• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Laboratory Method Files
• Laboratory Quality Assurance and Monitoring
• Data Processing and Editing
• Analytic Notes
• References
• Codebook
  • SEQN - Respondent Sequence Number
  • WTSC2YR - Two-year C subsample weights
  • URXMU1 - 1-methyluric acid (umol/L)
  • URDMU1LC - 1-methyluric acid comment code
  • URXMU2 - 3-methyluric acid (umol/L)
  • URDMU2LC - 3-methyluric acid comment code
  • URXMU3 - 7-methyluric acid (umol/L)
  • URDMU3LC - 7-methyluric acid comment code
  • URXMU4 - 1,3-dimethyluric acid (umol/L)
  • URDMU4LC - 1,3-dimethyluric acid comment code
  • URXMU5 - 1,7-dimethyluric acid (umol/L)
  • URDMU5LC - 1,7-dimethyluric acid comment code
  • URXMU6 - 3,7-dimethyluric acid (umol/L)
  • URDMU6LC - 3,7-dimethyluric acid comment code
  • URXMU7 - 1,3,7-trimethyluric acid (umol/L)
  • URDMU7LC - 1,3,7-trimethyluric acid comment code
  • URXMX1 - 1-methylxanthine (umol/L)
  • URDMX1LC - 1-methylxanthine comment code
  • URXMX2 - 3-methylxanthine (umol/L)
  • URDMX2LC - 3-methylxanthine comment code
  • URXMX3 - 7-methylxanthine (umol/L)
  • URDMX3LC - 7-methylxanthine comment code
  • URXMX4 - 1,3-dimethylxanthine(theophylline)umol/L
  • URDMX4LC - 1,3-dimethylxanthine comment code
  • URXMX5 - 1,7-dimethylxanthine(paraxanthine)umol/L
  • URDMX5LC - 1,7-dimethylxanthine comment code
  • URXMX6 - 3,7-dimethylxanthine(theobromine)umol/L
  • URDMX6LC - 3,7-dimethylxanthine comment code
  • URXMX7 - 1,3,7-trimethylxanthine(caffeine)umol/L
  • URDMX7LC - 1,3,7-trimethylxanthine comment code
  • URXAMU - 5-actylamino-6-amno-3-methyluracil(uM/L)
  • URDAMULC - 5-actylamno-6-amno-3-methylucl cmnt code

National Health and Nutrition Examination Survey

2011-2012 Data Documentation, Codebook, and Frequencies

Caffeine & Caffeine Metabolites - Urine (CAFE_G)

Data File: CAFE_G.xpt

First Published: January 2020

Last Revised: NA

Note: See Analytic Note describing the change in method in the 2011-2012 survey cycle.

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 the high prevalence of behaviorally active caffeine doses in the worldwide diet, 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 from a one-third sample were eligible.

Description of Laboratory Methodology

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. Then, 100 µL of the diluted urine is combined with 120 µL of a 0.2 N NaOH solution containing stable isotope labeled internal standards. The mixture is incubated 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 with 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 filtered and analyzed by use of 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.

There were no changes to the lab method, lab equipment, or lab site for this component during the NHANES 2011-2012 cycle.

Laboratory Method Files

Caffeine and Caffeine Metabolites (January 2020)

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 the appropriate frozen (-30^oC) conditions until they were shipped to the National Center for Environmental Health for testing. 

The NHANES quality assurance and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Amendments mandates. Detailed QA/QC instructions are discussed in the NHANES LPM.

Mobile Examination Center (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 2011-2012 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 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.   

Demographic and Other Related Variables

The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables.  The NHANES 2011-2012 Demographic File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample design variables.  The recommended procedures for variance estimation requires use of stratum and PSU variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.

This laboratory data can be linked to the other NHANES data files using the unique survey participant identifier (i.e., SEQN).

Starting in the 2011-2012 NHANES cycle, the variable URXUCR (urine creatinine) will not be reported with this file. URXUCR can be found in the data file titled “Albumin & Creatinine – Urine”.

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 = 1), 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]).

