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I am really confused on something and I hope someone can explain it
to me. We estimated inulin clearance for a group of patients. The
units are in mL/min. Normal ranges are reported in mL/min/1.73 m**2.
How do I convert mL/min to mL/min/1.73 m**2? Do I divide inulin CL
by (BSA/1.73)?
Thanks,
Pete Bonate
Peter L. Bonate, PhD
Genzyme Corporation
Senior Director, Pharmacokinetics
4545 Horizon Hill Blvd
San Antonio, TX 78229 USA
peter.bonate.aaa.genzyme.com
phone: 210-949-8662
fax: 210-949-8219
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The following message was posted to: PharmPK
Hi Peter- In a word, yes. Another way to think about it is: take
the patient's clearance in ml/min and divide it by their BSA--this
will yield ml/min/m^2. Then, multiply by 1.73 to get ml/min per
1.73m^2--otherwise known as ml/min/1.73m^2 --Larry
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The following message was posted to: PharmPK
This reminds me of the debacle with the MDRD equation...and is more
confusing than it looks.
The inulin GFR is corrected or normalized to 1.73m**2, the BSA of the
fictional average patient.
to convert mL/min to mL/min/1.73m**2 for a specific patient, divide
that patients inulin GFR by their BSA which will give you mL/min/m**2
(per m**2) , then multiply by 1.73 to normalize the value to an
"average" individual of 1.73 m**2 BSA.
ie
[(ml/min) / (subjects BSA m**2)] * 1.73 = x mL/min/1.73 m**2
Wolowich
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Pete:
The conversion is as follows:
1. Calculate patient's BSA in m**2
2. Multiply Inulin CL (mL/min) x (1.73/BSA)
Here, GFR values are standardized to a BSA of 1.73m**2. This approach
is commonly used in clinical research but not widely accepted in the
clinical setting (i.e. Cockcroft-Gault creatinine clearance is
reported in mL/min). For example, patients with BSA >> 1.73m**2 (i.e.
3.0 m**2) may have apparent GFR >> 200 mL/min, however when this
value is corrected to 1.73m**2, their result is usually in the normal
range of 90 - 140 mL/min/1.73m**2.
Recently, the MDRD equation has been proposed for estimating GFR in
the clinical setting, and its units are mL/min/1.73m**2.
Hope this helps,
Tom
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The standard formula for the calculation of the creatinine or inulin
clearance corrected by BSA is the following:
Clearance per standard surface area (mL/min/1.73 m2) = calculated
clearance * (1.73 / (BSA in square meters))
An interesting paper about the normalization of GFR is:
Peters AM, Gordon I, Sixt R. Normalization of glomerular filtration rate
in children: body surface area, body weight or extracellular fluid
volume?
J Nucl Med. 35(3):438-44.1994.
Regards,
Prof. Jose M. Lanao
Dpt. Pharmacy and Pharmaceutical Technology
University of Salamanca
Spain
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The following message was posted to: PharmPK
Hi,
Just a note on the use of m**2 as as a standardising factor. The only
theory I am aware of for the use of body surface area as a scale
factor for functional properties of the body such as GFR comes from a
disproven allometric hypothesis.
See http://www.cognigencorp.com/nonmem/nm/99oct042001.html
The current best allometric theories are based on a 3/4 power
function of weight.
"Commonly used in clinical research" does not mean it is a sensible
thing to do. Althought the BSA scaling method is better than a simple
per kg standardisation clinical researchers should use the 3/4 power
allometric function for GFR if they want to be scientific.
Nick
--
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
email:n.holford.aaa.auckland.ac.nz tel:+64(9)373-7599x86730 fax:373-7556
http://www.health.auckland.ac.nz/pharmacology/staff/nholford/
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