Lab Notes

What does a telomere test measure?

by Patricia Shelton, MD

The telomere test is a blood test.  While most of your blood cells don’t contain DNA, your white blood cells, or leukocytes, do contain DNA.  These are the cells that are used to measure your telomere length1.  The measurement you receive is called your leukocyte telomere length, or LTL.  The LTL is an average of the length of the telomeres in the cells from your blood sample; it’s been shown in studies to be highly correlated with the length of telomeres measured in other tissues of the body2.


Along with your actual leukocyte telomere length, you’ll usually receive some charts of average telomere lengths for various ages using that test, and either your “biological age” or your “percentile” (or both of those).  This allows you to compare your telomeres to other people of the same age, to get an idea of what would be expected in a person of average health at your age. 


You may get a report of your “percentile.”  This is the percentage of people your age who have telomeres the length of yours or shorter, so a higher percentile means your telomeres are longer relative to others of your age.  For instance, if your telomeres are in the 70th percentile, then you have telomeres as long as or longer than 70% of people your age, while 30% have longer telomeres than you.


The age at which your own telomeres match the average for the population is your “biological age.”  If your biological age is 49, then you have the telomere length of an average 49-year-old.  Your biological age will often be reported to you with the results of your telomere test, and may be significantly different from your chronological age (your actual age measured by the calendar).


Telomeres that are longer than expected for your age (i.e., a younger biological age, or a high percentile) generally indicate that you’re doing a great job supporting your body’s ability to heal and repair damage; telomeres that are shorter than expected (i.e., an older biological age, or a low percentile) may tell you that something in your lifestyle is significantly damaging your body.  Thus, a telomere test indirectly measures the healthfulness of your lifestyle, although there are also genetic influences on telomere length.  Some examples of factors that could cause shorter telomeres are smoking3, stress4, obesity5, lack of sleep6, and inflammation7.


Leukocytes differ from other cells of the body in that they contain telomerase, an enzyme that makes telomeres longer8.  This means that if you make significant changes in your lifestyle between one telomere test and the next, your telomeres may not just shorten more slowly; they may actually be longer on the second test.  Don’t assume this is a lab error; it might be your body getting healthier!





1Montpetit AJ, Alhareeri AA, Montpetit M, et al. Telomere length: a review of methods for measurement. Nurs Res. 2014 Jul-Aug;63(4):289-99.


2Starkweather 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.


3Chen X, Velez JC, Barbosa C, et al. Smoking and perceived stress in relation to short salivary telomere length among caregivers of children with disabilities. Stress. 2014 Sep 26:1-38.


4Theall KP, Brett ZH, Shirtcliff EA, et al. Neighborhood disorder and telomeres: connecting children’s exposure to community level stress and cellular response. Soc Sci Med. 2013 May;85:50-8.


5Valdes AM, Andrew T, Gardner JP, et al. Obesity, cigarette smoking, and telomere length in women. Lancet. 2005 Aug 20-26;366(9486):662-4.


6Liang G, Schernhammer E, Qi L, et al. Associations between rotating night shifts, sleep duration, and telomere length in women. PLoS One. 2011;6(8):e23462.


7Masi S, Nightingale CM, Day IN, et al. Inflammation and not cardiovascular risk factors is associated with short leukocyte telomere length in 13- to 16-year-old adolescents. Arterioscler Thromb Vasc Biol. 2012 Aug;32(8):2029-34.



8Lanca V, Zee RY, Rivera A, et al. Quantitative telomerase activity in circulating human leukocytes: utility of real-time telomeric repeats amplification protocol (RQ-TRAP) in a clinical/epidemiological setting. Clin Chem Lab Med. 2009;47(7):870-3.