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Ideal Body Weight & Adjusted Body Weight


Ideal body weight (IBW) and adjusted body weight are frequently used clinically to adjust drug dosing, help estimate renal function and the pharmacokinetics in morbidly obese patients. The most common IBW equation used is the Devine formula.
Height     Weight:
Gender:
Select option for calculating the IBW for patients under 60 inches
(default option - BMI method) - ignore for all other patients.

   [See reference section]

Background

Ideal body weight

Estimate Ideal body weight in (kg)
Males: IBW = 50 kg + 2.3 kg for each inch over 5 feet.
Females: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet.


Reference:
Devine BJ. Gentamicin therapy. DICP. 1974; 8:650–5.

Adjusted body weight

AjBW = adjusted body weight:
AjBW = IBW + 0.4( ABW - IBW)


Reference:
1) Bauer LA. Applied clinical pharmacokinetics. New York: McGraw Hill, Medical Publishing Division; 2001:93-179.

2) Winter, M.E., 2004. Basic pharmacokinetics. London: Lippincott Williams and Williams.


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Background info for height less than 60 inches top of page

If the actual body weight is less than any of the calculation methods, the actual body weight will be used.
Discussion of the various methods:   The output of this section is based on research I had completed ~20 years ago on this subject.  A quick review of the recent literature has not changed or added any new methods for estimating an ideal body weight for patients less than 60 inches tall.  Note: naming convention is based on my earlier work...

1] Intuitive Method:
Reference: Murdaugh LB. Competence Assessment Tools for Health-System Pharmacies. 5th ed. Bethesda, MD: ASHP; 2015. [Chap:29 Medication dosing in Patients with Renal Dysfunction]
IBW (Male) = 50kg - 2.3kg for each inch below 60 inches
IBW (Female) = 45.5kg - 2.3kg for each inch below 60 inches
Comments:  For patients just a few inches below 60 inches, the result is reasonable, however, 2.3 kg/inch is excessive when used for shorter heights.  At 38 inches for a male, and 40 inches for a female, the IBW is ZERO.   This provides support for the next method below.

2] Baseline Method
The baseline method starts with the initial ideal body weight baseline values e.g. 60 inch male patient - 50kg and 60 inch female patient - 45.5kg.   Male patient: 50kg /60 inches = 0.833 kg/inch.  Female patient = 45.5kg/60 inches = 0.758 kg/inch.   Therefore a male patient - 55 inches: IBW = 50kg - (0.833 x 5) = 45.8kg versus the first method = 50kg - (2.3 x 5) = 38.5 kg.
[Reference: reasonable assumption based on the standard ideal body weight equations and the baseline weights established for a height of 60 inches.
Also review: Murphy JE. Introduction. In: Murphy JE, ed. Clinical Pharmacokinetics, 5th ed. Bethesda, MD: American Society of Health-System Pharmacists, 2011:xxxiv. - Note: for patients who are less than 60 inches tall, the weight should be decreased more conservatively than 2.3kg/inch.]

3]  BMI method

References:
Wiggins, K. L. (2004). Renal care: Resources and practical applications. Chicago: American Dietetic Association. pg 12.

Barash, P. G., Cullen, B. F., & Stoelting, R. K. (1989). Clinical anesthesia. Philadelphia: Lippincott. chap:47:1231

Remember that BMI = weight(kg)/height2 (meters squared).   Next, we will establish an 'ideal' BMI based on values in the standard IBW equations: Male: 60 inches - 50kg - BMI= 21.53.   Female: 60 inches - 45.5kg - BMI= 19.59.  We can then use this association to generate an equivalent ideal weight based on this standardized BMI and the height of the patient.  Using the example above (55 inch male patient):
IBW = 21.53 (BMI value above)  x (55 x 0.0254)2  = 42 kg.
Background info: the body mass index quantifies the amount of tissue mass at a particular height (units: kg/m2).  Example: the following patients all have a BMI ~ 21:  130 lbs - 5'6",  163 lbs - 6'2",  107 lbs - 5'.

BMI

19

20

21

Height
(inches)
Body Weight (pounds)
58 91 96 100
59 94 99 104
60 97 102 107
61 100 106 111
62 104 109 115
63 107 113 118
64 110 116 122
65 114 120 126
66 118 124 130
67 121 127 134
68 125 131 138
69 128 135 142
70 132 139 146
71 136 143 150
72 140 147 154
73 144 151 159
74 148 155 163

4] Hume method
LBW (Males) = (0.3281 x Weight in kg) + (0.33939 x Height in cm) - 29.5336
LBW (Females) = (0.29569 x Weight in kg) + (0.41813 x Height in cm) - 43.2933
Using the example above:   (55 inch male patient):  IBW= 36.9 kg

Reference: Hume R.  Prediction of lean body mass from height and weight.  J Clin Path(1966), 19, 389.
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