- Dosing errors can have a significant impact on the patient. Neonates, infants and children are particularly vulnerable.
- There are risks associated with failing to ascertain and monitor accurate weights of children and adults. There are many online calculators to assist with imperial-metric conversions.
- One of the most common calculation errors occurs when the dose units are converted.
- Base your calculations on reputable and up-to-date reference sources and read dose recommendations carefully and in full.
- It is good practice for a second practitioner to independently check your calculations to minimise the risk of error.
- Calculated doses should not exceed the adult dose, or maximum daily dose, especially when dosing for neonates and children.
- Beware of medicines that require the dose, or the weight used to calculate the dose to be capped.
- If the product information requires the dose to be calculated on a 'per kilogram' basis, but does not specify a type of weight to use (such as lean or ideal body weight) then use actual body weight.
- Ensure the dose you have calculated and prescribed is measurable; round the dose if necessary to aid administration.
Common errors include:
- Miscalculating the concentration of a medicine in a solution.
- Moving the decimal point in the wrong direction when converting dose units, for example, milligrams to grams.
- Over or under estimating the time interval between doses.
- Miscalculating the number of tablets to be administered.
- Miscalculating the rate of administration.
Additional risks in paediatrics:
- Doses need to be calculated based on age and weight.
- Dosing recommendations can be misinterpreted if the information is not read in full (e.g. confusion between 50 mg/kg/day and 50 mg/kg/dose).
- Liquid preparations are often required, and these can vary in strength.
- There can be confusion between ml and mg for the prescriber or for the person administering the dose. Doses should be written in mg unless otherwise specified by the BNFC.
- If medicines are not licensed in a liquid form, the use of non-standard, unlicensed preparations can cause confusion.
- A small dosing mistake, with minimal effect in an adult, can have a significant effect in a child.
| Approach | Details |
|---|---|
| Independent self-check | Carried out at away from original prescriber Avoid leading questions like "this is right, isn't it?" |
| Sense check | What is the approximate answer you are expecting from the calculation? Ask yourself if the dose seems reasonable to prescribe for the drug in question. Check that the calculated dose does not exceed the adult dose, or maximum daily dose, especially when dosing for neonates and children. |
| Imperial | Metric |
|---|---|
| lb | 450 g |
| stone (14 lb) |
6.35 kg |
Check the SPC for which one to use
| Description | Represents... | Use for drugs that distribute into |
|---|---|---|
| Actual body weight (Total) |
Observed weight on a set of scales Good when patient not at extremes of cachexia or obesity |
Fat |
| Lean body weight | Subtract body fat weight from actual body weight | Water |
| Ideal body weight | Calculated based on formula to take into account patients height etc. | Water |
| Adjusted body weight | Fits inbetween Ideal and Actual | ? |
Emergency weight estimates
| Age | Formula |
|---|---|
| 0 to 12 months | (0.5 x age in months) + 4 (kg) |
| 1 to 5 years | 2 x age in years) + 8 (kg) |
| 6 to 12 years | (3 x age in years) + 7 (kg) |
Used for toxic therapies, Systemic Anti-cancer drugs and antibiotics
| Units | What is 1 % | Used in |
|---|---|---|
| % w/w | 1 g in 100 g | Solid and semi-solids |
| % w/v | 1 g in 100 mL | Solid mixed in Liquids |
| % v/v | 1 mL in 100 mL | Liquid mixed in Liquids |
1:1000 represents 1 g in 1000 ml 1:10000 represents 1 g in 10000 ml