WHAT IS ALREADY KNOWN ON THIS TOPICHOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICYIntroduction

Asthma is one of the most common chronic diseases, affecting 5.4% of the population globally.1 In 2021, the Centers for Disease Control and Prevention estimated that approximately 20.3 million adults (8.0%) had asthma in the USA. Asthma is an illness with a high economic burden on society, which accounted for $81.9 billion in total costs in 2013.2 Among healthcare expenses for asthma, prescribed medications were the most expensive types of care, constituting nearly half of asthma spending.3 Asthma can be properly managed with asthma medications.1 The goals of asthma management are to achieve good symptom control and minimise risks of asthma-related mortality, exacerbations and side effects from treatment.1 However, the prevalence of uncontrolled asthma remains high.2 Patients with uncontrolled asthma face increased risks of adverse outcomes, including emergency room (ER) visits, hospitalisations, and absenteeism from school and work.1

One factor that contributes to uncontrolled asthma is medication non-adherence.4 5 Adherence to asthma medications tends to be poor, with reported rates of non-adherence varying widely from 22% to 66%.5–7 A study with 5504 patients investigated the adherence with fluticasone/salmeterol combination therapy and showed that patients filled enough medication to cover about 22% of days in a 1-year follow-up period.6 Medication non-adherence is complex and multifactorial and represents a major issue in treating patients with asthma.4 Financial hardship is a major factor which leads to medication non-adherence.4 8 Previous studies suggest that individuals with a low income or low socioeconomic status were more likely to have asthma exacerbations.8 Patients might delay taking or even skip prescribed medications if they are unable to afford treatment costs.4 This phenomenon consequently results in cost-related non-adherence to medications (CRN) and adversely affects their asthma control.

The prevalence of CRN ranges 10%–25% among different chronic diseases in the USA.9–11 Prior evidence demonstrated that patients who reported CRN were at higher risk of hospitalisation and unplanned emergency visits than those without such problems.4 11 Several factors are associated with experiencing CRN, including age, sex, income and comorbidities.9–11 However, most previous studies conducted to investigate CRN were related to chronic diseases in general but were not specific to asthma. The extent to which CRN affects patients with asthma has not been well established. The determinants of CRN among individuals with asthma in the USA also remain unclear. Since economic conditions and medical management of asthma at the individual and population levels have changed over time,1 12 a better understanding of the trends in CRN is crucial in the USA, particularly given that a substantial proportion of the population has healthcare expenses far beyond their means.10

To bridge the study gap, this study aimed to (1) analyse the trend and prevalence of CRN, (2) investigate factors associated with CRN and (3) examine associations between CRN and asthma-related adverse outcomes among adults with asthma in the USA.

MethodsData source

We used data from the National Health Interview Survey (NHIS) from 2011 to 2022. Conducted by the National Center for Health Statistics (NCHS), the NHIS is a nationally cross-sectional survey which represents the US non-institutionalised population.13 Information of the NHIS is collected through questionnaires administered by trained interviewers from the NCHS.13 They collect a broad range of information including demographic, socioeconomic and health conditions.13 The NHIS uses a complex sampling design, which involves stratification, clustering and weighting to provide estimates for the non-institutionalised population in the USA.13 Data from the NHIS are publicly available as deidentified data through the NCHS.13

Study population

Adults with current asthma were included as the study population. We selected adult respondents aged 18 years or older from the Sample Adult file in the NHIS. Current asthma was defined as respondents who answered affirmatively to both of the following questions: ‘Has a doctor or other health professional ever told you that you have asthma?’ and ‘Do you still have asthma?’.14 15

Outcomes

In aims 1 and 2, CRN was the study outcome. CRN was defined as an affirmative response to any of the following questions: (1) ‘Did you skip medication doses to save money over the past 12 months?’; (2) ‘Did you take less medication to save money over the past 12 months?’; and (3) ‘Did you delay refilling medications to save money over the past 12 months?’.9–11 15 These three questions were defined as the components of CRN.

