Creatinine is essentially a metabolite of creatinine phosphate, a compound that acts as a source of energy in muscle. Men tend to have higher creatinine levels than women due to their greater skeletal mass. A high dietary intake of creatinine due to a meat-rich diet, for example, can also increase the creatinine level.
When applied to women, the reference range for serum creatinine is 0.5 to 1.0 mg/dL (about 45-90 μmol/L), while for men the range is 0.7 to 1.2 mg/dL (60-110 μmol/L). However, several variables are taken into consideration when interpreting the serum creatinine level and these include gender, age, weight and ethnicity. For example, a serum creatinine level of 2.0 mg/dL (150 μmol/L) in a male body builder with a high muscle mass would not necessarily indicate kidney disease, whereas a level of 1.2 mg/dL (110 μmol/L) may indicate marked kidney disease in an elderly woman.
Creatinine levels may also be raised in the presence of health conditions that require certain medications. The use of angiontensin converting enzyme (ACE) inhibitors or an angiotensin receptor blocker (ARB) in the treatment of high blood pressure can raise the creatinine level, for example. Using an ACE inhibitor and ARB together will increase the creatinine level more than the use of one of these drugs alone. Physicians can expect to see a rise in creatinine of as much as 30% with the use of an ACE inhibitor or ARB.
An alternative to using CrCl calculations 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 therefore also used as a measure of kidney health, with BUN rising the more kidney function decreases. This test can also be used as a marker of problems outside of those intrinsic to the kidney. A disproportionately high urea to creatinine ratio may suggest volume depletion, for example.
Reviewed by Sally Robertson, BSc