90 mg/mL at 20°C[2] Creatinine (/kriˈætɪnɪn, -ˌniːn/; from Ancient Greek κρέας (kréas) 'flesh') is a breakdown product of creatine phosphate from muscle and protein metabolism.

[7] Creatinine is removed from the blood chiefly by the kidneys, primarily by glomerular filtration, but also by proximal tubular secretion.

[8] Ketoacids, cimetidine, and trimethoprim reduce creatinine tubular secretion and therefore increase the accuracy of the GFR estimate, in particular in severe kidney dysfunction.

[9][10] An alternative estimation of kidney function can be made when interpreting the blood plasma concentration of creatinine along with that of urea.

Dehydration secondary to an inflammatory process with fever may cause a false increase in creatinine concentrations not related to actual kidney impairment, as in some cases associated with cholecystitis.

[6] A rise in blood creatinine concentration is a late marker, observed only with marked damage to functioning nephrons.

eGFR can be calculated without a 24-hour urine collection, using serum creatinine concentration and some or all of the following variables: sex, age, and weight, as suggested by the American Diabetes Association.

Algorithms to estimate GFR from creatinine concentration and other parameters are discussed in the renal function article.

Unfortunately, the MDRD Study equation was developed in people with chronic kidney disease, and its major limitations are imprecision and systematic underestimation of measured GFR (bias) at higher/normal values.

The IDMS method would result in comparative overestimation of the corresponding calculated GFR in some patients with normal renal function.

To counter the effect of changing to IDMS, new FDA guidelines have suggested limiting doses of carboplatin, a chemotherapy drug, to specified maxima.

Serum creatinine concentrations may increase when an ACE inhibitor (ACEI) is taken for heart failure and chronic kidney disease.

[25] In chemical terms, creatinine is a lactam and an imidazolidinone, a spontaneously formed cyclic derivative of creatine.