In aim 3, asthma-related adverse events were the study outcomes, including asthma attacks and ER visits for asthma. Asthma attacks were identified by an affirmative response to the following question: ‘During the past 12 months, have you had an episode of asthma or an asthma attack?’. ER visits for asthma were defined by an affirmative response to the following question: ‘During the past 12 months, have you had to visit an emergency room or urgent care center because of asthma?’.14

Covariates

In this study, covariates were categorised into the following groups: age (grouped into 18–40, 41–60 and >60 years old), sex (male and female), race/ethnicity (non-Hispanic white, Hispanic, non-Hispanic black and others), region (Northeast, Midwest, South and West), educational level (less than high school and high school/higher), health insurance (not covered and covered), poverty status, representing family incomes as multiples of the federal poverty level (FPL) (<100%, 100%–<200%, 200%–<400% and ≥400%), work status (employed and unemployed), smoking status (non-smoker, former smoker and current smoker), perceived health status (good and poor), marital status (married or living with partner and single or living alone) and comorbidities (obesity (body mass index of ≥30 kg/m2), hypertension, diabetes mellitus, coronary artery disease, hypercholesterolaemia, chronic obstructive pulmonary disease, liver disease, kidney disease and cancer).

Statistical analyses

Statistical analyses were implemented for each of the study aims. In aim 1, we estimated the prevalence and trend of CRN among US adults with asthma from 2011 to 2022. To obtain national estimates, survey-weighted proportions were used based on analytic guidelines of the NHIS.13 A joinpoint regression analysis was used to evaluate the statistical significance of the trend in CRN during the study period.16 The average annual percentage change (AAPC) was determined with a joinpoint regression analysis, and we reported whether the AAPC was significantly different from zero.16 In aim 2, the Rao-Scott χ2 test was used to assess differences in covariates among adults with asthma who reported CRN and those who did not. In addition, a multivariable logistic regression was used to investigate factors associated with reporting CRN. In aim 3, multivariable logistic regression models were used to examine associations between CRN and asthma-related adverse events, including asthma attacks and ER visits for asthma. Statistical significance was assessed with a two-tailed alpha of 0.05. The joinpoint regression analyses were performed with the Joinpoint Regression Program by the National Cancer Institute, V.5.0.2.17 Other analyses were performed with survey-specific tools using Stata, V18.0 (StataCorp).

Sensitivity analysis

To test the robustness of our study results, we conducted several sensitivity analyses. The three components of CRN were evaluated separately for each aim. In aim 1, we estimated trends in the prevalence of each component of the CRN separately. In aim 2, multivariable logistic regression models were used to assess factors associated with each component of the CRN. In aim 3, multivariable logistic regression models were used to examine associations between each component of the CRN and asthma-related adverse events.

Subgroup analysis

To identify a group that might benefit most from targeted policies or interventions, we conducted a subgroup analysis for Aim 1. Trends in the prevalence of CRN were stratified by respondents’ region (Northeast, Midwest, South and West).

Results

The 2011–2022 NHIS included 378 350 sample adults. Among them, 30 793 adults had asthma, representing approximately 8.1% (19.38 million) of the US population. Table 1 shows the weighted prevalence of CRN from 2011 to 2022 among US adults with asthma. Overall, 17.8% of US adults with asthma reported CRN, representing an estimated 2.99 million of the US population. Among the components of CRN, 11.6% (1.95 million) of US adults with asthma skipped medication doses to save money, 12.4% (2.06 million) took less medication to save money and 15.1% (2.54 million) delayed refilling medications to save money. Figure 1 and online supplemental table 1 show the trend in prevalence of CRN from 2011 to 2022 among US adults with asthma. The prevalence of CRN significantly decreased during the 12-year period (AAPC=−5.0, 95% CI −6.1 to −3.8), with a decline from 23.2% in 2011 to 13.1% in 2022. In sensitivity analyses, this declining trend was also observed in each component of CRN (skipped medication doses to save money: AAPC=−6.5, 95% CI −7.9 to −5.1; took less medication to save money: AAPC=−6.2, 95% CI −7.3 to −5.1; and delayed refilling medications to save money: AAPC=−5.1, 95% CI −6.2 to −4.0). Online supplemental table 2 provides detailed results from the joinpoint regression analyses.

Table 1

Prevalence of cost-related non-adherence to medications among US adults with asthma

Figure 1

Trends in the prevalence of cost-related non-adherence to medications among US adults with asthma. Arrows indicate the joinpoints identified from joinpoint regression analyses. (A) Overall. (B) By components of cost-related non-adherence to medications.

Table 2 demonstrates the characteristics of US adults with asthma. Adult patients with asthma aged over 60 years were the least likely to report CRN compared with those aged 18–40 and 41–60 years (21.5% vs 18.7% and 21.5%, p<0.01). Females were more likely to report CRN than males (19.5% vs 14.4%, p<0.01). Among the race/ethnicity groups, non-Hispanic blacks were the most likely to report CRN. In addition, characteristics significantly associated with higher percentages of CRN included residing in the South, having an educational level of less than high school, lacking health insurance coverage, having a family income of 100%–<200% of the FPL, being a current smoker, having poor perceived health status and being single or living alone. As to comorbidities in adults with asthma, liver disease was the most commonly reported among those who experienced CRN. Patients with two or more comorbidities were the most likely to report CRN compared with those with one comorbidity and those with no comorbidities (20.3% vs 16.4% and 14.3%, p<0.01).

