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One generally uses an IBW (ideal body weight) calculation for adults such as:
* males: 50 kg + (2.3 x height in inches over 5 feet)
*females: 45.5 kg + (2.3 x height in inches over 5 feet)
Or in children (1-18 yrs old):
*(height in cm)^2 x 1.65 /1000
My basic question is what are folks out there using to calculate an IBW for
the population of patients which would include the "little old lady from
Pasedena" who is 4 feet 6 inches tall and weighs 38 kg or worse yet, her
afternoon tea friend who overly indulges in pastries and is 4 feet 5 inches
tall but weighs 78 kg?
Any assistance would be helpful.
Marc Semprebon, RPh
APD Memorial Hospital
Lebanon, NH 03766
(603-448-7404
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[A few replies - db]
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Date: Wed, 17 Jun 1998 16:12:10 -0400
To: PharmPK.aaa.pharm.cpb.uokhsc.edu
From: Daro Gross
Subject: Re: PharmPK IBW Calculations - Second Mailing
IBW is a factor that is significant for some studies and not others. First,
establish the pharmacodynamic relationship between IBW on a purely
metabolic basis, then you will be in a position to normalize your patient
population in terms of IBW, i.e., you question assumes a linear
relationship between IBW and the other variables in the study, but this
assumption cannot be assumed to be correct. In many cases, IBW has no
statistically significant correlation to any of the other variables
measured.
Daro Gross
PharmaLogic
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From: "John E. Murphy, Pharm.D."
To: PharmPK.at.pharm.cpb.uokhsc.edu
Date: Wed, 17 Jun 1998 14:01:47 MST7
Subject: Re: PharmPK IBW Calculations - Second Mailing
Priority: normal
Since males would reach a weight of 0 kg at a height of 38 inches if
the formula is used as is, it makes sense that you deduct weight at
progressively less than 2.3 kg per inch the smaller the patient is.
Caveat 1 - this formula is for adults. There are very few adults a
lot shorter than 5 feet. You probably won't be too far off
deducting 2.3/inch if just an inch or two less than five feet
tall. Caveat 2 - whenever the guessing rate goes up for any
prediction, the justification for measuring a concentration sooner
also goes up (for drugs where there is good justification for
monitoring anyway).
Just a clinician's point of view.
John E. Murphy, Pharm.D.
Professor and Head
Department of Pharmacy Practice and Science
(520) 626-5730 (P), (520) 626-7355 (FAX)
---
From: "David Nix"
To: PharmPK.-a-.pharm.cpb.uokhsc.edu
Date: Wed, 17 Jun 1998 16:33:01 MST7
Subject: Re: PharmPK IBW Calculations - Second Mailing
Priority: normal
For the case where Actual BW < IBW it is standard practice to use the
Actual body weight
The obese case presents a more difficult problem. Organ weights and
lean mass do increase in obesity but not in proportion to the total
body mass. The formulas that you cite are rather empiric and do not
provide a very good estimate of true lean body mass. .
Your reason for asking the question is probably the fact that the
formula for IBW is only valid if the height is >=60 inches. Here is
a suggestion - sorry no references available!
Body mass index (BMI) is a well accepted measure of ideal body
weight. BMI=weight (kg) / height (m)^2. A normal BMI is considered
to be around 25 and is constant over a range of heights.
Based on IBW relationships:
For a 60 inch individual:
BMI = 50 kg / (60 X 0.0254)^2 = 21.5 male
BMI = 45.5 kg / (60 X 0.0254)^2 = 19.6 female
For a 72 inch individual
BMI = 73 kg / (72 X 0.0254)^2 = 21.8 male
BMI = 68.5 kg / (72 X 0.0254)^2 = 20.5 female
BMI is rather constant and suggests that LBW represents ~21.7/25 or
87% for a male of normal IBW and 20/25 or 80% for a female of normal
IBW. You can apply this relationship to individuals less than 60 in
height.
4 feet 6 inches tall and weighs 38 kg - Use 38 kg (actual BW)
4 feet 5 inches tall but weighs 78 kg?
LBW = LBMI X (height in meters)^2
For the lean BMI use 21.5 male and 19.6 for female
(arbitrary - you could also use the average over a range
of heights)
LBW = 19.6 (53 x 0.0254)^2 = 35.5 kg
I have seen this alternative used but I think it provides values
that are too low.
