Creatinine is a waste product from the normal breakdown of muscle tissue. As creatinine is produced, it’s filtered through the kidneys and excreted in urine. Doctors measure the blood creatinine level as a test of kidney function. The kidneys’ ability to handle creatinine is called the creatinine clearance rate, which helps to estimate the glomerular filtration rate (GFR) — the rate of blood flow through the kidneys.
Creatinine and Creatinine Clearance
Creatinine is a waste product that is produced continuously during normal muscle breakdown. The kidneys filter creatinine from the blood into the urine, and reabsorb almost none of it.
The amount of blood the kidneys can make creatinine-free each minute is called the creatinine clearance. Creatinine clearance in a healthy young person is about 95 milliliters per minute for women/120 milliliters per minute for men. This means that each minute, that person’s kidneys clear 95-120 mL of blood free of creatinine. The GFR can vary depending on age, sex, and size. Generally, the creatinine clearance is a good estimation of the glomerular filtration rate.
Doctors use creatinine and creatinine clearance tests to check renal function (kidney function). Testing the rate of creatinine clearance shows the kidneys’ ability to filter the blood. As renal function declines, creatinine clearance also goes down.
There are three main ways doctors use creatinine tests to measure kidney function:
- Creatinine clearance can be precisely determined by measuring the amount of creatinine present in a sample of urine collected over 24 hours. This method requires a person to place all his urine in a plastic jug for one day, then bring it in for testing. Although the urine creatinine measurement method is inconvenient, it may be necessary to diagnose some kidney conditions.
- GFR can be estimated using a single blood level of creatinine, which your doctor enters into a formula. Different formulas are available, which take into account age, sex, and sometimes weight and ethnicity. The higher the blood creatinine level, the lower the estimated GFR and creatinine clearance.
- Serum creatinine test, Serum Creatinine:Creatinine is a waste product in your blood that comes from muscle activity. It is normally removed from your blood by your kidneys, but when kidney function slows down, the creatinine level rises. Your doctor should use the results of your serum creatinine test to calculate your GFR.
For practical reasons, the blood test estimation method for GFR is used far more often than the 24-hour urine collection test for creatinine clearance. However, the use of 24-hour collections for creatinine clearance may still be useful in patients who have large muscle mass or a marked decrease in muscle mass.
Other Creatinine Related Tests
Blood Urea Nitrogen (BUN): Urea nitrogen is a normal waste product in your blood that comes from the breakdown of protein from the foods you eat and from your body metabolism. It is normally removed from your blood by your kidneys, but when kidney function slows down, the BUN level rises. BUN can also rise if you eat more protein, and it can fall if you eat less protein.
Urine Protein: When your kidneys are damaged, protein leaks into your urine. A simple test can be done to detect protein in your urine. Persistent protein in the urine is an early sign of chronic kidney disease.
Microalbuminuria: This is a sensitive test that can detect a small amount of protein in the urine.
Urine Creatinine: This test estimates the concentration of your urine and helps to give an accurate protein result. Protein-to-Creatinine Ratio: This estimates the amount of protein you excrete in your urine in a day and avoids the need to collect a 24-hour sample of your urine.
Serum Albumin: Albumin is a type of body protein made from the protein you eat each day. A low level of albumin in your blood may be caused by not getting enough protein or calories from your diet. A low level of albumin may lead to health problems such as difficulty fighting off infections. Ask your dietitian how to get the right amount of protein and calories from your diet.
Potassium: Potassium is a mineral in your blood that helps your heart and muscles work properly. A potassium level that is too high or too low may weaken muscles and change your heartbeat. Whether you need to change the amount of high- potassium foods in your diet depends on your stage of kidney disease.
WHAT ARE NORMAL BLOOD CREATININE LEVELS?
Normal levels of creatinine in the blood are approximately 0.6 to 1.2 milligrams (mg) per deciliter (dL) in adult males and 0.5 to 1.1 milligrams per deciliter in adult females. (In the metric system, a milligram is a unit of weight equal to one-thousandth of a gram, and a deciliter is a unit of volume equal to one-tenth of a liter.)
WHAT ARE CONSIDERED HIGH CREATININE LEVELS?
A person with only one kidney may have a normal level of about 1.8 or 1.9.
Creatinine levels that reach 2.0 or more in babies and 5.0 or more in adults may indicate severe kidney impairment.
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The need for a dialysis machine to remove wastes from the blood is based upon several considerations including the BUN, creatinine level, the potassium level and how much fluid the patient is retaining.
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