Creatinine is a breakdown product of muscle metabolism. The main route of creatinine excretion is through the kidneys, where it is filtered by the glomerulus and also secreted by the proximal tubule.
In a healthy kidney, there is little or no tubular reabsorption of creatinine. Creatinine is therefore a useful indicator of renal health because it is excreted in the urine as an unchanged and easily measured by-product of muscle metabolism. In a healthy kidney, little or no creatinine is reabsorbed, whereas in kidney disease, the creatinine concentration in the blood may increase. The creatinine concentration in the urine and blood can therefore be used to calculate the rate at which the kidney is clearing creatinine – the creatinine clearance (CrCl) rate. This CrCl rate is correlated with the glomerular filtration rate (GFR), which is important in the clinical assessment of renal function.
The ways in which creatinine is used to assess kidney health are described in more detail below.
- Serum creatinine – This is a measurement of the blood creatinine level, which provides a simple way of checking kidney function. However, the serum creatinine concentration only increases in cases of severe nephron dysfunction, meaning the test is not suitable for detecting early-stage kidney disease. When applied to women, the reference range for serum creatinine is 0.5 to 1.0 mg/dl (45-90 μmol/l), while for men the range is 0.7 to 1.2 mg/dl (60-110 μmol/l).
- A more accurate way of assessing kidney function is to calculate the estimated glomerular filtration rate (eGFR). The creatinine levels in the blood and urine can be used to calculate the rate at which creatinine is being cleared by the kidney. This is referred to as the CrCl rate. In turn, the CrCl can be used to calculate the eGFR.
- An eGFR can also be calculated using the serum creatinine level alone, using the following variables:
GFR is a useful indicator of kidney function, although calculations using the CrCl rate will give an overestimation of the GFR in cases of severe renal dysfunction because the proximal tubule secretes excess amounts of creatinine, thereby increasing the overall amount of total creatinine cleared. Drugs that can be used to minimize this excess secretion and therefore the accuracy of the eGFR include cimetidine and trimethoprim.
An alternative to using CrCl rate and eGFR to indicate renal function is to interpret the plasma concentration of creatinine along with the blood urea level. A test called the BUN (blood urea nitrogen)-to-creatinine ratio is also used as a measure of kidney health, with BUN rising the more kidney function decreases.