Factors associated with adverse drug reaction occurrence and prognosis, and their economic impacts in older inpatients in Taiwan: a nested case–control study

OBJECTIVE: Older patients are likely to have higher disease complexity and more drug prescriptions of which are associated with a higher incidence of adverse drug reactions (ADR). This study aimed to investigate factors associated with ADR occurrence, prognosis and medical expenses in older inpatients. DESIGN: A nested case–control study. SETTING: A medical centre located in north Taiwan. PARTICIPANTS: 539 reported ADR cases from a patient cohort containing 108 548 older inpatients were collected from 2006 to 2012. There were 1854 non-ADR matched controls; a maximum of 1:5 matched by age, sex and principal diagnosis were collected. EXPOSURE: Polypharmacy, the number of drugs prescribed, comorbidities and the admission department were factors associated with ADRs, as well as subsequent poor prognosis, length of stay and medical expenses. PRIMARY AND SECONDARY OUTCOME MEASURES: ADR occurrence and poor prognosis (mortality, discharge against medical advice in critical conditions, or admitted to intensive care unit) were the primary outcomes. Additional medical expenses and the length of hospital stay were the secondary outcomes. RESULTS: The admission department, number of comorbidities and number of drug prescriptions before ADRs were associated with ADR occurrence among older inpatients. ADR severity was a significant prognostic factor among ADR cases. The multivariate-adjusted OR of 1.63 (95% CI 1.36 to 1.95) for poor prognosis was found as the number of comorbidities increased. Patients prescribed ≥11 drugs including psychoactive drugs showed 2.45-fold (95% CI 1.40 to 4.28) poorer prognosis than other patients. ADRs caused the addition of US$1803.8, US$360.8 and 5.6 days in total medical expenses, drug expenses and length of stay among affected older inpatients, respectively. CONCLUSIONS: The number of comorbidities and polypharmacy including the use of psychoactive drugs has significant impacts on ADR occurrence and prognosis among older inpatients. The findings provide clues for future prescription modification and patient’s safety improvement in geriatric care.