Background Unintended consequences of computerized patient care system interventions may increase resource utilization, foster clinical errors, and reduce users' confidence.

Objective To evaluate three successive interventions designed to reduce serum magnesium test ordering through a care provider order entry system. The second, modeled after a previously successful intervention, caused paradoxical increases in magnesium test ordering rates.

Design A time-series analysis modeled weekly rates of magnesium test ordering, underlying trends, the impact of the three successive interventions and the impact of potential covariates. The first intervention exhorted users to discontinue unnecessary tests recurring more than 72 hours into the future. The second displayed recent magnesium, calcium and phosphorus test results, limited testing to one test instance per order, and provided education regarding appropriate indications for testing. The third targeted only magnesium ordering, displayed recent results, limited testing to one instance per order, summarized indications for testing, and required users to select an indication.

Participants Clinicians at Vanderbilt University Hospital, a 609-bed academic inpatient tertiary care facility, from 1998 through 2003.

Measurements Weekly rates of new serum magnesium test orders, instances, and results.

Results At baseline, there were 539 magnesium tests ordered per week. This decreased to 380 (P=0.001) per week after the first intervention, increased to 491 per week (P<0.001) after the second, and decreased to 276 per week (P<0.001) after the third.

Conclusion A clinical decision support intervention intended to regulate testing increased test order rates as a result of unintended decision support. CPOE implementers must carefully design resource-related interventions and monitor their impact over time.