Creatinine measures your muscle as much as your kidneys
Creatinine is a waste product of muscle, cleared by the kidneys. The amount in your blood therefore depends on two things: how well the kidneys are clearing it, and how much muscle is producing it.
That second half explains results that otherwise look contradictory. A heavily built young man and a small elderly woman with identical kidney function will not have the same creatinine, and the same person’s creatinine shifts after a period of intense training or after losing muscle.
It is also why the reference ranges differ by sex, and why a creatinine at the top of its range in someone with little muscle mass deserves more attention than the same figure in someone with a lot.
eGFR is a calculation, not a measurement
The estimated glomerular filtration rate printed on your report was not measured. It is computed from your creatinine together with your age and sex, using an equation, and it carries the uncertainty of that equation.
Reports usually indicate which equation was used, and the figure is standardised to a body surface area rather than being specific to your body. The practical consequence is that eGFR is excellent for tracking a trend and poor for reading a single decimal.
Two values from different laboratories, or from different equations, are not directly comparable. A step in your curve when you change laboratory belongs to the method, not to your kidneys.
One reading is never chronic kidney disease
A single reduced eGFR is a finding, not a diagnosis. Dehydration, a recent illness, intense exercise and several common medicines, including anti-inflammatories, all lower it temporarily.
What turns a finding into a diagnosis is persistence, which is why a reduced value is normally repeated after an interval rather than acted on immediately, and why the urine matters as much as the blood: albumin in the urine adds information that no blood line carries.
This is also where reference ranges mislead most. A range leaves a share of healthy people outside it on any given test, and eGFR declines slowly through adult life, so an eGFR of 85 in a seventy-year-old is a different object from the same figure at twenty-five.
Urea, and the rest of the panel
Urea, or BUN, rises with dehydration, with a high-protein diet and with bleeding in the digestive tract, not only with reduced kidney function. It is read alongside creatinine rather than on its own, and the relationship between the two carries information neither carries alone.
Electrolytes on the same panel describe the balance the kidneys maintain: sodium, potassium, chloride and bicarbonate. They move for many reasons, including several common medicines, and they are read as a group.