To our knowledge this is the first study examining primary prescription adherence with medications for obstructive lung disease in adults. It demonstrates robust primary adherence at 76%, which is higher than secondary adherence in adults with asthma of between 30 and 70%, and with COPD of between 40 and 60% [2]. Poor medication adherence in adults with asthma and COPD has been associated with increased disease morbidity, mortality, higher healthcare costs, and reduced quality of life [2, 3, 6].
Depression was associated with reduced prescription adherence, in keeping with previous studies in both asthma and COPD [3, 7]. Studies have documented that prevalence of depression is twice as high among adults with obstructive pulmonary disease [8]. Depression has been independently linked with poorer health outcomes and reduced quality of life, both of which could compound prescription nonadherence [3].
Certain age groups were associated with prescription non-adherence (18–44 years and 65–74 years). Further studies are required to validate these findings to determine if this association is consistent in other settings.
Previous studies have documented a higher income quintile/increased socioeconomic status to increase the rate of medication adherence [9]. In contrast, this study demonstrated lower income quintiles to be associated with increased prescription adherence. While typically lower income reduces access to health care and affordability of medications, Manitoba has means tested pharmaceutical coverage. Our data did not include drug insurance claims data, so we were unable to fully describe the impact of medication coverage.
Medical complexity increased the risk of prescription nonadherence. Medical comorbidities is relatively consistently associated with medication nonadherence [6, 7, 9]. Possible solutions to improve prescription adherence in medically complex patients include ensuring patients understand the importance of not treating one condition at the detriment of another, and improved patient education both in a medical facility and in pharmacies [9].
While ≥ 3 ED visits in the prior year increased the likelihood of prescription adherence, other measures of possible lung disease severity including 1–2 ED visits in the previous year, number of ambulatory visits in the previous year, and number of hospitalizations in the previous year did not increase prescription adherence. Medication adherence in asthma and COPD have been associated with a reduced risk of exacerbations [2, 3], and reverse causation is possible whereby reduced prescription adherence led to an increased likelihood of exacerbations, although the reasons for the ED visits and hospitalizations were not captured in the database. Another possible explanation is poor perception of disease, higher medical complexity, or reduced access to medications, among those at higher risk.
A limitation of our retrospective analysis is that our study population is from a single Canadian province, which may affect external validity. We did not capture medications prescribed by specialists (e.g., pulmonologists), from whom primary adherence rates may be different. As a result of these limitations, it is possible that primary adherence is underestimated. Race/ethnicity, smoking status, health literacy, and level of education were not measured and are possible confounders. A further limitation is that the data cannot separate out whether this medication was prescribed for COPD versus asthma, or for another reason (such as viral infection), nor can we ascertain disease severity and its impact on adherence. We also cannot ascertain whether samples of medications may have been provided. In addition, some who do not meet criteria for pharmaceutical coverage may still find the cost of medications to be a significant barrier to adherence.
In conclusion, this study provides important insights about factors associated with prescription nonadherence and is the first study examining primary medication adherence with medications for obstructive lung disease in adults.
