A new patient is to be treated with theophylline (actually administering its salt aminophylline; S = 0.8). He is of fairly normal build and weighs 80Kg. Target concentration for theophylline will be 15mg/L. We would calculate an appropriate intravenous dose of aminophylline in three stages.

– First calculate the patient’s likely volume of distribution for theophylline as:

V= 0.48 L/Kg x 80 Kg = 38.4 L.

-Then calculate a theophylline dose as: D=CxV = 15 mg/L x 38.4 L = 576 mg

– Correct for the salt factor Dose = D / S = 576 mg / 0.8 = 720 mg aminophylline

Predicted volumes, based on body weight, tend to be precise enough for practical purposes, so long as patients are of fairly average build. However, with very over-weight patients, their ratio of fat to muscle will be increased. A relatively water soluble drug like digoxin will show limited distribution into fat and a volume estimated from total body weight is likely to be biased upwards. With such drugs, it is better to estimate volume of distribution from a patient’s ideal rather than actual weight.