Drug Dosage Based on Adjusted Body Weight
Adjusted Body Weight is also used for determining drug dosage for some medications. Adjusted body weight (IBWadj) is an adjusted version of IBW. The rationale for using IBWadj is that people above IBW may have a certain percentage of excess weight as lean muscle mass as compared to individuals at IBW. For certain medications IBWad is used if the ABW is 125% of IBW.
Calculation of IBWadj is performed by first determining IBW, calculating the difference between ABW and IBW, multiplying the difference between ABW and IBW with a correction factor, usually 10, 20, 25 or a 40 %. Some common equations used to calculate IBWadj are:
$$ IBW_{adj } = { IBW + (0.1)(ABW - IBW)} $$
$$ IBW_{adj } = { IBW + (0.2)(ABW - IBW)} $$
$$ IBW_{adj } = { IBW + (0.25)(ABW - IBW)} $$
$$ IBW_{adj } = { IBW + (0.4)(ABW - IBW)} $$
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Solved Problem: A physician writes the following order: gentamicin 1.5 mg/kg/dose q8h for a 33-year old man who is 190 kg and 70 inches tall. Gentamicin is a medication dosed based on IBW, however, if a patient is above 125% of their IBW, it is recommended that Gentamicin should be dosed using adjusted body weight with a dose correction factor of 0.4. Calculate the dose of gentamicin for the patient per day. Approach: Calculate IBW and determine if ABW is 125% of IBW. $$ ABW : { 190 \ kg } $$ $$ IBW: 50 kg + (2.3 kg)(10) = { 73 \ kg } $$ $$ {190 \ kg \ (ABW) \over 73 \ kg \ (IBW) } { \times 100 } = { 260 \% }$$ The patient’s ABW is 260 % of his IBW, therefore adjusted body weight needs to be determined. $$ IBW_{adj} = {73 \ + \ (0.4)(190 - 73)} $$ $$ IBW_{adj} = 119.8 \ kg $$ $$ Gentamicin dose = { 119.8 \ kg \ \times }{ 1.5 \ mg \over kg} = 179.7 = 180 mg $$ 180 mg q8h, so in a say 180 * 3 = 540 mg Answer: 540 mg |