National Health and Nutrition Examination Survey

Between 2009 - 2010, the hepatic steatosis (fatty liver) was assessed by re-reviewing the archived gall bladder ultrasound video images originally obtained in NHANES III between 1988 and 1994. The examination made it possible to grade the presence of fat within the hepatic parenchyma. The NHANES III Second Exam Sample was a sub-study of NHANES III, conducted for research purposes. These research files are intended to provide additional data for use with special statistical methods to improve estimates from the main survey data and for methodologic investigations. Be sure to read the documentation of the primary exam carefully before attempting any analysis.

No statistical sampling design was applied for the second exam. However, a nonrandom sample of about five percent was obtained by selecting approximately 20 participants from the roughly 400 sample persons examined at each survey location. The following general guidelines were used by the NHANES-Mobile examination center staff to select participants for the second exam: 1) select mainly adults, 2) half between the ages of 20-39 years, and half over 40 years; 3) select about half men and half women. The Hepatic/Gallbladder Ultrasound Examination Second/Replicate Examination sample consists of 1,121 adults 20-74 years of age, with ultrasounds performed on two separate occasions. Fasting was preferred but not required.

The second exams were scheduled after the first or primary exams, when possible at the same time of day as the first exam. The second exams were conducted over the same time period as the primary exams for a particular survey location by the same MEC staff, although priority was given to scheduling and completing primary exams. The second exams were administered following the same protocols as for the primary exam.

Ultrasound examinations were originally recorded using a Toshiba Sonolayer SSA-90A and Toshiba video recorder. The examinations were performed using standardized procedures (Gallbladder Ultrasonography Procedure Manual, Third National Health and Examination Survey, https://wwwn.cdc.gov/nchs/data/nhanes3/manuals/gallblad.pdf), which had been developed to ensure that each examination was performed in a consistent manner and that the results of each examination were accurate and reliable.

For the hepatic steatosis assessments, the video tapes of the NHANES III ultrasound examinations were digitized using a DVD-VHS Video cassette Recorder (SONY RDR-VX560) onto a Recordable DVD (Memorex, +R or +RW). DVD video-images were reviewed using a Dell Flat Panel Monitor (2408WFP, active matrix, thin-film transistor, liquid crystal display, 24-inch (609.6 mm) viewable area display, 1920 x 1200 resolution).

Three ultrasound readers were trained and approved by a board certified radiologist who is specialized in hepatic imaging.

All ultrasound assessments for hepatic steatosis were conduct by readers who had no access to any other participant data and who were not aware if the study was a second exam, nor were aware of the results of the first exam.

For practical reasons, NHANES III Phase 2 ultrasounds were reviewed before Phase 1.

For more detailed information regarding the training and data collection data users are encouraged to review the Hepatic Steatosis Procedures Manual, NCHS 2010. https://wwwn.cdc.gov/nchs/data/nhanes3/manuals/hepatic_steatosis_ultrasound_procedures_manual.pdf

A radiologist, with 21 years of experience in the interpretation of ultrasound images, trained, observed, and approved the readers. Routine monitoring of the reliability (both inter- and intra-reader) was conducted every three months, throughout the study to detect if there was any need for re-training.

A more detailed description of the Training, Quality Assurance and Quality Control is included in the Procedure Manual, Hepatic Steatosis, NCHS, 2010. https://wwwn.cdc.gov/nchs/data/nhanes3/manuals/hepatic_steatosis_ultrasound_procedures_manual.pdf

The primary finding variable (GURHSPF), is a 4 level variable. A primary finding recode (GURHSPFR) variable was created from this variable. This is a two category variable: hepatic steatosis yes or no. The recode value of "Yes" indicates that moderate or severe hepatic steatosis was present. A recode value of "No" indicates that the liver was normal or had mild hepatic steatosis.

Due to the research nature of these data, special caution should be used in
analysis. All analyses should include thorough investigation of the potential selection bias of this small non-random sub-sample. Careful attention to identifying and evaluating differences in important characteristics (e.g., age and race-ethnicity) between the subsample and the main sample should be considered along with other issues.

The second exam data can be linked to the primary exam data and the household interview data using the unique identifier (SEQN). This is necessary to obtain the demographic data for the sample. NCHS recommends that the survey design variables (e.g., sample weights) not be linked with the second exam data, since the survey design variables were created for the full sample. There are no sample weights or other design variables specifically created for the second exam sample.

Because the second exams were identical to the primary exams, the file structure for the second exams is the same as for the primary exam files. The variable nomenclature is the same with the following important distinction: "R" was added to the primary exam variables names in the third positition.

Users are encouraged to use the primary finding recode (GURHSPFR) variable for all the analyses. This recode will help to avoid potential overlap between mild and moderate hepatic steatosis. The data file provides all the different parameters that were obtained for the assessment of hepatic steatosis: liver to kidney contrast, parenchymal brightness, deep beam attenuation, vessel walls definition, and gallbladder wall definition and the overall confidence in the assessment. These parameters should allow more experienced users to explore alternative criteria and to conduct sensitivity analyses.

GURHSQC (Hepatic Steatosis assessment status comment) is a quality control variable that allowed NCHS to monitor the status of the assessment and the reasons readers provided why an exam was not analyzed or missing. The variable is provided in the data release file to allow analysts to have some information on missing data. Image is present, but ungradable may indicate any of the following: VHS tape was damaged, the ultrasound has a very poor quality that impairs the assessment of hepatic steatosis (e.g. presence of gas, patient examined in a wheelchair). No image reflects the fact that there was no ultrasound available for the participant.

In addition, for each of the above mentioned parameters (liver to kidney contrast, parenchymal brightness, deep beam attenuation, vessel walls definition, and gallbladder wall definition), in cases where the ultrasound technique and coverage did not include the parameter, then we marked "cannot assess" (coded as 9).