LBW = 50 kg + (Height(in)-60) x 2.3 male
LBW = 45.5 kg + (height(in)-60) x 2.3 female
4 feet 5 in tall -- weight 78 kg female
LBW = 45.5 + (53-60) x 2.3 = 29.4 kg
I hope this helps
David Nix
The University of Arizona
nix.-at-.pharmacy.arizona.edu
---
From: Stephen Duffull
To: "'PharmPK.aaa.pharm.cpb.uokhsc.edu'"
Subject: RE: PharmPK IBW Calculations - Second Mailing
Date: Thu, 18 Jun 1998 08:26:59 +0100
MIME-Version: 1.0
Dear Marc
While we tend to use metric units (ie cm) to determine IBW we use the same
equations. Therefore a LOL from Pasedena who is 54 inches tall would have
a calculated IBW of approx 32kg [ie 45.5 + 2.3*(height-60)]. The same
would be true for your 78 kg lady who is clearly obese. The question
therefore is on what estimate of size should dose be adjusted to, and that
will depend on the drug, the disease/pathophysiological changes/presence of
multiple pathology, the usual dosing protocol, desirable outcomes, and the
perceived need of the patient for the drug ... ie time to dose
individualise (but not necessarily TDM).
Sorry this is not more helpful.
Steve
PS For example, for gentamicin dosing I generally use the lower of total
body weight and IBW, or sometimes for obese patients an adjusted body
weight [IBW+ 0.4*(total body weight-IBW)], ie 50 kg for your 78 kg lady.
The rationale and evidence supporting an adjusted body weight is debatable.
========================
Stephen Duffull
School of Pharmacy
Manchester University
Ph +44 161 275 2355
Fax +44 161 275 2355
Email: sduffull.-a-.fs1.pa.man.ac.uk
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Thanks to all who responded to my question! Daro Gross, "John E. Murphy, Pharm.D." , "David Nix" ,
Stephen Duffull.
Some of the patients I may come in contact with who require
aminoglycosides are truly the" Little Old Ladies from Pasedena"(acutally
from Lebanon, NH and surrounding towns) and do weight less than 45.5 kg if
they are females. In addition those that are less than 5 feet tall who are
at the same time obese are the ones who are difficult to determine how to
dose. How does one determine the difference between the ABW and the IBW in
these patients if one does not have an accurate or semi-accurate method to
determine what that IBW is in the first place? What about those that
exhibit 3rd spacing? What if they are also malnourished? All these
factors as you know affect Vd.
Part of what I am trying to do is to establish simple guidelines for
non-pharmacists (i.e., MDs and ARNPs, etc.) to prescribe aminoglycosides.
I therefore feel that I definitely need to know how I arrive at a method
for IBW (or DBW, etc) in these patients. What I need is a good starting
point, and I guess experience with the formulae that you folks sent me may
help. Unfortunately I see very few patients who are actually prescribed
aminoglycosides in my practice to begin with.
I just though of another patient population that might help answer the
question (or make it worse!!), depending on if anyone has experience:
How do you determine IBW or aminoglycoside dosing in patients
who are dwarfs or midgets (are those the politically correct names?).
Marc Semprebon, RPh
APD Memorial Hospital
Lebanon, NH
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Marc, The IBW equations you described for adults were based on linear
regressions of the height weight data from the New York Metropolitan
Life
tables from 1959 (and a later table published a few years later, same
source). These were performed by Ben Devine, PharmD about 1972 or 1974
when he was a resident at USC School of Pharmacy as an easy way to
perform a weight adjustment when dealing with digoxin and/or gentamicin
pharmacokinetics. The tables did not include adults shorter than 5 feet
(as I recall) and did break down IBW based on frame size (small, medium,
or large). A few years later, I replicated Ben's work for my own
interest using the frame sizes and using height measured in centimeters
as at the time it appeared that the world would convert to metric
measure. I have found that extrapolating the regression line downwards
for shorter females produces reasonable results. Note that this takes
care of the shorter female who is obese fairly nicely. Typically for
individuals who have IBW (calculated) less than their actual body
weight, the actual body weight is generally used. This later seems to
work for many corrections for distribution volume but has not appeared
to work well for me in those uncommon situations in which one is
relating renal clearance to body mass. Good luck!
Addenda. I still have the regressions I did back then and have found
them useful (not for frame size which requires some effort to determine)
but for the standard errors of the estimate for the predicted
values--this gives one a range for the guess at IBW.
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