Finding errors in dosage calculation

Abstract

T topic of this month’s Calculation Skills is checking for errors in calculation, which will focus on intravenous drug administration, a high-risk activity, usually involving two people. One person selects the products (eg. drug and diluent) and performs any associated calculations. The second person must then carry out a second check as a separate and independent action to reduce the risk of missing an error. It is vital that health practitioners can not only perform calculations accurately themselves but also confirm the accuracy of calculations made by colleagues. As part of a second check, health practitioners must start at the beginning and recheck information used as the basis for the calculation, such as the original dosage statement and the strength of the injection solution, in addition to the calculation itself. The strength of parenteral injection solutions is usually stated as an amount per unit volume, such as milligrams per millilitre (mg/ml). The dose of an injection is usually calculated as an amount per kilogram of body weight (mg/kg). Health practitioners are often required to calculate the volume of an injection that contains the dose required for the patient. The National Patient Safety Agency (2007) defines a complex calculation as one where more than one step is required for the preparation and/ or administration of a medicine. Particular care is needed for very potent drugs where the drug strength and/or the dose may be stated in smaller dosage units, such as micrograms (mcg, one-thousandth of a milligram) or even nanograms (ng, one-millionth of a milligram). Conversion between different dosage units may be needed, increasing the complexity of the calculation. The risk is also higher for particular patient groups, such as neonates, children and patients receiving high-intensity care, because individualized dosage calculation is more likely to be required. There is some evidence that health practitioners may become confused when trying to identify the information relevant to the calculation in practice, even if theoretically competent to perform the calculation on paper (Galligan, 2001). It is vital to remain vigilant for unexpected errors. For example, the health practitioner should check that the patient’s body weight used in the calculation is feasible, given a prior knowledge of the patient. If a calculated Alison Eggleton