Doctors Lounge - Nephrology Answers
"The information provided on www.doctorslounge.com is designed to support, not replace, the relationship that exists between a patient/site visitor and his/her physician."
Forum Name: Renal Failure
Question: Conversion from g/24 hours to ML/Min
|tscales - Thu Mar 17, 2005 8:02 pm|
My wife is a long-term diabetic who is starting to experience chronic renal failure. She recently had a 24 creatinine clearence test and the results were:
.56 g/24 hours
Her serum creatinine is 2.6 and the volume was 3500 ml.
Everything we find in our research discusses creatinine clearance in ml/min, not g/24 hours. We're having trouble finding the formula to convert from one to the other.
Can anyone point us to the proper formula?
|Rhonda P, CEP - Thu Mar 17, 2005 9:17 pm|
Here is a link for you to contact the American Kidney Fund and the National Kidney Foundation. Their addresses and phone numbers are listed as well as a bit of information on how the test is measured. It's hard to explain conversions like this but I think they can answer your questions the best.
If you can't open the link, here are the addresses:
National Kidney Foundation
30 East 33rd Street
New York, NY 10016
Phone: 1-800-622–9010 or (212) 889-2210
American Kidney Fund
6110 Executive Boulevard
Rockville, MD 20852
Phone: 1-800-638–8299 or (301) 881-3052
|Dr. Anthony Solomon - Sun Mar 27, 2005 7:18 pm|
If the laboratory supplied you with these values and expected your wife to calculate the Creatinine Clearance Rate (CCR), you are entitled to a refund or to a repeat test free of charge. As Web-Medic correctly explained in the first reply, it is difficult to explain conversions. It is not possible to convert g/hr to ml/min because one is a weight, and the other a volumetric measurement. Even doctors sometimes find the formula for calculation confusing and difficult to remember. The laboratory is expected to provide you with a calculated value. I have calculated your wife’s CCR to be 149.54ml/min (normal range 90 –130 ml/min) assuming that the figures you supplied are accurate.
Here is the formula for calculating CCR based on the values you have given:
CCR = A multiplied by B divided by C
A = The average volume in milliliters of urine excreted in one minute. In your wife's case, this would be 3500ml divided by 1440 minutes (24 hours). This equals 2.43ml/min.
B = The average amount of creatinine in milligrams in one liter of excreted urine. (0.56g is equal to 560mg). This would be 560mg divided by 3500ml multiplied by 1000ml. This equals 160mg/litre.
C = The average amount of creatinine in milligrams in 100ml of blood. The result is 2.6mg/100ml (incorrectly reported as 2.6 without the units).
Now, let us get down to simple mathematics: CCR = A x B divided by C = 149.54ml/min
Normal values for adult males range from about 100 to 140 milliliters per minute.
Normal values for adult females range from about 90 to 130 milliters per minute.
I wish you and your wife a Happy Easter and hope that her CCR improves with treatment.
Dr Anthony Solomon
MB BS DTM&H DIP.VEN FRSM
Consultant Physician, Tropical & Genitourinary Medicine
|tscales - Sun Mar 27, 2005 7:44 pm|
Thank you so much for your reply. I had continued to research and finally found a similar formula, except that B was represented in milligrams per deciliter, resulting in 14.95 rather than 149.5. Obviously quite a difference. I guess I am surprised at the 149, which is ABOVe normal, given her high serum creatinine.
Not questioning your calculation, just awfully confused.
|Dr. Anthony Solomon - Sun Mar 27, 2005 8:46 pm|
The accuracy of the Creatinine Clearance Test is affected by many factors:
Incomplete or faulty urine collection
Inaccurate estimations of serum and urinary creatinine
Vigorous exercise prior to the test
Certain medications that interfere with the test
I was also surprised at the value obtained by the calculation, and can only attribute it to inaccurate laboratory results or faulty urine collection, not the formula or calculation. If your wife had a CCR of 14.95 with your B interpretation, she would be in a very serious condition. Try another laboratory for comparison.
Dr Anthony Solomon
|tscales - Sun Mar 27, 2005 9:02 pm|
Yes, she would and I believe she is. We have an appointment with a Nephrologist on Friday. It's complicated by the fact that she has a nephroureteral stint and external bag, as she is not draining any urine through her ureter.
Thanks again for your help.
|Dr. Anthony Solomon - Tue Mar 29, 2005 10:19 am|
You may use any of the two methods described herein to obtain the CALCULATED CCr, which differs from the MEASURED CCr. The measured CCr utilises urine values; the calculated CCr uses the patient's age and weight which you have supplied to me by PM.
On the home page of our website (www.doctorslounge.com), click MEDICAL CALCULATORS (same column as Ask a Doctor); then click Creatinine Clearance Calculator; tick female circle; insert 2.6 in first box and change units to mg/dl ; insert 47(weight of patient) in second box and ensure unit is in kg; insert 43 in third box (age of your wife)
You should have a CCR of 20.70 ml/min.
From the link you sent to me by PM, click the Cockcroft-Gault formula in the last paragraph and calculate CCR as follows (USING THIS FORMULA)
(140 - 43) x 47
72 x 2.6
Multiply the value of above by 0.85 (for female): CCR = 24.35ml/min
These two calculated values are more realistic, assuming again that the serum creatinine
figure of 2.6mg/dl is accurate.
Dr Anthony Solomon
|tscales - Tue Mar 29, 2005 10:43 am|
Thank you for your reply. That matches what I had found in my research. It's probably a more realistic number (certainly than 149!).
|| Check a doctor's response to similar questions|
Are you a Doctor, Pharmacist, PA or a Nurse?
Join the Doctors Lounge online medical community
Editorial activities: Publish, peer review, edit online articles.
Ask a Doctor Teams: Respond to patient questions and discuss challenging presentations with other members.