Objective To determine risk factors of medical injury, defined as untoward injury from diagnostic or therapeutic healthcare interventions. Identifying risk factors for medical injuries could inform the development of preventive interventions.

Methods A hospital-based case–control study. Cases and controls were selected among patients discharged from a large Midwestern teaching hospital between 1 January 2003 and 31 December 2004. Cases (n=268) were selected in a three-step process. First, medical injuries in the discharge database were identified using the Wisconsin Medical Injury Prevention Programme Surveillance Criteria. Second, provisional cases were randomly chosen from patients flagged with a medical injury. Provisional controls were randomly selected from patients not flagged with a medical injury, matching for Diagnosis Related Group of the provisional cases. Third, a chart review determined ultimate case–control status. Severity of illness and risk of mortality were calculated using the All Patients Refined–Diagnosis Related Groups system. Zahn's comorbidity score was calculated. Conditional logistic regressions were run with injury status as the dependent variable.

Results Among the 268 cases, 47.8% were procedure-related injuries and 44.8% were medication-related injuries. Conditional logistic regressions found higher severity of illness and higher risks of mortality were related to risk of medical injury (OR 3.29 (95% CI 1.05 to 10.31) and OR 5.16 (95% CI 1.42 to 18.79), respectively). Additional regressions showed the Zahn comorbidity score related to the risk of medical injury (OR 1.63, 95% CI 1.31 to 2.02).

Conclusions Patients with higher severity of illness, higher risk of inpatient mortality and multiple comorbidities are at increased risk for a medical injury.