Component Description
Caffeine is an alkaloid that is known to have psychoactive
stimulatory effects. Caffeine naturally occurs in plants (e.g., coffee beans,
tea leaves, cocoa beans, cola nuts), and the dietary consumption of caffeine originates mainly from derivative
beverages (e.g., coffee, tea, cola drinks) and foods (e.g., chocolate).
Caffeine is also used as a food additive in beverages (e.g., caffeinated soft
drinks, “energy” drinks) and as a drug either on its own or as an adjuvant in
certain medications (e.g., analgesics).
Given caffeine’s high prevalence in the worldwide diet at
behaviorally active doses, significant scientific interest in the health
effects of caffeine has developed. As a psychoactive stimulant, the behavioral
effects of caffeine, such as its effect on mental alertness, have been studied
extensively, and topics such as caffeine tolerance, addiction, and withdrawal
have also been examined. Caffeine
consumption has been studied as a risk factor for many diseases and conditions,
including hypertension, bone density, cardiovascular diseases, various cancers,
reproduction and developmental abnormalities, and mental and behavioral
disorders.
Eligible Sample
Examined participants aged 6 years and older 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.
Refer to the Laboratory Method Files section for a
detailed description of the laboratory methods used.
This is the new component in the 2009-2010 survey cycle.
Laboratory Quality Assurance and Monitoring
Urine samples were processed, stored and shipped to the
Division of Laboratory Sciences, National Center for Environmental Health,
Centers for Disease Control and Prevention, Atlanta, GA for analysis.
Detailed
instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM).
Vials
were stored under appropriate frozen (-30oC)
conditions
until they were shipped to National Center
for Environmental Health for testing.
The NHANES quality assurance and quality control protocols
(QA/QC) meet the 1988 Clinical Laboratory Improvement Amendments mandates. Detailed
QA/QC instructions are discussed in the NHANES LPM.
Mobile Examination Centers (MECs)
Laboratory team performance is monitored using several
techniques. NCHS and contract consultants use a structured competency
assessment evaluation during visits to evaluate both the quality of the
laboratory work and the quality-control procedures. Each laboratory staff member
is observed for equipment operation, specimen collection and preparation;
testing procedures and constructive feedback are given to each staff member.
Formal retraining sessions are conducted annually to ensure that required skill
levels were maintained.
Analytical Laboratories
NHANES uses several methods to monitor the quality of the
analyses performed by the contract laboratories. In the MEC, these methods
include performing blind split samples collected during “dry run” sessions. In
addition, contract laboratories randomly perform repeat testing on 2% of all
specimens.
NCHS developed and distributed a quality control protocol
for all CDC and contract laboratories, which outlined the use of Westgard rules
(Westgard,
et al. 1981)
when running NHANES specimens. Progress reports containing any problems
encountered during shipping or receipt of specimens, summary statistics for
each control pool, QC graphs, instrument calibration, reagents, and any special
considerations are submitted to NCHS quarterly. The reports are reviewed for
trends or shifts in the data. The laboratories are required to explain any
identified areas of concern.
All
QC procedures recommended by the manufacturers were followed. Reported results
for all assays meet the Division of Laboratory Sciences’ quality control and
quality assurance performance criteria for accuracy and precision, similar to the
Westgard rules (Caudill, et al. 2008).
Data Processing and Editing
The data were reviewed. Incomplete data or improbable values were sent
to the performing laboratory for confirmation.
Analytic Notes
Refer to the 2009-2010 Laboratory Data Overview for general information on
NHANES laboratory data.
Please refer to the NHANES Analytic Guidelines and
the on-line NHANES Tutorial for details on the use of sample
weights and analytic issues.
Subsample Weights
Urinary caffeine and caffeine metabolites were measured
in a one-third subsample of participants 6 years and older. 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.
Demographic and Other Related Variables
The analysis of
NHANES laboratory data must be conducted with the key survey design and basic
demographic variables. The NHANES
2009-2010 Demographics file contains demographic data, health
indicators, and other related information collected during household interviews
as well as the 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.
This laboratory data file can be linked to the other NHANES
data files using the unique survey participant identifier (i.e., SEQN).
Detection Limits
The detection limits were constant for all of the analytes
in the data set. Two variables are
provided for each of these analytes. The variable name ending in “LC” (ex.,
URDMU1LC) indicates whether the result was below the limit of detection: the
value “0” means that the result was at or above the limit of detection, “1”
indicates that the result was below the limit of detection. The other variable
prefixed URX (ex., URXMU1) provides the analytic result for that analyte. For
analytes with analytic results below the lower limit of detection (ex.,
URDHU1LC), an imputed fill value was placed in the analyte result field. The
value is the lower limit of detection divided by the square root of 2
(LLOD/sqrt[2]).