Biological age: what the blood test actually shows

A biological age test takes your blood values, runs them through a model, and hands back a number. PhenoAge is often used, weighing markers for inflammation, metabolism, blood counts and organ function. The number can be compared with your chronological age. It is not your real age, and it is not a diagnosis.

It is easy to read the number as a grade. A lower figure feels like good news, a higher one like bad. That is not how the model works. It calculates using the markers that particular test chose. Two labs can therefore give two different answers for the same person, the same week, without anything having happened in the body. The difference sits in the model.

Some tests hand over only the score. Others hand over the raw values — the individual markers the calculation is built on. Without the raw values you cannot see whether the number is driven by one marker, such as inflammation or blood sugar, or by several small deviations. Nor can you compare the next test with the first in a way that means anything.

The retest is half the purchase. A single test is a snapshot. What you want to know is whether the number moves when sleep, strength and diet have been steady for a few months. So check what a second test costs before you order the first, and whether the same model is used again. A different lab next time makes the comparison useless.

Before you order, three questions. Which markers are included. Whether you get the raw values or only the score. What the retest costs, and whether anyone explains the result or you just get a PDF. Without those three answers, the number is hard to use.

An out-of-range value in the test is not something to interpret yourself. It is something to take to a doctor. The test does not replace a health check-up.