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Drug units and calculations

Please note OSCEstop content is for educational purposes only and not intended to inform clinical practice. OSCEstop and authors take no responsibility for errors or the use of any information displayed. 

Unit conversions

NB: prior to performing calculations, always convert all values the same units.

Drug dilutions and concentrations

  • Ratios = unit of solute to units of solvent or grams of solvent in mls of solution (because 1g = 1ml)
    • Ratio = grams of drug in mls of solution
    • e.g. 1:10,000 adrenaline = 1 unit adrenaline to every 10,000 units saline or 1g adrenaline in every 10,000ml saline
  • % solutions = grams of solvent per 100ml solute (because 1ml = 1g)
    • % = grams of drug per 100ml of solution 
    • e.g. 0.01% adrenaline = 0.01g adrenaline per 100ml saline (or 1g adrenaline per 10,000ml)

Weight-dosage calculations

  • Dose required (mg) = weight-dosage (mg/kg) x weight (kg)
    • e.g. the weight-dosage of a phenytoin infusion in status epilepticus is 20mg/kg and the patient weighs 80kg. The dose required is therefore 20 x 80, which is 1600mg.

Infusion rate calculations

Infusion time/rate

  • Infusion time (minutes) = total dose required (mg) / drug rate required (mg/min)
    • e.g. the infusion rate of phenytoin in status epilepticus is 50mg/minute and the total dose required for an 80kg patient is 1.44g (1600mg). The infusion time is therefore 1600 / 50, which is 32 minutes.
  • Infusion rate (ml/hour) = dose rate (mg/hour) / concentration (mg/ml)
    • e.g. a 5g (5000mg) magnesium infusion is prescribed to run at 1g/hour (1000mg/hour). The 500ml infusion bag contains magnesium at a concentration of 10mg/ml. The infusion rate is therefore 1000 / 10, which is 100ml/hour.

Infusion set drop rate

  • Drops per minute = total drops in bag / infusion time (minutes)
    • Where total drops in bag = bag volume (ml) x drops per ml (usually 20 but depends on giving set)
    • e.g. a 1L (1000ml) bag of normal saline is prescribed over 8 hours (480 minutes). The giving set runs fluid through at 20 drops per ml and so the total drops in the bag are 1000 x 20, which is 20,000. To run the infusion over 8 hours, the rate required in drops per minute is 20,000 / 480, which is 42 drops per minute. This is approximately 10 drops in 15 seconds (42 / 4), and the giving set clamp can be adjusted to give that rate.

Number of tablets/liquids needed

What you need to give (number of tablets or ml) = dose you want / dose of what you’ve got (dose per tab or dose per ml)

  • What you need tp gove = dose you want / dose of what you have
  • e.g. you want to give 40mg prednisolone. Only 5mg tablets are available. The amount you need to give is therefore 40 / 5, which is 8 tablets.

Test yourself

You would like to load a patient with IV phenytoin for status epilepticus. The patient has a low BMI and weighs 37.5kg. You look up the phenytoin loading dose and it is 20mg/kg.

How many milligrams of phenytoin does this patient require?

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The IV injection needs to be diluted in normal saline at 10mg/ml. How many mls of normal saline is required?

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You look up the rate and determine the infusion should be administered at 50mg/minute. At what rate should you set the infusion machine to run the infusion at (in ml/minute)?

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If an infusion machine was not available, how many drops per minute would you set the infusion giving set to run at? The giving set specifies 20 drops/ml.

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