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Testing · 6 min readWhat a blood test can and cannot tell you
An honest account of where the numbers help, and where they mislead.
What it is genuinely good at
A blood panel is very good at answering specific, bounded questions. Is your ferritin low? Is your vitamin D where it should be in February? Is your thyroid doing its job? Are your lipids where you would want them at your age? These are real questions with real answers, and guessing at them wastes months.
It is also good at settling arguments with yourself. A woman who is convinced she is deficient in everything, and a woman convinced she is fine, are both better served by a number than by a feeling.
of a healthy reference population
In someone perfectly healthy
Same markers, same terms
Where it misleads
Reference ranges are the middle 95 per cent of a reference population, not a definition of health. Sitting just inside a range is not proof of nothing wrong, and sitting just outside is not proof of something. With twenty markers on a panel, statistics alone will push one or two outside the range in a perfectly healthy person.
Single measurements also mislead. Many markers move with time of day, hydration, recent training and recent illness. A ferritin drawn two days after a hard block reads high because inflammation lifts it. This is why context matters more than the flag in the margin.
A reference range is conventionally set to include the central 95 per cent of a healthy reference population. On a twenty-marker panel, the probability that at least one result falls outside range by chance alone is about 64 per cent. Context, not the flag, decides whether a result means anything.
How to use one well
Test for a reason, not for reassurance. Read the result alongside how you actually feel and train. Change one or two things deliberately. Re-test the same markers at twelve weeks, on the same terms — same lab, similar time of day, not straight after a hard week.
The re-test is the part that turns a number into knowledge, and it is the part almost nobody does. A single panel tells you where you are. Two tell you whether what you changed worked.
- 01
Test to answer a question, not for reassurance
Why - 02
Read the number next to how you actually feel
How - 03
Re-test the same markers on the same terms
The part that matters
General nutrition education, not medical advice or diagnosis. Results outside a reference range should be discussed with a qualified clinician, and nothing here replaces that conversation. Full health disclaimer →
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