• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Laboratory Quality Assurance and Monitoring
• Data Processing and Editing
• Analytic Notes
• References
• Codebook
  • SEQN - Respondent sequence number
  • WTSSCTJ2 - Surplus specimen SSCH_J_R 2 year weights
  • SSCTIG - Anti-C. trachomatis pan-IgG detection
  • SSCTIGO - Anti-CT pan-IgG optical density
  • SSCTIG1 - Anti-C. trachomatis IgG1 detection
  • SSCTIG1O - Anti-CT IgG1 optical density
  • SSCTIG3 - Anti-C. trachomatis IgG3 detection
  • SSCTIG3O - Anti-CT IgG3 optical density

National Health and Nutrition Examination Survey

2017-2018 Data Documentation, Codebook, and Frequencies

Chlamydia Serology - Serum (Surplus) (SSCH_J_R)

RDC Only Data File: SSCH_J_R.xpt

First Published: August 2025

Last Revised: NA

At this time, this dataset is not available publicly. Access may be obtained through the NCHS Research Data Center (https://www.cdc.gov/rdc).

Component Description

Chlamydia serologic assays can be used to identify whether someone has been infected with Chlamydia trachomatis (CT, the bacteria that causes chlamydia). IgG (pan-IgG, reactive against IgG1, IgG2, IgG3, and IgG4), IgG1, and IgG3 comprise the predominant targets in serologic assays used to estimate CT seroprevalence (Geisler, et. al., 2012; Waters, et. al., 2024). There are several CT serologic assays (Waters, et. al., 2024); some have been used to estimate prior infection with chlamydia in nationally representative data (Petersen, et. al., 2021; Anyalechi, et. al., 2021). However, few assays can distinguish a recent infection from past infection.  

For participants with available serum specimens, three mixed-peptide ELISAs (enzyme linked immunosorbent assays) were performed to measure serum antibodies to CT. Specimens with a positive test for either IgG, IgG1, or IgG3, indicate an antibody response to a previous infection with or exposure to CT. It is hypothesized that specimens with a positive test for IgG3 indicate a recent prior infection with or recent exposure to CT (in the past 1-6 months) (Geisler, et. al., 2012).

Eligible Sample

All examined partcipants aged 14-59 years were eligible.

Description of Laboratory Methodology

The Hybiske Laboratory at the University of Washington in Seattle, Washington, USA performed the mixed peptide ELISAs on serum specimens received from the National Center for Health Statistics.

Prior to this study, positive and negative cutoffs for IgG, IgG1, and IgG3 for the mixed peptide ELISAs were established by testing a receiver operating characteristic (ROC) curve on 48 serum samples with a known history of CT and preexisting CT serologic data. The mixed peptide ELISAs were repeated five to six times per serum sample to obtain several optical density (OD) 450 nm values. The mean of these OD values for each serum sample was used in the ROC curve where the sum of sensitivity and specificity was maximized. The ‘cutpointr’ package (Thiele, et. al., 2021) in R/RStudio (Posit Team, 2023; R Core Team, 2022) was used to calculate the ROC curve. Background subtracted OD values greater than or equal to 0.0646, 0.0222, and 0.0108 for IgG, IgG1, and IgG3, respectively, are considered positive and OD values below these cut points are considered negative.

Assays were run according to standardized, published protocols for the CT mixed peptide ELISA assays (Rahman, et. al., 2018a; Rahman, et. al., 2018b). All results were quality checked and summarized in the accompanying data file.

Laboratory Quality Assurance and Monitoring

Serum specimens were received and accessioned by the Hybiske Laboratory at the University of Washington. After being accessioned, the specimens were stored in a secure −80°C freezer until assaying.

Prior to assaying the specimens, the CT mixed peptide ELISAs were verified in-house using previously-tested specimens for which CT infection and serology were established. After validation, the NHANES 2017-2018 serum specimens were analyzed by the CT mixed peptide ELISAs. ELISA serologies were performed in singlet.

ELISA absorbance values were normalized by Hybiske lab staff, entered into a spreadsheet database, and quality checked by other staff. All data entries were reviewed each week as part of quality control.

For samples that had abnormally high absorbance readings for control wells (non-CT peptides), samples were rerun and a manual quality check was performed. The value of the second reading was used for final reporting when results were discordant. Finalized individual absorbance results were tabulated and variables denoting serostatus (seropositive vs. seronegative) for each mixed peptide ELISA were created; both individual absorbance results and serostatus data were provided to NCHS.

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 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. Additionally, availability of specimens for surplus projects is lower than for other laboratory tests performed on NHANES participants. The specimen availability can also vary by age or other population characteristics. 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.

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.

Subsample Weights

The analytes included in this dataset were measured in examined participants aged 14–59 years and older. Special sample weights are required to analyze these data properly. Specific sample weights for this subsample, WTSSCTJ2, are included in this data file and should be used when analyzing these data. The sample weights created for this file used the examination sample weight, i.e., WTMEC2YR, as the base weight. The base weight was adjusted for additional nonresponse to these lab tests and re-poststratified to the population total using sex, age, and race/Hispanic origin. Participants who were part of the eligible population but who did not provide a serum specimen, or did not have sufficient volume of biospecimens, or who did not give consent for their specimens to be used for future research are included in the file, but they have a sample weight assigned “0” in their records.

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.

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

References

• Anyalechi G.E., Hong J., Danavall D.C., et al. High plasmid gene protein 3 (Pgp3) Chlamydia trachomatis seropositivity, pelvic inflammatory disease, and infertility among women, National Health and Nutrition Examination Survey, United States, 2013-2016. Clin Infect Dis. (2021) 73(8):1507-16.
• Geisler W.M., Morrison S.G., Doemland M.L., et al. Immunoglobulin-specific responses to Chlamydia elementary bodies in individuals with and at risk for genital chlamydial infection. J Infect Dis. (2012) 206:1836-43.
• Petersen M.R., Patel E.U., Grabowski M.K., Gaydos C.A., Quinn T.C, Tobian A.A.R. Seroprevalence of Chlamydia trachomatis among female adults in the United States: The National Health and Nutrition Examination Surveys. Clin Infect Dis. (2021) 73(3):e629-37.
• Posit team. RStudio: Integrated Development Environment for R. Published online 2023. Accessed January 13, 2024. http://www.posit.co/
• R Core Team. R: A language and environment for statistical computing. R Foundation for Statistical Computing. (2022) https://www.R-project.org/
• Rahman K.S., Darville T., Russell A.N., et al. Comprehensive molecular serology of human Chlamydia trachomatis infections by peptide enzyme-linked immunosorbent assays. mSphere. (2018a) 3.
• Rahman K.S., Darville T., Wiesenfeld H.C., et al. Mixed Chlamydia trachomatis peptide antigens provide a specific and sensitive single-well colorimetric enzyme-linked immunosorbent assay for detection of human anti-C. trachomatis antibodies. mSphere (2018b) 3.
• Thiele C., Hirschfeld G. cutpointr: improved estimation and validation of optimal cutpoints in R. J Stat Softw. (2021) 98(11):1-27.
• Waters M.B., Hybiske K., Ikeda R., et al. Chlamydia trachomatis seroassays used in epidemiologic research: a narrative review and practical considerations. J Infect Dis. (2024) 230(1):250-62.

Codebook and Frequencies