A serum creatinine test measures the amount of creatinine in the blood. When the chemical Creatine Phosphate is metabolised by the muscles to produce energy, creatinine is produced as a waste product and carried by the blood to the kidneys from where it is excreted in the urine.
Creatinine production is linked to muscle mass, which varies little from day to day, so if the kidneys are functioning properly the creatinine level should remain constant.
However that is assuming you eat a normal diet. Eating large quantities of meat can cause temporary increases in blood creatinine levels. Likewise taking creatine supplements can also increase creatinine levels.
What is the purpose of this test?
1. It is a used to evaluate the way the kidneys are working. It is an inexpensive test that can help detect kidney disease. If kidney function is impaired then creatinine levels will rise in the blood and decrease in the urine. However, because blood creatinine levels only rise if there is a reasonable amount of damage to functioning nephrons this test is not suitable for detecting early kidney disease.
Sometimes a combination of blood and urine creatinine levels may be used to calculate a ‘creatinine clearance’.
See Creatinine clearance test
2. It is a used to monitor the kidney function of people who take medications that can cause kidney damage.
3. It is a used to detect whether the patient is suffering from severe dehydration.
How is this test done?
A blood sample is taken and sent to the lab for interpretation. It is suggested you avoid strenuous exercise for 48 hours before the test and eat no more than 8 oz of protein for 24 hours prior to it.
Also, if you are taking prescribed medications or vitamin and herbal supplements you need to tell your doctor. It is possible he may require you to stop taking them for a 24 hour period prior to the test in case they affect the test results.
Examples of such medicines include ascorbic acid (vitamin C), barbiturates, and diuretics.
How to interpret the results.
Remember the US measures in mg/dl while elsewhere we use µmol/l.
1 mg/dL of creatinine is 76.26 µmol/l and remember that results also vary between labs.
As females have less muscle than males their creatinine levels are lower.
Likewise if you are a 3 foot midget you will have lower levels than a 7 foot giant and if you are the winner of the Mr Universe competition all those above average muscles will give you above average creatinine levels.
Older people tend to have decreased muscle mass and therefore lower creatinine levels.
Levels may be slightly lower during pregnancy due to increased kidney filtration rates.
Normal range for new-borns is 0.3-1.2 mg/dL (22.9-91.5µmol/l)
Normal range for infants is 0.2-0.4 mg/dL (15.3 - 30.5µmol/l)
Normal range for children is 0.3 to 0.7 mg/dl (22.9 to 53.4µmol/l)
Normal range for adolescents 0.5-1.0 mg/dL (38.1 - 76.3µmol/l).
Normal range for women is 0.5 to 1.1 mg/dL (38.1-83.9µmol/l).
Normal range for men is 0.9 to 1.4 mg/dL (68.6-106.8 µmol/l).
Note - The maturity of a baby and its ability to breath can be estimated by measuring the amount of creatinine in the amniotic fluid. If the baby has mature kidneys and lungs it will have more creatinine. This knowledge can be invaluable in a decision as to whether to deliver a baby prematurely due to complications in the mother.
So what relationship does this number have to actual kidney damage?
Your doctor will be better able to accurately interpret the results based on a number of tests done but below is a guideline.
Serum Creatinine level of 0.6 - 1.5 mg/dL (45.8 - 114.4µmol/l) means you can have up to 50% kidney nephron loss.
Serum Creatinine level of 1.6 to 4.6 mg/dL (77.9 - 350.8µmol/l) means you can have over 50% kidney nephron loss and therefore impaired kidney function.
Serum Creatinine level of 4.7 to 9.9 mg/dL (358.4 - 755.0µmol/l) means you can have 75% kidney nephron loss and seriously impaired kidney function.
Serum Creatinine level of 10 mg/dL and over (762.6µmol/l and over) means you can have 90% kidney nephron loss and end-stage kidney disease.
The upward or downward trend of creatinine levels is more important than the level of one single test.
Creatinine tests are usually done in conjunction with a Blood Urea Nitrogen (BUN) test and are a better indicator of long term kidney function because creatinine levels rise and fall at a slower pace than BUN levels.