The lower limit of detection (LLOD in umol/L) for caffeine and caffeine metabolites: 

VARIABLE	SAS LABEL	LLOD
URXMU1	1-methyluric acid (umol/L)	0.05
URXMU2	3-methyluric acid (umol/L)	0.1
URXMU3	7-methyluric acid (umol/L)	0.04
URXMU4	1,3-dimethyluric acid (umol/L)	0.02
URXMU5	1,7-dimethyluric acid (umol/L)	0.02
URXMU6	3,7-dimethyluric acid (umol/L)	0.03
URXMU7	1,3,7-trimethyluric acid (umol/L)	0.005
URXMX1	1-methylxanthine (umol/L)	0.03
URXMX2	3-methylxanthine (umol/L)	0.04
URXMX3	7-methylxanthine (umol/L)	0.02
URXMX4	1,3-dimethylxanthine (theophylline) (umol/L)	0.01
URXMX5	1,7-dimethylxanthine (paraxanthine) (umol/L)	0.006
URXMX6	3,7-dimethylxanthine (theobromine) (umol/L)	0.004
URXMX7	1,3,7-trimethylxanthine (caffeine) (umol/L)	0.003
URXAMU	5-acetylamino-6-amino-3-methyluracil	0.1

Caffeine and caffeine metabolite regression equations recommended when comparing or combining 2011-2012 and 2009-2010 data:

A method validation (bridging) study was performed to assess possible differences resulting from an instrument change in the 2011-2012 cycle. Urine caffeine and caffeine metabolites were measured by use of HPLC-ESI-MS/MS for both NHANES 2009-2010 and 2011-2012 cycles, but with different instruments for each cycle. In both NHANES cycles, urine caffeine and caffeine metabolites were analyzed using both polarity modes to ensure best possible measurement sensitivity, i.e., some compounds were measured as positive ions and others as negative ions. For NHANES 2009–2010, the instrument used was not suitable for analyzing both positive and negative ions simultaneously so each urine specimen was analyzed twice: once to obtain data for positive ion compounds, and a second time to obtain data for negative ion compounds. For NHANES 2011–2012, the instrument that was used had improved polarity-switching capabilities and could be used to measure both positive and negative ions in a single analysis.

A convenience sample (n=180) of surplus urine specimens from NHANES 2009–2010 participants were measured using the 2011-2012 instrument in 2014. These data were then compared with their original 2009-2010 results measured during August 2011 through October 2012. Data from the bridging study indicated that the correlation coefficient (r) between the measurements for all caffeine analytes ranged between 0.990 to 0.998. To account for observed proportional differences in variability, weighted Deming regressions were chosen to compare the urine caffeine results (umol/L). The list of analytes and the results from the bridging study for each analyte, including the number of valid specimens (n), median of the differences (% Diff, in %) and correlation coefficients (r) between the two measurements, as well as the forward and backward adjustment equations are provided in the table below. Analyses were performed using Analyse-it, v4.30.4.

The method validation study tested specimens from the same participants on both instruments; however, this testing was not performed concurrently.  Although the analysis laboratory has a comprehensive set of policies and procedures in place to ensure the quality and consistency of specimen storage, handling, and analysis over time, the non-concurrent nature of the bridging study may have been affected by unknown or unidentified temporal factors.

These regression equations are recommended when examining trends of urine caffeine data across the 2009-2010 and 2011-2012 cycles or combining data from these two cycles for analysis.

