Gaps in follow-up of abnormal test results are known to contribute to delays in diagnosis in primary care, yet primary care practices still lack standard processes to detect and manage abnormal test results. In this study, investigators identified specific abnormal test results requiring follow-up and tested an escalating strategy of communicating with primary care physicians about test results. The study team first sent a secure email with test results to providers, and if the appropriate diagnostic follow-up action did not occur within one week, they made up to three attempts to reach providers by telephone. Email spurred about 11% of providers to act, and more than two-thirds of providers followed up after receiving telephone calls. For the handful of providers who did not act in response to the email or telephone calls, investigators contacted clinic directors. However, even with this patient-specific communication intervention, follow-up of abnormal test results remained incomplete. These results demonstrate that communicating abnormal results to primary care providers is not sufficient to achieve optimal follow-up. As recommended in the Improving Diagnosis report, team-based results management or technological approaches may be needed to assist primary care providers in tracking and following up on outpatient results to promote timely and accurate diagnosis.