United States of America. The ISMP encourages prescribers to include in written orders the mg/kg or mg/m2 dose with the calculated drug dose as an effective error prevention strategy for geriatric, paediatric and oncology patients. Thus an independent calculation can easily confirm accuracy, as long as it is performed. The importance of double checking dosages was highlighted by a case reported to the ISMP.

A recent error caused a significant overdose of methylprednisolone even though the prescriber (the physician’s assistant) included the mg/kg dose. After an 80-year-old patient was admitted to the hospital for cervical neck surgery, the prescriber wrote an order for “methylprednisolone 10.6 g (30 mg/kg) over one hour IVPB prior to surgery”. The patient weighed about 70 kg. However, neither the pharmacist who entered the order into the computer system, nor the nurses who transcribed the order and administered the medication, independently checked the prescriber’s calculated dose. Had they done so, they would have identified that the correct dose was 2.1 g, not the ordered dose of 10.6 g.

Additionally, the same prescriber wrote a second order for “methylprednisolone 1.7 g (5.4 mg/kg) over 8 hours IVPB intra-op and post-op”. In this case, the correct dose was 5.4 mg/kg/hr. However, the prescriber did not include the per hour designation. Thus the total calculated dose was incorrect. Again, the pharmacist and nurses failed to verify the dose calculation, which should have been 3 g (not 1.7 g) infused over 8 hours. The surgeon discovered both errors later that day when he co-signed the physician’s assistant’s orders. The patient had already received a massive overdose from the initial infusion which resulted in significant hyperglycaemia and hypokalaemia. The patient was monitored for additional adverse effects and recovered without injury.

All prescribers should be encouraged to include the mg/kg or mg/m2 dose upon which they base the calculated drug dose. Just as important, pharmacists and nurses must take responsibility for independently double- checking the calculated dose mathematically rather than just by looking at it. Practitioners should also initial the order to show that they have verified the dose. Additionally, these errors might have been detected during order entry if the pharmacy computer system had been instructed to include warnings for doses that are either too low or too high.

Reference: ISMP Medication Safety Alert!, Vol. 4, Issue 8, 21 April 1999.