C-reactive protein (CRP) is an acute phase protein synthesized in the liver. It is involved in the activation of complement, enhancement of phagocytosis, and detoxification of substances released from damaged tissue. It is one of the most sensitive, though nonspecific, indicators of inflammation. CRP levels may rise within six hours of an inflammatory stimulus. Measurement of CRP concentrations by this highly sensitive method is performed primarily to ascertain the level of cardiovascular disease risk in individuals who have no existing inflammatory conditions. Increases in CRP concentration are non-specific and should be used in conjunction with traditional clinical laboratory evaluation of acute coronary syndromes.
Examined participants aged 1 year and older were eligible.
This is a two-reagent, immunoturbidimetric system. The specimen is first combined with a Tris buffer, then incubated. The second reagent (latex particles coated with mouse anti-human CRP antibodies) is then added. In the presence of circulating CRP the latex particles aggregate, forming immune complexes. These complexes cause an increase in light scattering that is proportional to the CRP concentration. The light absorbance resulting from this light scatter is read against a stored CRP standard curve. The concentration of CRP is determined from this line. Turbidity is measured at a primary wavelength of 546 nm (secondary wavelength 800 nm).
Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.
There were changes to the lab method, lab equipment and lab site for this component in the NHANES 2017-2018 cycle.
HS-CRP Laboratory Procedure Manual (February 2020)
Serum specimens are processed, stored, and shipped to the University of Minnesota – Advanced Research Diagnostics Laboratory (ARDL), Minneapolis, MN for analysis.
Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM). Vials were stored under appropriate frozen (–30°C) conditions until they were shipped to Collaborative Laboratory Services for testing.
The NHANES quality control and quality assurance protocols (QA/QC) meet the 1988 Clinical Laboratory Improvement Act 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.
The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.
Refer to the 2017-2018 Laboratory Data Overview for general information on NHANES laboratory data.
There are over 800 laboratory tests performed on NHANES participants. However, not all participants provided biospecimens or enough volume for all the tests to be performed. The specimen availability can also vary by age or other population characteristics. For example, in 2017-2018, approximately 80% of children aged 1-17 years who were examined in the MEC provided a blood specimen through phlebotomy, while 95% of examined adults age 18 and older provided a blood specimen. Analysts should evaluate the extent of missing data in the dataset related to the outcome of interest as well as any predictor variables used in the analyses to determine whether additional re-weighting for item non-response is necessary.
Demographic and Other Related Variables
The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2017-2018 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.
The Fasting Questionnaire File includes auxiliary information, such as fasting status, length of fast and the time of venipuncture.
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 limit was constant for the analyte in the data set. Two variables are provided for this analyte. The variable name ending in “LC” (ex., LBDHRPLC) 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. For the analyte with analytic results below the lower limit of detection (LBDHRPLC=1), an imputed fill value was placed in the analyte results field. This value is the lower limit of detection divided by square root of 2 (LLOD/sqrt [2]). The variable prefixed LBX (ex., LBXHSCRP) provides the analytic result for that analyte.
The lower limit of detection (LLOD, in mg/L) for High-Sensitivity C-Reactive Protein:
|
Variable Name |
Analyte Description |
LLOD |
|
LBXHSCRP |
High Sensitivity C-Reactive Protein (mg/L) |
0.15 |
High Sensitivity C-Reactive Protein (HSCRP) regression equations to compare 2017-2018 and 2015-2016 data:
Method validation (bridging) studies were performed to compare results from a laboratory, instrument, and method change between the 2015-2016 and 2017-2018 survey cycles using NHANES samples from late 2016. During the 2015-2016 cycle the Beckman Coulter UniCel DxC 800 Synchron was upgraded to the Beckman Coulter UniCel DxC 660i Synchron Access chemistry analyzer (DxC 660i). Previous analyses indicated that no statistical adjustment is needed for results obtained between the two Beckman UniCel® analyzers (i.e., DxC 800 and DxC 660i). During the 2017-2018 cycle the Roche Cobas 6000 chemistry analyzer (Cobas 6000) was used for the entire 2017-2018 cycle. Analyses comparing results obtained from the Cobas 6000 to those obtained from the DxC 660i using selected serum samples (n=207) from NHANES participants in 2016 indicate that adjustments are needed for some analytes as described below. Statistical analyses were performed using Analyse-it, v4.30.4.
On average, HSCRP values measured with the Cobas 6000 (2017-2018 instrument) were 19.6% lower than values measured with the DxC 660i (2015-2016 instrument) (n=207, p<.0001) and the correlation coefficient (r) between the measurements was 0.997.
Data from the bridging study showed mixed difference in variability between measurement procedures. For samples with HSCRP levels equal to or less than 20 mg/L based on Cobas 6000 (or 23 mg/L based on DxC 660i) proportional differences were observed and a weighted Deming regression was chosen to describe the relation between the two instruments The equations are given below:
Forward (applicable to DxC 660i values ≤ 23 mg/L) : Y (Cobas 6000) = 0.8695 (95%CI: 0.8419 to 0.8971) * X (DxC 660i) + 0.2954 (95%CI: 0.2786 to 0.3121)
Backward (applicable to Cobas 6000 values ≤ 20 mg/L): Y (DxC 660i) = 1.150 (95%CI: 1.114 to 1.186) * X (Cobas 6000) – 0.3397 (95%CI: -0.3663 to -0.3130)
For HSCRP levels above 20 mg/L based on Cobas 6000 (or 23 mg/L based on DxC 660i), the relationship between the two instruments appeared opposite that of the lower values, with 2017-2018 measurements higher than 2015-2016 measurements. However, with limited specimens within this range (n=7), we do not have enough statistical power to recommend an adjustment for these results.
The regression equations derived from the bridging study are recommended when examining trends of high sensitivity CRP data across 2015-2016 and 2017-2018 cycles. For data points above the specified range (i.e., Cobas 6000 value > 20 mg/L or DxC 660i value > 23 mg/L), unadjusted values should be used with caution. For analysis involving HSCRP data from the NHANES 2015-2016 cycle, please refer to the documentation accompanying the 2015-2016 HSCRP data (HSCRP_I) for additional details.
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0.11 to 182.82 | Range of Values | 7250 | 7250 | |
| . | Missing | 1116 | 8366 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0 | At or above detection limit | 7175 | 7175 | |
| 1 | Below lower detection limit | 75 | 7250 | |
| . | Missing | 1116 | 8366 |