The kidneys filter wastes from the blood; these wastes are produced as all our cells use nutrients and oxygen to generate the energy they need to function. “Kidney function” is a measurement of how well the kidneys are able to do this. Even if one kidney is removed, overall kidney function remains normal if there is no other disease to affect it.
Another term for kidney function is “renal function,” since “renes” is the Latin word for kidney. The specialist who deals with abnormal kidney function is a Nephrologist, because the “nephros” is the Greek word for kidney (we have a million nephrons in each kidney to filter blood). However, when dealing with kidney infections, kidney stones, or cancer, the Urologist is the specialist involved. Those diseases usually have nothing to do with “kidney function.”
To measure kidney function exactly, we’d need to collect all urine over a 24-houor period. That’s not convenient or routine. So we use an estimate called the “estimated glomerular filtration rate” (eGFR), based mainly on a blood test for creatinine. The higher the creatinine, the lower the eGFR, and the worse the kidney is functioning.
An average eGFR for healthy people is around 100 milliliters per minute (multiplied by standard measurement of body weight & height), but this decreases with age. Anything >60 is normal. An eGFR 45-59 = mild chronic kidney disease, 30-44 is moderate. Such patients rarely have symptoms. An eGFR <30 means severe kidney disease, and patients usually see a Nephrologist in preparation for dialysis. Dialysis is usually begun when the eGFR approaches 15. See our topic Kidney Failure).