- On 17 Jun 1998 at 14:31:33, Marc.A.Semprebon.at.Hitchcock.ORG (Marc A. Semprebon) sent the message

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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 - On 18 Jun 1998 at 11:52:00, David_Bourne (david.-at-.pharm.cpb.uokhsc.edu) sent the message

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[A few replies - db]

X-Sender: mentor.-a-.hardlink.com

Mime-Version: 1.0

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 - On 19 Jun 1998 at 11:18:30, Marc.A.Semprebon.aaa.Hitchcock.ORG (Marc A. Semprebon) sent the message

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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 - On 20 Jul 1998 at 11:44:12, radfloyd.-a-.pacbell.net sent the message

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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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