Greater-than-normal levels may indicate:-
1. Your kidneys are not functioning properly.
2. Urinary tract obstruction such as a kidney stone or an enlarged prostrate.
3. You have a kidney infection.
5. Congestive heart failure
6. Decreased renal blood flow
7. Eclampsia (a condition of pregnancy that includes seizures)
8. Pre-eclampsia (pregnancy-induced high blood pressure)
9. Diabetic nephropathy
10. Glomerulonephritis, - a type of kidney disease caused by inflammation of the internal kidney structures (glomeruli).
11. Muscular dystrophy - a disease in which muscles of the body get weaker and may slowly stop working.
12. Pyelonephritis - a bacterial infection of one or both kidneys.
13. Rhabdomyolysis - the breakdown of skeletal muscle due to injury
14. Medications eg:-
ACE inhibitors such as Captopril and Enalapril, taken to control blood pressure.
Nonsteroidal anti-inflammatorys like Ibuprofen and Naproxen.
Diuretics such as Spironolactone (Aldactone), used as a diuretic in cases of congestive heart failure, cirrhosis of the liver, and kidney disease. Also to treat elevated blood pressure in combination with other drugs and in resistant cases of low potassium.
Furosemide(Lasix) a powerful diuretic used to treat edema of the body caused by heart failure, cirrhosis and chronic kidney failure. It is sometimes used along with other meds to treat high blood pressure.
Some antibiotics - Aminoglycosides (Garamycin), an antibiotic used to treat or prevent a skin infection.
Cephalosporins (cefoxitin) which are used to treat a wide variety of bacterial infections.
Cimetidine (Tagamet) - used for the treatment of ulcers, gastroesophageal reflux disease (GERD), heartburn and the prevention of gastrointestinal bleeding.
Cisplatin - a chemo drug used to treat cancer.
Lower-than-normal levels may indicate:-
1. Muscular dystrophy (late stage)
2. Myasthenia gravis - an autoimmune disease that causes muscle weakness.
3. Some types of severe liver disease.
4. A diet very low in protein.
5. Pregnancy can also cause low blood creatinine levels.
This test does not give the whole picture on its own. A normal creatinine test does not mean there is no kidney impairment.
Together these will give the doctor a pretty good idea of how your kidneys are functioning.
A measurement termed the BUN-to-creatinine ratio can be calculated by dividing the measured BUN level by the creatinine level.
This measurement enables your doctor to decide whether your lack of kidney function is due to dehydration or kidney disease.
If it is due to dehydration the BUN levels rise more than creatinine ones, causing an increased BUN to creatinine ratio.
However if there is either kidney disease or something that is blocking the flow of urine from the kidneys the BUN and creatinine levels rise at a very similar pace so the BUN to creatinine ratio is more or less normal.
And then there is something else that can help your doctor decide whether it is dehydration or damage that is causing the problem.
This is called the Fractional excretion of sodium (FENA) calculation. It involves a onetime urine sample and is used to measure urine creatinine and urine sodium.
Lastly are the ‘scanning’ tests
These are used to access the physical state of the kidney and its vessels rather than how they are functioning.
Here sound waves are used to show a picture of the kidneys. The nice part about this test is that it is one of the least invasive. However, as stated above, it is more useful to rule out physical issues such as congenital abnormalities, stones or tumours or show up changes in size, like swelling, or position than how the kidneys are actually working.
If there is worry that there may be narrowing of the veins and arteries or maybe clots then a newer technique called Colour Doppler may be used. Like the ultrasound it produces a picture of the blood vessels but then it goes a step further and using a computer the sound is converted into colours. These are then overlaid on the picture of the blood vessel. The colours represent the speed and the direction of the flow through the vessel.
2. CT Scan (Computerized tomography)
Again this is a test that is mainly used to look for abnormalities or obstructions. Basically it makes use of a contrast dye to show a picture of the kidneys.
3. Kidney Biopsy
If there really is a need to evaluate the amount of damage suffered by the kidneys then the doctor may ask for a kidney biopsy to be done. A thin needle is inserted and tiny pieces of kidney tissue are removed for microscopic examination. A biopsy is usually done only if there is any doubt about the diagnosis.
(Checked and updated April 2013)