National Health and Nutrition Examination Survey

NHANES at a Glance

NHANES at a Glance
National Health and Nutrition Examination Survey
Data Type National self-report and examination survey
Sample Nationally representative sample
VEHSS Topics Included Visual Function
Vision Exam Measures
Eye Health Conditions
Service Utilization
Examination Measures
Years Analyzed 1999–2008
Approximate Size 5,000 people per year

The National Health and Nutrition Examination Survey (NHANES) is a program of studies designed to assess the health and nutritional status of adults and children in the United States. The survey is unique in that it combines interviews and physical examinations. NHANES is a major program of the National Center for Health Statistics (NCHS). NCHS is part of the Centers for Disease Control and Prevention (CDC) and has the responsibility for producing vital and health statistics for the nation. In 1999, the survey became a continuous program that has a changing focus on a variety of health and nutrition measurements to meet emerging needs.

Sample Design

The survey examines a nationally representative sample of about 5,000 people per round, with 2-year data sets available for analysis. These people are located in counties across the country, 15 of which are visited each year. The sample for the survey is selected to represent the US population of all ages. To produce reliable statistics, NHANES over-samples people aged 60 years and older, African Americans, Hispanics, and low-income non-Hispanic White and Other Race adults. In 1999–2006, NHANES oversampled Mexican-Americans, African Americans, low-income White and Other Race people, adolescents aged 12-19, and non-Hispanic White and Other Race adults aged 70 and over. A supplemental sample included pregnant women. In 2007–2008, NHANES oversampled Hispanics, non-Hispanic African Americans, low-income non-Hispanic White and Other Race adults, and non-Hispanic White and Other Race adults aged 80 and over. NHANES generally top-codes age at 85 years, but top-coded at age 80 years in the 2007-2008 data. We therefore top-code all NHANES data at age 80.

Data Collection Procedures

The NHANES interview includes demographic, socioeconomic, dietary, and health-related questions. The examination component consists of medical, dental, and physiological measurements, as well as laboratory tests administered by highly trained medical personnel.

All participants visit the physician. Dietary interviews and body measurements are included for everyone. All but the very young have a blood sample taken and have a dental screening. Depending on the age of the participant, the rest of the examination includes tests to assess various aspects of health. In general, the older the individual, the more extensive the examination.

Health interviews are conducted in respondents’ homes. Health measurements are performed in specially-designed and equipped mobile centers, which travel to locations throughout the country. The study team consists of a physician, medical and health technicians, and dietary and health interviewers. Many of the study staff are bilingual (English/Spanish).

NHANES included visual examinations from 1999–2008.  From 2005–2008 NHANES included ophthalmology examinations, including retinal imaging and Frequency Doubling Technology (FDT) tests for visual field loss from eye diseases.

Analysis Overview

For the VEHSS team, NORC estimated the prevalence rate and sample size for each survey instrument selected for inclusion. We merged samples from the 1999–2008 and 2005–2008 rounds for analysis to maximize the available sample sizes for more detailed levels of stratification.

For binary response questions included in the analysis, prevalence rate was defined as the number of people who gave an affirmative response to the question divided by the total number of respondents who gave an affirmative or negative response. The result was then multiplied by 100 to obtain a percentage. For scaled responses (e.g., mild, moderate, severe), the data value was the proportion of respondents that selected one of the possible response options, and all responses should sum to 100%. We estimated upper and lower confidence intervals and the relative standard error (RSE) of the prevalence estimate. Results in this report calculate confidence intervals using the Clopper-Pearson method.[1] Initially, the cross-tabulated data in the VEHSS data visualization application calculates confidence intervals using standard Wald methods. We plan to change to the Clopper-Pearson method in 2019. Estimates that were based on a sample size less than 30 and/or with a relative standard error greater than 30% were suppressed.

A detailed description of the analytical steps is described in the report “VEHSS Survey Analysis Plan Cdc-pdf[PDF – 480 KB]External.” Full analysis documentation is included in the “VEHSS NHANES Data Report Cdc-pdf[PDF – 753 KB]External.”

