding venipuncture. The Demographic, Questionnaire and Phlebotomy Examination files may be linked to the laboratory data file using the unique survey participant identifier SEQN.
Subsample Weights
Urinary caffeine and caffeine metabolites were measured in a one third subsample of persons 6 years and over. Special sample weights are required to analyze these data properly. Specific sample weights for this subsample are included in this data file and should be used when analyzing these data.
Variance Estimation
The analysis of NHANES laboratory data must be conducted with the key survey design and basic demographic variables. The NHANES Demographic Data File contains demographic and sample design variables. The recommended procedure for variance estimation requires use of stratum and PSU variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.
Links to NHANES Data Files
This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier SEQN.
Detection Limits
The detection limits were constant for all of the caffeine measurements in the data set.
The variable named LBD__LC indicates whether the result was below the limit of detection. There are two values: “0,” and “1.” “0” means that the result was at or above the limit of detection. “1” indicates that the result was below the limit of detection. In cases where the result was below the limit of detection, the value for that variable is the detection limit divided by the square root of two.
Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial for further details on the use of sample weights and other analytic issues.