Serum creatinine and urea levels are elevated in patients with renal malfunction especially decreased glomerular filtration. In the early stages of kidney damage, the rise in the serum urea levels usually precedes the increase in serum creatinine. The advantage is offset by the fact that serum urea levels are affected by factors such as diet, degree of hydration and protein metabolism. Serum creatinine levels on the other hand tend to be constant and unaffected by factors affecting serum urea levels. Thus serum creatinine is a significantly more reliable renal function-screening test than serum urea. A considerably more sensitive test for measuring glomerular filtration is the creatinine clearance test.

The original Jaffé technique is still the most widely used today. After deproteinization creatinine in alkaline solution, forms a yellow-red complex with picrate.

The intensity of the color produced is directly proportional to creatinine concentration. It is determined by measuring the increase in absorbance at 500 – 550 nm.