Objective To evaluate the effectiveness of a computerised self-adjusting anticoagulant algorithm to predict appropriate warfarin dosing and to assess its use in clinical practice.

Design A 3-year audit of anticoagulant control in patients managed by doctors and pharmacists using computer decision support and an evaluation of the impact of dose adjustments made by the users.

Participants 3660 patients on oral anticoagulants; one-third of patients managed by doctors and two-thirds by pharmacists.

Setting Anticoagulant supervision in primary care and pharmacies at 60 sites in New Zealand.

Main outcome measures The time in the therapeutic range (TTR), the outcome of adherence to the computer dosing algorithm, the percentage of time the clinicians over-ride the algorithm and the impact of their intervention on anticoagulant control.

Results A TTR of 72.9% was achieved for all patients. The TTR was significantly better in patients managed by pharmacists than doctors (75.1% versus 67.4%, p<0.0001). The computer algorithm provides appropriate dose recommendations for INR results from 1.5 to 4. Users administered a dose that differed from the computer recommendation 23.3% of the time. The doctors adjusted the dose more frequently (28.2% versus 21.1% of tests) and made larger dose changes than the pharmacists.

Conclusions The clinicians predominantly change the dose when the INR is below the therapeutic range. The changes are not necessary to correct for inaccuracies in the algorithm. The most likely explanation is the clinician's belief that their own dose adjustment would achieve better control; however, in practice, their changes tend to underdose patients. The doctors achieved poorer control than the pharmacists; this is in part due to the action of the doctors over-riding the algorithm. Our results imply that clinicians could achieve better anticoagulant control if they more closely followed the computer algorithm.