Serum Creatinine Level
Serum Creatinine Level
Creatinine is actually a chemical waste product that is produced by the muscles. The chemical “creatine” is an important chemical involved in the production of energy needed for muscle contraction. During the course of every day, about 2% of the body’s creatine becomes creatinine. Creatinine enters the bloodstream and goes to the kidneys. Healthy kidneys filter out this waste material from the blood. It passes into the urine and out of the body. Unhealthy kidneys are unable to filter out the creatinine from the blood. The creatinine remains circulating in the bloodstream, and levels rise as the muscles continue to produce more and more.
The serum creatinine level is used to predict how the kidneys are functioning. In many cases, the serum creatinine level will begin to rise before a patient is even aware of any symptoms of kidney malfunction. High creatinine levels indicate the need for further investigation into the possibility that kidney failure is ensuing. If a creatinine level is elevated, then other tests such as blood urea nitrogen (BUN) or urine creatinine will be performed. Calculations involving serum and urine creatinine levels will give the creatinine clearance, a figure which reflects the capacity of the kidneys to filter small molecules out of the bloodstream. Calculations involving the serum creatinine level and the individual’s gender, height, weight, and age will allow estimation of the glomerular filtration rate, which can screen for kidney damage and disease.
Serum creatinine level is tied to muscle contraction, therefore, the normal value of an individual’s serum creatinine level will be dependent on the individual’s size and their overall muscle mass. In general, the normal serum creatinine level for men is higher than the normal serum creatinine level in either women or children. Because athletes tend to have greater muscle mass, their normal creatinine level may be higher than that of non-athletes.
A serum creatinine level is usually drawn as part of a larger metabolic panel or screen. Other tests performed in this panel include electrolytes (sodium, potassium, chloride, and carbon dioxide), as well as calcium, glucose, and BUN. A serum creatinine level is usually checked during a routine physical examination, as well as to evaluate patients for the presence of kidney disease, to monitor patients who have illnesses or who are taking medications that might affect the functioning of their kidneys, or to make sure that treatment for kidney disease is effective.
Serum creatinine levels can be affected by a number of medications. Patients who are on these medications should inform their doctor, so that test results can be interpreted appropriately. Medications that may affect serum creatinine levels include methyldopa, trimethoprim, vitamin C, cimetidine, certain diuretics, and cephalosporin antibiotics. Additionally, if the serum creatinine level is going to be used in calculations with the urine creatinine or the BUN levels to evaluate kidney functioning, results may be skewed by the following medications: vitamin C, phenytoin, cephalosporin antibiotics, captopril, aminoglycosides, trimethoprim, cimetidine, quinine, quinidine, procainamide, amphotericin B, steroid medications, and tetracycline antibiotics.
Patients who are taking anticoagulant medications should inform their healthcare practitioner, since this may increase their chance of bleeding or bruising after a blood test.
Blood urea nitrogen (BUN)— Blood urea nitrogen is a chemical waste product of protein metabolism that circulates in the bloodstream. Healthy kidneys remove urea from the bloodstream and it leaves the body in the urine. When the kidneys are not functioning properly, they are unable to filter the urea out of the blood, and blood urea nitrogen levels become elevated.
Creatine— Creatine is a substance produced by proteins and stored in the muscles. Creatine is a source for energy, allowing muscle contraction to take place. Some creatine is converted to creatinine, and enters the bloodstream, where it is filtered out by healthy kidneys and leaves the body in the urine. When the kidneys are not functioning properly, creatinine levels in the blood become abnormally elevated.
Diabetic nephropathy— Kidney damage or disease brought on by the long-term effects of diabetes.
Glomerulonephritis— A condition in which the filtering structures within the kidneys become damaged, limiting the kidneys’ ability to filter waste products from the blood.
Preeclampsia— A condition occurring in pregnancy in which high blood pressure leads to a number of complications, including a decreased ability of the kidneys to appropriately filter wastes from the blood.
Urine creatinine level— A value obtained by testing a 24-hour collection of urine for the amount of creatinine present.
This test requires serum to be drawn from a vein (usually one in the forearm), generally by a nurse or phlebotomist (an individual who has been trained to draw serum). A tourniquet is applied to the arm above the area where the needle stick will be performed. The site of the needle stick is cleaned with antiseptic, and the needle is inserted. The serum is collected in vacuum tubes. After collection, the needle is withdrawn, and pressure is kept on the serum draw site to stop any bleeding and decrease bruising. A bandage is then applied.
In the 24-48 hours prior to a serum creatinine level, patients should be advised to avoid strenuous exercise and to limit the amount of protein they ingest. Creatinine is a waste product of muscle contraction and, therefore, vigorous exercise in the 48 hours prior to a serum creatinine level could alter the results of the test. Similarly, ingesting more than eight ounces of meat (particularly beef) or other protein sources in the 24 hours prior to the serum creatinine level may affect the results.
As with any blood tests, discomfort, bruising, and/or a very small amount of bleeding is common at the puncture site. Immediately after the needle is withdrawn, it is helpful to put pressure on the puncture site until the bleeding has stopped. This decreases the chance of significant bruising. Warm packs may relieve minor discomfort. Some individuals may feel briefly woozy after a serum test, and they should be encouraged to lie down and rest until they feel better.
Basic serum tests, such as serum creatinine levels, do not carry any significant risks, other than slight bruising and the chance of brief dizziness.
In adult men, a normal serum creatinine level is 0.6-1.2 milligrams per deciliter (mg/dL) or 53-106 micromoles/L (mcmol/L). In adult women, a normal serum creatinine level is 0.5-1.1 mg/dL or 44-97 mcmol/L. In teenagers, a normal serum creatinine level is 0.5-1.0 mg/ dL. In children, a normal serum creatinine level is 0.3–0.7 mg/dL. In newborn babies, a normal serum creatinine level is 0.3-1.2 mg/dL.
High serum creatinine levels suggest that the kidneys are suffering from damage or disease. Kidneys can be damaged by severe infections, shock, cancer, or conditions that limit the blood flow reaching the kidneys. High serum creatinine levels can also occur when the urinary tract is blocked, or due to:
- obstruction of the urinary tract from a kidney stone or tumor;
- acute tubular necrosis;
- diabetic nephropathy;
- pre eclampsia;
- heart failure;
- extreme blood loss;
- muscular dystrophy;
- rhabdomyolysis (conditions resulting in the abnormal breakdown of muscle tissue);
- myasthenia gravis;
- acromegaly; or
Low serum creatinine levels may be due to:
- abnormally low muscle mass, as may occur in muscle wasting diseases like muscular dystrophy, or due to aging;
- liver disease;
- extreme low-protein diets; or
Brenner, B. M., and F. C. Rector, eds. Brenner & Rector’s The Kidney, 7th ed. Philadelphia: Saunders, 2004.
Goldman L., and D. Ausiello, eds. Cecil Textbook of Medicine, 23rd ed. Philadelphia: Saunders, 2008.
McPherson R.A., and M. R. Pincus, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods, 21st ed. Philadelphia: Saunders, 2006.
Medical Encyclopedia. Medline Plus. National Institutes of Health. http://www.nlm.nih.gov/medlineplus/encyclopedia.html (February 10, 2008).
American Association for Clinical Chemistry, 1850 K Street, NW, Suite 625, Washington, DC, 20006, (800) 892-1400, http://www.aacc.org.
Rosalyn Carson-DeWitt, M.D.
Serum electrolyte tests seeElectrolyte tests
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