Analyte	n1	Forward Equation [CI]	Backward Equation [CI]	% Diff2	r3
URXMU14	171	N/A	N/A	-1.40	0.995
URXMU2	137	Y(11-12) = -0.013 [-0.036, 0.010] + 1.119 [1.067, 1.170] * X(09-10)	Y(09-10) = 0.012 [-0.009, 0.032] + 0.894 [0.853, 0.935] * X(11-12)	10.3	0.990
URXMU3	165	Y(11-12) = 0.003 [-0.033, 0.040] + 1.023 [1.008, 1.038] * X(09-10)	Y(09-10) = -0.003 [-0.039, 0.033] + 0.978 [0.964, 0.992] * X(11-12)	1.58	0.997
URXMU4	163	Y(11-12) = -0.012 [-0.027, 0.003] + 0.980 [0.968, 0.993] * X(09-10)	Y(09-10) = 0.012 [-0.003, 0.027] + 1.020 [1.007, 1.033] * X(11-12)	-3.15	0.996
URXMU5	167	Y(11-12) = -0.002 [-0.020, 0.016] + 1.014 [1.003, 1.026] * X(09-10)	Y(09-10) = 0.002 [-0.015, 0.019] + 0.986 [0.975, 0.997] * X(11-12)	1.49	0.998
URXMU64	158	N/A	N/A	2.53	0.996
URXMU7	153	Y(11-12) = -0.002 [-0.011, 0.007] + 1.033 [1.017, 1.048] * X(09-10)	Y(09-10) = 0.002 [-0.006, 0.010] + 0.968 [0.954, 0.983] * X(11-12)	2.96	0.998
URXMX1	170	Y(11-12) = -0.025 [-0.072, 0.021] + 1.021 [1.009, 1.034] * X(09-10)	Y(09-10) = 0.025 [-0.020, 0.070] + 0.979 [0.967, 0.991] * X(11-12)	1.49	0.997
URXMX2	165	Y(11-12) = -0.005 [-0.029, 0.019] + 1.134 [1.115, 1.153] * X(09-10)	Y(09-10) = 0.005 [-0.016, 0.025] + 0.882 [0.867, 0.896] * X(11-12)	12.9	0.995
URXMX3	169	Y(11-12) = 0.203 [-0.073, 0.479] + 1.183 [1.155, 1.210] * X(09-10)	Y(09-10) = -0.172 [-0.411, 0.068] + 0.846 [0.826, 0.865] * X(11-12)	18.4	0.997
URXMX4	158	Y(11-12) = 0.005 [-0.005, 0.016] + 1.036 [1.017, 1.054] * X(09-10)	Y(09-10) = -0.005 [-0.015, 0.004] + 0.965 [0.948, 0.983] * X(11-12)	3.41	0.995
URXMX5	165	Y(11-12) = -0.006 [-0.022, 0.010] + 1.029 [1.014, 1.044] * X(09-10)	Y(09-10) = 0.006 [-0.009, 0.022] + 0.972 [0.958, 0.986] * X(11-12)	3.06	0.996
URXMX6	168	Y(11-12) = -0.026 [-0.043, -0.008] + 1.019 [1.009, 1.029] * X(09-10)	Y(09-10) = 0.025 [0.008, 0.043] + 0.981 [0.971, 0.991] * X(11-12)	1.20	0.998
URXMX7	148	Y(11-12) = 0.026 [-0.002, 0.053] + 1.045 [1.028, 1.062] * X(09-10)	Y(09-10) = -0.025 [-0.051, 0.002] + 0.957 [0.941, 0.973] * X(11-12)	5.58	0.998
URXAMU	165	Y(11-12) = 2.116 [1.714, 2.517] + 0.976 [0.954, 0.997] * X(09-10)	Y(09-10) = -2.168 [-2.618, -1.718] + 1.025 [1.002, 1.047] * X(11-12)	3.45	0.998

¹ Records with values < LOD or missing were not included in the analysis

² The median of the percent differences (calculated as [(New-Old)/Old] * 100) was presented to indicate the crude difference between the two measurements.

³ Pearson’s correlation coefficient (r) between the measurements.

⁴ The weighted Deming regression equations did not show a significant slope or intercept (i.e., 95% confidence interval [CI] for slope included 1 and for intercept included 0), therefore, it was determined that the results for URXMU1 and URXMU6 did not need to be adjusted.

 

References

• Caudill SP, Schleicher RL, Pirkle JL. 2008. Multi-rule quality control for the age-related eye disease study. Stat Med 27:4094-4106.

• Westgard J.O., Barry P.L., Hunt M.R., Groth T. A multi-rule Shewhart chart for quality control in clinical chemistry. Clin Chem (1981) 27:493-501.

Codebook and Frequencies