Background The most common way of introducing decision support in computerised physician order entry (CPOE) systems is warnings on drug combinations that should be avoided, such as drug–drug interactions or duplicate prescriptions. To allow for the CPOE to provide decision support, the system needs structured drug registers and indexes. One index that is commonly used in CPOE solutions is the Anatomical Therapeutic Chemical Classification (ATC) System developed by World Health Organisation (WHO).

Purpose The purpose of this study was to evaluate the use of ATC codes as an index in a CPOE decision support system for duplicate prescribing and other critical warnings.

Material and methods The study demonstrates how the ATC classification as an index gives some incorrect warnings and lack of other relevant warnings for prescribing as there is no 1:1 relation between the ATC code and the active ingredients. This was shown using prescribing examples and knowledge on algorithms for duplicate prescribing and critical warnings in CPOE systems.

Results Some of the identified problems using the ATC classifications can be illustrated following a patient admitted to hospital with the medicines prescribed on admission shown in the first table.

The patient was allergic to paracetamol (ATC: N02BE01) and penicillin (ATC: J01CE02). During the stay 2 new drugs were prescribed to the patient (as shown in the second table).

These examples of new prescriptions will not give rise to any warnings to the doctor for penicillin allergies or duplicate prescriptions of losartan as there is no match between the ATC codes of the drugs involved.

Conclusion Use of the ATC classification as a single index for warnings on duplicate prescriptions and other critical warnings will lead to lack of clinically relevant warnings in the CPOE systems. Manual connexion of ATC codes or using different levels of the ATC code could be a solution, but is time consuming and will not be very reliant over time.

No conflict of interest