Sunday, December 26, 2021

Age-related trends in health markers may indicate survival advantages: The case of platelet counts

Platelets () are particles that circulate in the blood of mammals. They react to blood vessel injuries by forming clots. Platelet counts are provided in standard blood panels, and are used by medical doctors to diagnose possible health problems. At the time of this writing, the refence range for platelet counts is 150,000 to 450,000 per cubic millimeter.

The figure below has two graphs, and is based on an article by Balduini and Noris, published in 2014 in the prestigious journal Haematologica (). The graph on the left shows the distribution of platelet counts by age and sex. The one on the right shows the reference ranges for platelet counts by age and sex.

The reference ranges within the bars include all individuals in the same age-sex group, whereas the ones outside the bars are based on groups of individuals in specific areas (i.e., geographic regions) where the age-sex reference ranges are wider. A reference range is essentially an interval, derived from statistical analyses, in which one would expect individuals who are disease-free to fall into.

A clear pattern that emerges from these graphs is that platelet counts go down with age, in both men and women. A tendency to form clots is generally associated with health problems at more advanced ages, even though blood clotting is necessary for good health. Given this, one could interpret the graphs as indicating that older individuals have lower blood clot counts because those lower counts are associated with a survival advantage.

This interpretation is not guaranteed to be correct, of course. Nevertheless, one of the key conclusions by the authors of the study seems quite correct: “[…] using 150–400×10^9/L as the normal range for platelet count, a number of old people of some areas could be at risk of receiving a wrong diagnosis of thrombocytopenia, while young inhabitants of other areas could be at risk of an undue diagnosis of thrombocytosis.”

If you get a platelet count in a standard blood panel that is out of the reference range, your doctor may tell you that this could be an indication of a frightening underlying health problem. If this happens, you may want to discuss this blog post, and the article on which it is based, with your doctor. There are follow-up tests that can rule out serious underlying problems. Having said that, a low count may in fact be a good sign.

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