This section includes information about units, converting dosage units and the basic calculation skills you need as a prescriber. There are lots of activities to test your understanding and give you an opportunity to practice calculations for common scenarios encountered in practice. The most common type of medication error is using the wrong dose, strength or frequency so it is key to get these basic principles right, first time, every time. See the section on medication errors for more detail on this important area.

The Prescribing Skills Assessment (PSA) exam also includes questions on calculation skills so practicing these will help you prepare for the exam. It is worth noting under normal circumstances you would not have time pressure to complete calculations and in practice you should take your time and get a second check if unsure


Using unit abbreviations

 

Table: Clinical Purpose

 

Unit abbreviation Unit
kg kilogram
g gram
mg milligram
microgram should always be written as microgram not mcg
nanogram should always be written as nanogram not ng
L litre
ml or mL millilitre
units or international units should always be written as units or international units; never use ‘iu’ or 'u' e.g. insulin units
mMol or mmol millimoles

 

Units of Lantus


Unit abbreviations

Look at the real example to illustrate the use of units of Lantus (a type of long-acting insulin) - is it 100 units or 10 units?

A note on decimal points; where possible avoid using fractional doses e.g. levothyroxine 0.05mg should be written as 50 microgram or digoxin 0.125mg should be written as 125 micrograms. Never use decimal points unnecessarily e.g. bisoprolol 5.0mg. Sometimes you may need to convert between imperial measurement and the metric system. These are included in the table below for information (e.g. in paediatrics parents will often talk about ounces (oz) of milk in feeds so you would need to know how this converts to mls).

 

Table: Equivalent units and conversions

 

Weight
1kg = 1000 g
1g = 1000 mg
1mg = 1000 micrograms
1 microgram = 1000 nanograms
1 oz = 28g
1 Ib = 16 oz = 450g = 0.45kg
Volume
1L = 1000 ml
1ml = 1g (approx.) so 1L weighs 1kg or 1000g
1 fl oz = 28ml
1pint = 20 fl oz = 568ml
Length
1 inch = 2.54cm
1 foot = 12 inches = 30.48cm

TIP: When converting dosage units always use metric units for weight and volume

Take great care with decimal placement as incorrect conversion can result in 10 to 100 fold errors which could prove fatal.

Copyright eBook 2019, University of Leeds, Leeds Institute of Medical Education.