Objective The objectives of this cross-sectional study were to estimate the prevalence of unintended discrepancies between three sources of prescription information and to describe the types of electronic prescribing system vulnerabilities identified.

Methods Staff from community pharmacies identified approximately 200 new prescriptions written at three participating ambulatory care clinics (2 adult, 1 paediatric). Unintended discrepancies were identified by comparing three sources of prescription information: (1) the prescriber's note as documented in the patient's chart; (2) the electronic prescription (e-prescription) entered into the clinic's electronic prescribing software; (3) the medication that was ultimately dispensed by the pharmacy as indicated on the prescription label. The discrepancy rate was calculated by dividing the number of discrepancies identified by the number of prescriptions evaluated.

Results A total of 602 prescriptions written by 33 prescribers were evaluated from the 3 ambulatory care clinics. The discrepancy rate between the prescriber's note and the e-prescription was 1.7%, 0.6% and 3.9% for the three clinics. The discrepancy rate between the e-prescription (clinic) and the prescription label (pharmacy) was 4.2%, 0.9% and 1.5%. Differences between directions for administration was the most common type of discrepancy identified.

Conclusions Discrepancy rates between the prescriber's note and the e-prescription were similar to the discrepancy rates between the e-prescription and pharmacy label. To reduce outpatient medication errors, a better understanding is needed of the sources of discrepancies that occur within the prescriber's clinic, and those that occur between the clinic and pharmacy.