National Health and Nutrition Examination Survey

An association between Human Leukocyte Histocompatibility Antigen B-27 (HLA-B27) and major arthritis syndromes such as Ankylosing Spondylitis and Reactive Arthritis is well documented (Khan et al., 2007). HLA B-27 has also has been shown to be to be associated with disorders of the heart valves and cardiac conduction system, with ocular disorders such as Iritis, and also with disorders of the immune system (Lautermann and Braun, 2002; Mathieu et al., 2009). Some studies indicate an increase in mortality among persons who have HLA-B27 associated disorders, however there are no mortality studies that relate directly to the HLA-B27 trait itself (Zochling and Braun, 2008).

HLA-B27 has been extensively studied clinically but there is limited population-level data available. A U.S. national-level HLA-B27 prevalence estimate is required for public health planning purposes because of the recent significant improvements in the medical therapy for HLA-B27 related disorders. The B27_F_R data release provides data for the estimation of the U.S. national prevalence of HLA-B27 based on samples collected during the 2009 U.S. National Health and Nutrition Examination Survey (NHANES) survey year. Also in NHANES 2009-10, questionnaire data related to inflammatory back pain and Spondyloarthritis (ARQ_F) and physical examination data for selected arthritis-related body measures (ARX_F) were collected.

Adult participants ages 20 to 69 years. There were no specific exclusions other than the standard exclusions for NHANES phlebotomy.

Whole blood sampes were obtained from NHANES 2009 survey participants. HLA-B27 testing was performed on these samples using a polymerase chain reaction (PCR) to specifically replicate the DNA sequences encoding the HLA loci of interest. Exons 2 and 3 of the HLA-B locus were amplified with locus specific primers and the amplified DNA was arrayed onto seven replicate nylon membranes and immobilized by UV cross-linking. The cross-linked sample DNA was hybridized with sequence-specific oligonucleotide probes to identify HLA-B27 allele sequences. A consensus probe was used as a positive control for PCR amplification. Positive probe hits were detected by imaging chromogenic dots. Duplicate interpretations of positive probe hits were uploaded into a laboratory information system which interpreted the hit pattern to determine if the HLA-B27 was positive or negative. No allelic or HLA-B27 subtype analysis was performed and none is reported. Please see the HLA-B27 component Laboratory Manual for more detailed documention of laboratory testing methods and quality control procedures.

Please read the General Documentation of Laboratory Data file for detailed data processing and editing protocols. The HLA-B27 trait is reported as a catagorical variable (present or absent). No data editing was performed on the dataset.

All HLA-B27 positive specimens and 2.5% of HLA-B27 negative samples were confirmed using a second, independent DNA preparation in order to identify any sample discrepancies. A negative control was included in each amplification run as a quality control check for contamination of the reactions by specimens or amplified product DNA. The oligonucleotide positive control dots were required to be positive for each probe and to be clearly distinguishable from all negative controls. The dots that were expected to be negative for a given probe were required not to show any signal above background.

The NHANES 2009 B27_F_RDC dataset is a special use data file for participants 20-69 years is available only through the National Center for Health Statistics (NCHS) Research Data Center (RDC).

The analysis of NHANES laboratory data must be conducted using the key survey design variables and basic demographic variables. The NHANES Household Questionnaire Data Files contain demographic data, health indicators, and other related information collected during the NHANES household interviews. The phlebotomy file includes auxiliary information such as the conditions precluding venipuncture. The household questionnaire and phlebotomy files may be linked to the laboratory data file using the unique survey participant identifier SEQN.

The HLA-B27 data was collected only for a single year (2009) rather than a typical NHANES 2-year data collection period. Therefore,

• The single year examination weight, WTSFM, should be used in estimating the prevalence of and in testing statistical hypotheses concerning HLA B27.
• The delete 1 Jackknife method rather than Taylor Series Linearization is recommended for variance estimation(Wolters, 2007).
• The delete 1 Jackknife method is a replication method and therefore a design based method for variance estimation.
• Fifteen delete 1 jackknife replicate weights, WTSFM01-WTSF15, are provided for variance estimation using the delete 1 Jackknife method.
• To calculate the nominal degrees of freedom of the estimated variance, unmasked variance units (which differ from the Masked Variance Unit codes provided for the two year public-use files) are provided. These unmasked variance units are needed to determine the critical value for the Student’s t statistic used to construct confidence intervals.

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. Both of these are available on the NHANES web site.