What sex changes, and why

Haemoglobin and haematocrit are the clearest case: the ranges for men sit higher than those for women, and a haemoglobin that is unremarkable in a man can be flagged low in a woman. Ferritin follows the same pattern, for reasons connected to menstruation over decades of adult life.

Creatinine differs too, but for a different reason: it comes from muscle, and average muscle mass differs. That is why the range moves with sex rather than with anything about the kidneys.

The practical consequence is that a range is never just a range. It is a range for a group, and a report prints the one for the group you were assigned.

What age shifts, and how slowly

Children are not small adults for laboratory purposes. Almost every line has its own paediatric ranges, changing through infancy and adolescence, which is why a child’s report should never be read against an adult table.

In adults, the shifts are slower and fewer. The estimated glomerular filtration rate declines gradually through adult life, so the same figure means different things at twenty-five and at seventy. Some hormone ranges move with age, and several lines drift in ways that are better followed against your own history than against a population band.

Pregnancy has a separate set of ranges entirely, trimester by trimester, for haemoglobin, thyroid function and several others. A pregnancy report read against non-pregnant ranges will look alarming and will be wrong.

What tables like this cannot tell you

A range published anywhere, including here, is a reading aid. The one that applies to your value is printed on the same page as the value, because it depends on the analyser, the reagent lot, the calibration and the population that laboratory serves.

Ranges also leave healthy people outside them by design: a small share of any healthy group falls outside on any given test. A review in family medicine notes that the number of results outside the normal range far exceeds the number that are clinically meaningful.

So a table answers the question of what is usual. It does not answer the question of what is happening to you.

The comparison that beats every table

Your own previous result. A value that has sat just outside its range for three years, unchanged, is a different object from the same value arriving for the first time, and no population band can make that distinction for you.

If you keep your reports, keep them in order. The trend is the part of your file that nobody else has.