Can a blood test show how old your organs are?

Three types of blood tests now provide a reasonable estimate of biological ageing, but none of them directly measures the age of individual organs. They work at the population level: they say something about how you are doing on average compared with people your age.

The DunedinPoAm test measures chemical changes to DNA in white blood cells and estimates how quickly a person is ageing biologically. In the underlying study, people with a higher rate of ageing had more chronic diseases, scored worse on balance, strength and walking speed, and had a greater chance of dying early. They also looked older to trained assessors. The test was developed on one specific birth cohort and is a research instrument, not yet a clinical test for individual use.

Blood chemistry scores such as PhenoAge and similar methods calculate biological age from routine blood values. In the CALERIE study, caloric restriction measurably slowed biological ageing as captured by these scores, and in several studies they predicted healthy lifespan even slightly better than DNA methylation tests. Here too, the result reflects a population average, not a specific organ.

For organ-specific risks, protein profiles in the blood are more promising. A combination of 11 proteins predicted significantly better than existing clinical models which patients with diabetes would go on to develop chronic kidney disease. That is more concrete than a general 'biological age', but is limited to one organ and one condition.

Blood magnesium is not suitable as an ageing marker: it poorly reflects the amount of magnesium inside cells and is strongly dependent on kidney function. Researchers conclude that it is not a useful biomarker of ageing.