Prescribing Calculations

A prescribing calculation converts a therapeutic order into a dose, volume, rate, or schedule that can be delivered safely to one patient. The arithmetic is only one part of the work. A mathematically correct result can still be unsafe if the weight was estimated, the concentration was misread, the order used an ambiguous abbreviation, the renal function was interpreted incorrectly, or the calculated amount exceeds a drug-specific maximum. The clinician must therefore connect the number to the patient. Ask whether the dose fits the indication, route, formulation, age, body size, organ function, current medications, and expected clinical response. A very large volume, an unexpectedly tiny dose, or a tenfold change from the usual range is not merely a calculation problem; it is a signal to stop and investigate the order. Prescribing authority is separate from calculation and administration authority. Nurse practitioners prescribe within the medication standards and scope set by their provincial regulator. Registered nurse prescribing exists only in some provinces and within defined limits. A nurse who does not have prescribing authority may calculate and administer an authorized order...