Kidney function is usually assessed from a blood test, not from how you feel — early kidney impairment typically causes no symptoms. The three numbers that appear most often are creatinine, eGFR and urea. Each is an indirect estimate rather than a direct measurement, and each can be moved by factors that have nothing to do with the kidneys. Knowing those limits prevents both false reassurance and unnecessary alarm.
One important principle runs through this whole topic: a single result matters far less than the trend. Clinicians generally look for changes confirmed on repeat testing over at least three months before describing kidney function as chronically reduced.
Creatinine: a useful but imperfect marker
Creatinine is a waste product of muscle metabolism, released at a fairly steady rate and removed almost entirely by the kidneys. When filtration falls, creatinine accumulates, so a rising creatinine suggests worsening filtration. A typical reference range is about 60-110 micromol/L (0.7-1.2 mg/dL) for men and somewhat lower for women, varying by laboratory.
The catch is production: muscular people make more creatinine, and people with low muscle mass — including many older adults — make less. A bodybuilder and a frail elderly person can share a creatinine value while having quite different filtration. Cooked meat eaten shortly before the test, creatine supplements, and some medicines (such as trimethoprim) can also raise creatinine without any change in the kidneys themselves.
eGFR: creatinine adjusted into an estimate
Estimated glomerular filtration rate (eGFR) converts creatinine into an estimated filtration rate using equations that adjust for age and sex. It is reported in mL/min/1.73 m², with values of 90 or above considered normal in the absence of other findings. Persistent eGFR below 60 for more than three months is one criterion for chronic kidney disease, and staging systems grade severity from there.
eGFR inherits creatinine’s limitations — it is an estimate, least precise at higher values, and skewed by extremes of muscle mass, amputation, severe illness and pregnancy. A related marker, cystatin C, is less dependent on muscle and is sometimes used to check an eGFR that does not fit the clinical picture.
Urea: the least specific of the three
Urea is produced when the body breaks down protein and is also cleared by the kidneys. It rises when filtration falls, but it also rises with high protein intake, dehydration, gastrointestinal bleeding and steroid treatment, and falls in liver disease and pregnancy. Because of this, urea alone is a poor kidney test; clinicians read it alongside creatinine, where the pattern between the two can hint at dehydration versus intrinsic kidney problems.
Why one low reading is not a diagnosis
Dehydration from a stomach bug, a new blood-pressure medicine, or a urinary infection can temporarily depress kidney numbers; this is acute kidney injury, and it often recovers. Conversely, urine testing matters as much as blood: albumin leaking into urine (measured as the albumin-to-creatinine ratio) is an early sign of kidney damage that can be present while eGFR is still normal. Modern kidney assessment uses both.
Practical takeaways
Ask for your trend, not just your latest number, and make sure repeat testing confirms any change before worrying. Tell your clinician about creatine supplements, high-protein diets and recent illness or dehydration when results are reviewed. If your eGFR is persistently reduced, the highest-value steps are blood-pressure control, diabetes management where relevant, and reviewing medicines that strain the kidneys — all decisions to make with your care team, not from a single lab line.
