It is a waste product of the normal breakdown of muscle tissue.
As creatinine is produced, it is filtered through the kidneys and excreted in the urine. Doctors measure the level of creatinine in the blood as a test of kidney function.
The kidneys’ ability to handle creatinine is called the creatinine clearance rate, which helps estimate the glomerular filtration rate (GFR), the rate of blood flow through the kidneys.
Normal kidney function and GFR
All the blood in the body flows through the kidneys hundreds of times a day. The kidneys push the liquid part of the blood through tiny filters (called nephrons) and then reabsorb most of the liquid back into the blood.
Fluid and waste products that are not reabsorbed by the kidneys are eliminated like urine.
The rate of blood flow through the kidneys is the glomerular filtration rate or GFR. (The glomeruli are microscopic bundles of blood vessels within the nephrons, and they are crucial parts of the filtration system.)
Glomerular filtration rate cannot be measured directly, that’s where the measurement of creatinine clearance comes in.
What is creatinine clearance?
Creatinine is a waste product that is continually produced during normal muscle breakdown. The kidneys transfer creatinine from the blood to the urine and reabsorb little of it.
The amount of blood that the kidneys can release creatinine per minute is called creatinine clearance.
Creatinine clearance in a healthy young person is approximately 95 milliliters per minute for women / 120 milliliters per minute for men. This means that every minute, that person’s kidneys remove 95-120 ml of creatinine-free blood. The GFR can vary based on age, gender, and size.
Measurement of creatinine clearance and kidney function
There are two main methods:
By measuring the amount of creatinine present in a urine sample collected over 24 hours. This method requires a person to put all of their urine in a plastic jug for one day, then bring it in for testing.
Glomerular filtration rate can be estimated using a single blood creatinine level, which your doctor enters into a formula. There are different formulas available, which take into account age, gender, and sometimes weight and ethnicity. The higher the creatinine level in the blood, the lower the estimated GFR.
Understanding an Abnormal Creatinine Test Result
Doctors determine the severity of chronic kidney disease with a staging system that uses GFR:
- Stage 1 : GFR 90 or higher (normal kidney function).
- Stage 2: GFR 60-89 (mild decrease in kidney function).
- Stage 3a: GFR 45-59 (mild to moderate decrease in kidney function).
- Stage 3b: GFR 30-44 (moderate to severe decrease in kidney function).
- Stage 4: GFR 15-29 (severe decrease in kidney function).
- Stage 5: GFR less than 15 ( kidney failure , usually requiring dialysis).
People over the age of 60 may have a seemingly normal blood creatinine level, but still have a low GFR and creatinine clearance. The 24-hour urine collection method, or one of the GFR estimation formulas, can more accurately identify decreased kidney function .
What to do with a low creatinine clearance
If you have a low GFR or creatinine clearance, your doctor will devise an action plan with you to address the problem.
The main causes of chronic kidney disease are high blood pressure and diabetes. If you have these conditions, the first step is to control them with improved diet, exercise, and medications. If these conditions are not present, more tests may be needed to identify the cause of kidney disease.
Regularly monitoring your GFR or creatinine clearance allows you and your doctor to track any decline in kidney function over time. Your doctor may need to make changes to your medications to accommodate any decline in kidney function.
Because over-the-counter medications (especially medications for minor aches, pains, and headaches), herbs, and supplements can affect your kidneys, do not take any of these without first checking with your doctor.
Most people don’t need dialysis until their GFR and creatinine clearance drop very little. However, because kidney function naturally declines with age, it is important to act early to preserve as much kidney function as possible.