Variables Analyzed in VEHSS

NHANES contains several vision-related questions and examinations relevant to VEHSS. Table 1 includes the NHANES self-report questions analyzed in VEHSS, while Table 2 includes NHANES examination measures calculated by VEHSS.

Table 1. Overview of NHANES Self-Report Variables Included in VEHSS

Overview of NHANES Self-Report Variables Included in VEHSS
VEHSS Indicator Topic VEHSS Indicator Category NHANES Variable Name Years Available
(Analyzed)
Question Response Options
Visual Function Blind or Difficulty Seeing VIQ017 2005–2008
(2005–2008)
Are you/Is survey participant blind in both eyes?
  • 1 Yes
  • 2 No
Service Utilization Cataract Surgery VIQ071 1999–2008
(2005–2008)
Have you/Has survey participant ever had a cataract operation?
  • 1 Yes
  • 2 No
Eye Health Conditions Self-report glaucoma VIQ090 2005-2008 (2005–2008) Have you/Has survey participant ever been told by an eye doctor that {you have/s/he has} glaucoma (gla-co-ma), sometimes called high pressure in {your/his/her} eyes?
  • 1 Yes
  • 2 No
Eye Health Conditions Self-report age-related macular degeneration VIQ310 2005–2008 (2005–2008) Have you/Has survey participant ever been told by an eye doctor that {you have/s/he has} age-related macular (mac-u-lar) degeneration?
  • 1 Yes
  • 2 No
Eye Health Conditions Self-report diabetic retinopathy DIQ080 2005–2008 (2005–2008) Has a doctor ever told you/survey participant that diabetes has affected {your/his/her} eyes or that {you/s/he} had retinopathy (ret-in-op-ath-ee)?
  • 1 Yes
  • 2 No

Table 2. Overview of NHANES Examination Variables Included in VEHSS

Overview of NHANES Examination Variables Included in VEHSS
VEHSS Indicator Topic VEHSS Indicator Category NHANES Variables Used Years Available
(Analyzed)
Eye Health Conditions Exam-based glaucoma OPASCST2 – Exam status;
OPXDGLAU – Glaucoma, right eye;
OPXSGLAU – Glaucoma, left eye
2005–2008
(2005–2008)
Eye Health Conditions Exam-based age related macular degeneration OPDUARMA – Any retinopathy, worse eye 2005–2008
(2005–2008)
Eye Health Conditions Exam-based diabetic retinopathy DIQ010 – Doctor told you have diabetes;
LBXGH – Glycohemoglobin (%);
OPDURET – Retinopathy level, worse eye
2005–2008
(2005–2008)
Visual Acuity Measures Presenting Visual Acuity VIDRVA – Right visual acuity, presenting;
VIDLVA – Left visual acuity, presenting
1999–2008
(1999–2008)
Visual Acuity Measures Best-corrected Visual Acuity VIDROVA – Right visual acuity, w/ obj. refraction;
VIDLOVA – Left visual acuity, w/ obj. refraction
1999–2008
(1999–2008)
Visual Acuity Measures Uncorrected Refractive Error Presenting visual acuity ≤20/50
Best-corrected visual acuity <20/40 with refraction
1999–2008
(1999–2008)
Limitations

This analysis is limited in a number of ways. NHANES does not include state-level data, and NHANES stopped collecting vision and eye health data in 2008. Ophthalmologic exams were only conducted in two two-year cycles, from 2005–2008. NHANES includes approximately 5,000 respondents per year, which means that the available samples are smaller than other surveys included in VEHSS. Additionally, the NHANES sample excludes group quarters including nursing facilities, which may contain a disproportionate share of persons with vision problems. The advantage of NHANES is the presence of examination data, but the exams were limited and did not employ gold-standard dilated examinations.

Additional information about NHANES can be found on the CDC website.

[1] Parker JD, Talih M, Malec DJ, et al. National Center for Health Statistics data presentation standards for proportions. Vital Health Stat. 2017;2(175).