Creatinine Clearance Test.

This is a finer, more precise measure of how your kidneys are working than the serum creatinine test and is better at detecting early kidney disease. However, because it involves collecting ones urine over a 24 hour period it is awkward (now there's an understatement!) and is not routinely done as part of ones normal health check-up.

Preparing for the test.
It definitely requires a day off work - there is no way you want to be entertaining your co-workers by carrying around beakers full of urine, to say nothing of cluttering up the communal fridge with it! And needless to say it does not fit in with any form of entertaining - the effect on ones social life when a guest goes to the fridge for a beer and discovers a large jar of urine is likely to be catastrophic!

Remember to drink plenty of fluids during the 24-hour urine collection period, but avoid coffee and tea which act as diuretics. As always water is best.

Remember that special container holds at least a gallon (3.8 litres) and they want a reasonable amount in it!

Also limit the amount of protein you eat, for 24 hours prior to the test and during the test period, to around 8 oz (227 g).

Finally avoid strenuous exercise for 48 hours prior to having creatinine tests.

How this test is done.
First you have to collect the 24 hour urine sample. You start in the morning when you first get up. The first urine of the day is not saved (basically you are emptying your bladder and flushing it ready for the test) but every drop in the next 24 hours is. You will be given a container big enough to hold it all and that container will have some preservative in it. None the less you need to keep it in the fridge.

You urinate into a separate container and pour it into the large one, being careful not to touch the inside with your fingers. Do not get any foreign matter into the samples eg. Toilet paper or pubic hair.

A serum creatinine test is performed on a blood sample taken from your vein during the 24 hour period so a comparison can be made between the amount of creatinine in your blood and that in your urine.
The results of these are inverse - the amount of creatinine in your urine decreases when its level in your blood increases.

How are the results calculated?
The creatinine clearance value is calculated from the amounts of creatinine in the urine and blood and from the volume of urine. This value is reported as the amount of blood cleared of creatinine per minute, adjusted for your size.

What are the normal values?
Normal value ranges vary slightly among different laboratories.
As a general rule:-
Male: 85 to 137 ml/min. (milliliters per minute)
Female: 80 to 128 ml/min.
Pregnant women: 150 to 200 ml/min.
Values usually drop as you get older. It has been suggested a drop of 6 mL/min for every 10 years past the age of 20 is normal.

A creatinine clearance value of approximately 10 ml/minute is an indication that dialysis may be necessary

High values can be caused by:-
1. Strenuous exercise,
2. Muscle injury (especially crushing injuries),
3. Burns,
4. Carbon monoxide poisoning,
5. Hypothyroidism
6. Pregnancy.

Low creatinine values can be caused by:-
1. Serious kidney damage.
2. Urinary tract obstruction such as a kidney stone or an enlarged prostrate.
3. A kidney infection.
4. Dehydration
5. Congestive heart failure
6. Decreased renal blood flow
7. Liver disease (cirrhosis)
9. Diabetic nephropathy
10. Cancer

Factors which can cause inaccurate results:-
1. Incomplete urine collection
2. Polluted urine sample
3.Vigorous exercise
4. Pregnancy.
5. Medications:-
Vitamin C (ascorbic acid)
Phenytoin (Dilantin) - an anti-seizure medication (anticonvulsant),
Cephalosporin antibiotics - used to treat a wide variety of bacterial infections.
Captopril - an ACE (angiotensin converting enzyme) inhibitor used to lower blood pressure.
Aminoglycosides (Garamycin) - an antibiotic used to treat or prevent a skin infection.
Trimethoprim (Proloprim, Trimpex) - antibiotic used for the treatment of urinary tract infections, diarrhoea, respiratory and ear infections.
Cimetidine (Tagamet) - used for the treatment of ulcers, gastroesophageal reflux disease (GERD), heartburn and the prevention of gastrointestinal bleeding.
Quinine - used to treat or prevent malaria and may also be used for leg cramps.
Quinidine (Cardioquin, Quinaglute, Quinidex) - an antiarrhythmic drug used in the treatment of abnormal heart rhythms.
Procainamide (Procan, Pronestyl) - another antiarrhythmic drug used in the treatment of abnormal heart rhythms.
Amphotericin B - an antifungal medication.

Further information.
This test does not give the whole picture on it’s own. A normal creatinine clearance test does not mean there is no kidney impairment. To evaluate that, the doctor will order a serum creatinine, creatinine clearance and the blood urea nitrogen (BUN). 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.

What is the ratio?
Normal BUN to Creatinine Ratio is :-
Under 12 months old – up to 30:1
Over 12 months old – 10:1 to 20:1

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 finally 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 one time urine sample and is used to measure urine creatinine and sodium.

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