National Health and Nutrition Examination Survey

Blood samples are obtained from participants in the NHANES surveys. Most of the findings from the analyses of the blood samples are available immediately such as the data on cholesterol and triglyceride levels in the population. A small portion of the blood samples is stored to conduct later analyses on DNA samples. DNA provides important information on genetic or hereditary patterns of disease or risk of disease. This information leads to advances in the prevention and treatment of disease.

Telomeres are a protective nucleoprotein structure at each chromosome end. Telomeres naturally shorten with age. The telomere length assay was performed in the laboratory of Dr. Elizabeth Blackburn at the University of California, San Francisco, using the quantitative polymerase chain reaction (PCR) method to measure telomere length relative to standard reference DNA (T/S ratio), as described in detail elsewhere (Needham et al, 2013; Cawthon, 2002). Each sample was assayed 3 times on 3 different days. The samples were assayed on duplicate wells, resulting in 6 data points. Sample plates were assayed in groups of 3 plates, and no 2 plates were grouped together more than once. Each assay plate contained 96 control wells with 8 control DNA samples. Assay runs with 8 or more invalid control wells were excluded from further analysis (< 1% of runs). Control DNA values were used to normalize between-run variability. Runs with more than 4 control DNA values falling outside 2.5 standard deviations from the mean for all assay runs were excluded from further analysis (< 6% of runs). For each sample, any potential outliers were identified and excluded from the calculations (< 2% of samples). The mean and standard deviation of the T/S ratio were then calculated normally. The interassay coefficient of variation was 6.5%.

All participants aged 20 years and older examined in 1999-2000 or in 2001-2002 who had blood collected for DNA purification were eligible.

Five 96-well quality control plates which represent 5% of the complete set were provided. These duplicate samples are blinded to the investigators.

If more than 5% of the duplicate samples on the quality control plates are discordant with their pair in the complete set, the variant fails quality control (i.e., >95% duplicate concordance required).

N/A

TeloMean: Mean T/S ratio which is measured telomere length relative to standard reference DNA.

For researchers who wish to convert T/S ratio to base pairs (bp), the formula is (3,274 + 2,413 * (T/S)). The conversion from T/S ratio to bp is calculated based on comparison of telomeric restriction fragment (TRF) length from Southern blot analysis and T/S ratios using DNA samples from the human diploid fibroblast cell line IMR90 at different population doublings. It is important to note that there is wide variance in telomere length measures across labs and types of assays. While they tend to be highly correlated, they often have different means. Base pair estimates are only comparable for T/S ratio data produced with the same reference standard and the same lab procedures. In this case, results from NHANES are comparable to other studies performed in the Blackburn Lab and may be similar to other PCR-derived measures of telomere length if they used the same methods as written here. While comparisons across studies of telomere length in base pairs are commonly done, it is not highly accurate.

TeloSTD: Asso. Std. Dev. For Mean T/S Ratio refers to the standard deviation corresponding to the mean of the 3 T/S ratio values obtained for each sample.

MEC exam weights are appropriate for this subsample.