Measurement of telomere length is done on white blood cells, or leukocytes. These cells are used because they’re the only blood cells that contain DNA. Isolation of the DNA from these cells allows the measurement of leukocyte telomere length, or LTL. Studies have shown that LTL is correlated with telomere length in other body tissues1, making the measurement of LTL a reliable method of determining overall average telomere length.
In order to measure LTL, the leukocytes are first collected, usually from the blood. This may involve venipuncture (collection of blood from a vein through a needle) or a fingertip prick, from which blood is collected in a small tube or a blood spot collected on special paper2. In some tests, a cheek swab is used3.
Once the blood is collected, the lab isolates the white blood cells, extracts their DNA, and processes it. Because not every telomere is identical, the lab reports the average length of the telomeres in the sample. There are a variety of techniques to determine this measurement, and each one produces slightly different results4,5,6. This means that if you’ve had a telomere test done more than once, you should only try to directly compare the telomere length results to each other if they were done in the same laboratory using the same technique. (To find out what technique was used in your test, check the paperwork from your lab; you might see the acronyms TRF, qPCR, or FISH.) The methods that use larger volumes of tissue are generally more accurate than those that use smaller volumes1; however, there are advantages and disadvantages to each method.
The results may be reported back to you in a number of ways. Your actual LTL will be reported, but without a context, it would be hard to interpret. For context, you’ll usually be given information the average lengths of telomeres for people of different ages, using the same technique as your test.
You can compare your own telomere length to the telomere lengths of others to find out whether your telomeres are shortening at an average rate, or faster or slower than average. You may receive a report of your “percentile,” which indicates the number of people in the population who have telomeres that are the same as or shorter than yours. For instance, if you’re in the 75th percentile, then 75% of people have telomeres as long as or shorter than yours, while 25% of people have longer telomeres than you.
There may also be a reporting of your “biological age.” This is the age at which your telomeres match the average for people of that age. In other words, if your telomere length is the same as the telomere length of an average 48-year-old, then your biological age is 48. Your biological age may or may not match your actual chronological age. This can give you a sense of whether your lifestyle is supporting your health or damaging it, though there are also genetic influences on telomeres.
1Starkweather AR, Alhaeeri AA, Montpetit A, et al. An integrative review of factors associated with telomere length and implications for biobehavioral research. Nurs Res. 2014 Jan-Feb;63(1):36-50.
2Zanet DL, Saberi S, Oliveira L, et al. Blood and dried blood spot telomere length measurement by qPCR: assay considerations. PLoS One. 2013;8(2):e57787.
3Canela A, Vera E, Klatt P, et al. High-throughput telomere length quantification by FISH and its application to human population studies. Proc Natl Acad Sci U S A. 2007 Mar 27;104(13):5300-5.
4Montpetit AJ, Alhareeri AA, Montpetit M, et al. Telomere length: a review of methods for measurement. Nurs Res. 2014 Jul-Aug;63(4):289-99.
5Gohring J, Fulcher N, Jacak J, et al. TeloTool: a new tool for telomere length measurement from terminal restriction fragment analysis with improved probe intensity correction. Nucleic Acids Res. 2014 Feb;42(3):e21.
6Aubert G, Hills M, Lansdorp PM. Telomere length measurement – caveats and a critical assessment of the available technologies and tools. Mutat Res. 2012 Feb 1;730(1-2):59-67.