The main indicator of kidney function is your blood level of creatinine, a waste product of the body produced by muscles and excreted by the kidneys. If kidney function is reduced, creatinine accumulates in the blood leading to an elevated level when a blood test is checked.
Kidney function is best measured by an indicator called GFR (Glomerular Filtration Rate) which measures the blood filtration rate by kidneys. This indicator allows doctors to determine if the kidney function is normal, and if not, to what level the reduced kidney function has deteriorated. In everyday practice, GFR can easily be estimated (eGFR), from measurement of the blood creatinine level, and taking into account, age, ethnicity and gender.