# PharmPK Discussion - IBW Calculations

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• On 17 Jun 1998 at 14:31:33, Marc.A.Semprebon.at.Hitchcock.ORG (Marc A. Semprebon) sent the message
` 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 forthe population of patients which would include the "little old lady fromPasedena" who is 4 feet 6 inches tall and weighs 38 kg or worse yet, herafternoon tea friend who overly indulges in pastries and is 4 feet 5 inchestall but weighs 78 kg?   Any assistance would be helpful.Marc Semprebon, RPhAPD Memorial HospitalLebanon, NH  03766(603-448-7404`
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• On 18 Jun 1998 at 11:52:00, David_Bourne (david.-at-.pharm.cpb.uokhsc.edu) sent the message
`[A few replies - db]X-Sender: mentor.-a-.hardlink.comMime-Version: 1.0Date: Wed, 17 Jun 1998 16:12:10 -0400To: PharmPK.aaa.pharm.cpb.uokhsc.eduFrom: Daro Gross Subject: Re: PharmPK IBW Calculations - Second MailingIBW is a factor that is significant for some studies and not others. First,establish the pharmacodynamic relationship between IBW on a purelymetabolic basis, then you will be in a position to normalize your patientpopulation in terms of IBW, i.e., you question assumes a linearrelationship between IBW and the other variables in the study, but thisassumption cannot be assumed to be correct. In many cases, IBW has nostatistically significant correlation to any of the other variablesmeasured.Daro GrossPharmaLogic---From: "John E. Murphy, Pharm.D." To: PharmPK.at.pharm.cpb.uokhsc.eduDate: Wed, 17 Jun 1998 14:01:47 MST7Subject: Re: PharmPK IBW Calculations - Second MailingPriority: normalSince males would reach a weight of 0 kg at a height of 38 inches ifthe formula is used as is, it makes sense that you deduct weight atprogressively less than 2.3 kg per inch the smaller the patient is.Caveat 1 - this formula is for adults. There are very few adults alot shorter than 5 feet.  You probably won't be too far offdeducting 2.3/inch if just an inch or two less than five feettall.  Caveat 2 - whenever the guessing rate goes up for anyprediction, the justification for measuring a concentration sooneralso goes up (for drugs where there is good justification formonitoring anyway).Just a clinician's point of view.John E. Murphy, Pharm.D.Professor and HeadDepartment of Pharmacy Practice and Science(520) 626-5730 (P), (520) 626-7355 (FAX)---From: "David Nix" To: PharmPK.-a-.pharm.cpb.uokhsc.eduDate: Wed, 17 Jun 1998 16:33:01 MST7Subject: Re: PharmPK IBW Calculations - Second MailingPriority: normalFor the case where Actual BW < IBW it is standard practice to use theActual body weightThe obese case presents a more difficult problem.  Organ weights andlean mass do increase in obesity but not in proportion to the totalbody mass.  The formulas that you cite are rather empiric and do notprovide a very good estimate of true lean body mass.   .Your reason for asking the question is probably the fact that theformula for IBW is only valid if the height is >=60 inches.  Here isa suggestion - sorry no references available!Body mass index (BMI) is a well accepted measure of ideal bodyweight.  BMI=weight (kg) / height (m)^2.   A normal BMI is consideredto 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   femaleFor 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   femaleBMI is rather constant and suggests that LBW represents ~21.7/25 or87% for a male of normal IBW and 20/25 or 80% for a female of normalIBW.   You can apply this relationship to individuals less than 60 inheight.   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 kgI have seen this alternative used but I think it provides valuesthat 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 kgI hope this helpsDavid NixThe University of Arizonanix.-at-.pharmacy.arizona.edu---From: Stephen Duffull To: "'PharmPK.aaa.pharm.cpb.uokhsc.edu'" Subject: RE: PharmPK IBW Calculations - Second MailingDate: Thu, 18 Jun 1998 08:26:59 +0100MIME-Version: 1.0Dear MarcWhile we tend to use metric units (ie cm) to determine IBW we use the sameequations.  Therefore a LOL from Pasedena who is 54 inches tall would havea calculated IBW of approx 32kg [ie 45.5 + 2.3*(height-60)].  The samewould be true for your 78 kg lady who is clearly obese.  The questiontherefore is on what estimate of size should dose be adjusted to, and thatwill depend on the drug, the disease/pathophysiological changes/presence ofmultiple pathology, the usual dosing protocol, desirable outcomes, and theperceived need of the patient for the drug ... ie time to doseindividualise (but not necessarily TDM).Sorry this is not more helpful.StevePS  For example, for gentamicin dosing I generally use the lower of totalbody weight and IBW, or sometimes for obese patients an adjusted bodyweight [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 DuffullSchool of PharmacyManchester UniversityPh +44 161 275 2355Fax +44 161 275 2355Email: sduffull.-a-.fs1.pa.man.ac.uk`
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• On 19 Jun 1998 at 11:18:30, Marc.A.Semprebon.aaa.Hitchcock.ORG (Marc A. Semprebon) sent the message
`     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 requireaminoglycosides are truly the" Little Old Ladies from Pasedena"(acutallyfrom Lebanon, NH and surrounding towns) and do weight less than 45.5 kg ifthey are females.  In addition those that are less than 5 feet tall who areat the same time obese are the ones who are difficult to determine how todose.  How does one determine the difference between the ABW and the IBW inthese patients if one does not have an accurate or semi-accurate method todetermine what that IBW is in the first place?  What about those thatexhibit 3rd spacing?  What if they are also malnourished?  All thesefactors as you know affect Vd.    Part of what I am trying to do is to establish simple guidelines fornon-pharmacists (i.e., MDs and ARNPs, etc.) to prescribe aminoglycosides.I therefore feel that I definitely need to know how I arrive at a methodfor IBW (or DBW, etc) in these patients.  What I need is a good startingpoint, and I guess experience with the formulae that you folks sent me mayhelp.  Unfortunately I see very few patients who are actually prescribedaminoglycosides in my practice to begin with.     I just though of another patient population that might help answer thequestion (or make it worse!!), depending on if anyone has experience:             How do you determine IBW or aminoglycoside dosing in patientswho are dwarfs or midgets (are those the politically correct names?).Marc Semprebon, RPhAPD Memorial HospitalLebanon, NH`
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• On 20 Jul 1998 at 11:44:12, radfloyd.-a-.pacbell.net sent the message
`Marc,  The IBW equations you described for adults were based on linearregressions of the height weight data from the New York MetropolitanLifetables from 1959 (and a later table published a few years later, samesource).  These were performed by Ben Devine, PharmD about 1972 or 1974when he was a resident at USC School of Pharmacy as an easy way toperform a weight adjustment when dealing with digoxin and/or gentamicinpharmacokinetics.  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 owninterest using the frame sizes and using height measured in centimetersas at the time it appeared that the world would convert to metricmeasure.  I have found that extrapolating the regression line downwardsfor shorter females produces reasonable results.  Note that this takescare of the shorter female who is obese fairly nicely.  Typically forindividuals who have IBW (calculated) less than their actual bodyweight, the actual body weight is generally used.  This later seems towork for many corrections for distribution volume but has not appearedto work well for me in those uncommon situations in which one isrelating renal clearance to body mass.  Good luck!Addenda.  I still have the regressions I did back then and have foundthem useful (not for frame size which requires some effort to determine)but for the standard errors of the estimate for the predictedvalues--this gives one a range for the guess at IBW.`
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