Drug Dosage Based on Body Weight
Drug dose for certain drugs is based on the weight of the patient. The dose is expressed as the quantity of drug per unit of patient weight such as milligrams of drug per kilogram of body weight or mg/kg. A useful equation for calculating drug dosage based on body weight is:
$$ { Patient's \ Dose \ (mg) = Patient's \ Weight (kg) \times } { Drug \ dose \ (mg) \over 1 \ (kg)} $$
Although mg/kg is a commonly used unit for expressing drug concentration, other units such as μg/kg are also used. In addition, patient’s weight is reported in pounds. The above equation may be used for different units as long as the terms used are consistently applied.
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Solved Problem: The dose of a drug is 20 μg/kg of body weight. What would be the dose for a 200 pound patient? Approach: Convert the pounds to kilograms, use the equation used for calculating drug dosage based on weight. $$ Patient' s \ Weight (kg) = { 200 \over 2.2 } = 90.9 \ Kg $$
$$ { Patient's \ Dose \ (μg) = Patient's \ Weight (kg) \times } { Drug \ dose \ (μg) \over 1 \ (kg)} $$ $$ { Patient's \ Dose \ (μg) = 90.9 \ (kg) \times } { 20 \ (μg) \over 1 \ (kg)} $$ Answer: 1818 μg |
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Solved Problem: The dose of clindamycin for treating a skin abscess is 8-16 mg/kg/day divided into three or four equal doses. Capsule strength available includes 150 mg and 300 mg. What strength, directions and how many capsules should be dispensed for a 10 day course for an 80-lb child. Approach: Convert the pounds to kilograms, use the equation used for calculating drug dosage based on weight. $$ Patient' s \ Weight (kg) = { 80 \over 2.2 } = 36.4 \ Kg $$
$$ { Patient's \ Dose \ (mg) = Patient's \ Weight (kg) \times } { Drug \ dose \ (mg) \over 1 \ (kg)} $$ $$ { Patient's \ Dose \ (mg) = 36.4 (kg) \times } { 8 - 16 \ (m;g) \over 1 \ (kg)} $$ $$ { Patient's \ Dose \ (mg) = 291.2 mg - 582.4 mg } $$ The above dose range has to be divided into 3-4 equal doses. $$ { Patient's \ Dose \ (mg) } = { 291.2 mg \over 3-4} - { 582.4 mg \over 3-4}$$ $$ {Patient's \ Dose \ (mg) } = { 97 - 194 \ mg \ po \ tid \ or \ 73 - 146 \ po \ qid } $$ For this patient, 150 mg capsules, taken three times daily will comply with the dosing guidelines. Answer: Clindamycin 150 mg, # 30 capsules Take 1 capsule by mouth three times a day. |
The above examples, took into consideration patients actual body weight (ABW) to calculate drug dose for the patient. However, many medications do not distribute well into fat because they are hydrophilic, if an obese patient is given a dose based on ABW, it would result in an overdose. Such medication are dosed on the patients ideal body weight (IBW) which is defined as the weight of a patient’s lean body mass. Following equations are used for calculating IBW for males and females:
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Solved Problem: A 6’ man weighs 195 lb. What is his ABW and his IBW in kg? Approach: Convert the pounds to kilograms, use the equation used for calculating IBW for a male $$ Patient's \ Weight \ (kg) \ = { 195 \over 2.2 } = 88.6 \ kg \ (ABW) $$ $$ Male: {50 \ kg + (2.3 \ kg)(12) } = 77.6 \ kg \ (IBW) $$ Answer: 88.6 kg, 77.6 kg |