Table 2

Characteristics among US adults with asthma: descriptive statistics

Figure 2 and online supplemental table 3 present results of the multivariable logistic regression model. After the adjustment for all covariates and the survey year, patients with asthma who were younger, female, resident of the Midwest, South or West, with an educational level of less than high school, without health insurance coverage, with a family income of <400% of the FPL, a current or former smoker and with comorbidities were associated with higher odds of reporting CRN. Those with a good perceived health status were associated with lower odds of reporting CRN. In the sensitivity analysis, we investigated factors associated with the three components of CRN (online supplemental table 4). Results were similar to those in the main analysis and showed that age, sex, region, educational level, health insurance coverage, family income, work status, smoking status, perceived health status and comorbidities were predictors for reporting CRN.

Figure 2

Characteristics associated with cost-related non-adherence to medications among US adults with asthma. FPL, federal poverty level.

Figure 3 shows associations between CRN and asthma-related adverse events. More than half of the patients reporting CRN experienced asthma attacks. Approximately 19.0% of patients reporting CRN visited the ER for asthma. Adults with asthma who reported CRN had significantly higher odds of having asthma attacks (adjusted OR, 1.95; 95% CI 1.78 to 2.13) and ER visits for asthma (adjusted OR, 1.63; 95% CI 1.44 to 1.84) compared with patients without CRN. These results were consistent with those in the sensitivity analysis examining the three components of CRN (online supplemental table 5). Adults with asthma who skipped medication doses, took less medication and delayed refilling medications to save money had nearly two times the odds of having an asthma attack than those without CRN. Similarly, adults with asthma who reported any components of CRN had approximately 1.6-times higher odds of visiting the ER for asthma than those without CRN.

Figure 3

Associations between cost-related non-adherence to medications and asthma-related adverse events among US adults with asthma. aOR, adjusted OR; CRN, cost-related medication non-adherence; ER, emergency room.

In the subgroup analysis, decreasing trends in the prevalence of CRN over time were found across all regions among patients with asthma. Notably, those residing in the South exhibited the smallest decline in CRN prevalence (AAPC=−5.0, 95% CI −7.0 to −3.2) (online supplemental tables 6 and 7).

Discussion

Using nationally representative data, we provided insights into the prevalence, trend, factors and impacts of CRN among patients with asthma. We found that nearly one in six patients with asthma was non-adherent to medications due to cost. Over the past 12 years, there has been a declining trend in the prevalence of CRN among patients with asthma in the USA. Several factors, including demographics and comorbidities, were associated with CRN among asthma patients. Patients with CRN were at a high risk of experiencing asthma-related adverse events.

In this study, we estimated that 17.8% of patients with asthma experienced CRN, which was higher than the prevalence observed in patients with other chronic diseases.9 10 Kang et al used Behavioral Risk Factor Surveillance System data from 2013 to 2014 and found that 16.5% of US adults with diabetes experienced CRN.9 Previous studies have showed that adherence to medications was poor in patients with asthma.5–7 With a nationally representative population, our findings support that cost might be an important consideration. Xia et al also reported that 15.8% of patients with a history of asthma were non-adherent to medications due to cost considerations, which was comparable to our estimates.15 The present study reinforces the recommendation that patients with asthma are best controlled when they are prescribed and take medications that are strongly recommended by clinical guidelines.1

From 2011 to 2022, we observed a declining trend in the prevalence of CRN among patients with asthma. This trend might be attributed to healthcare policies in the USA. In 2010, the Patient Protection and Affordable Care Act was enacted which aimed to improve access to health insurance and services.12 18 A significant strategy in the Affordable Care Act was the expansion of Medicaid, up to 138% of the FPL, with the aim of improving health outcomes for minorities.12 18 Shi et al found that Medicaid expansion was associated with increased utilisation of asthma medications among low-income patients with asthma in the USA from 2007 to 2018.12 With fewer barriers to accessing healthcare, including medications, patients may be more willing to fill prescriptions for medications to control their asthma than before, potentially leading to a decrease in CRN.18 Further research comparing data before and after the implementation of the Affordable Care Act is needed to evaluate its impact on CRN among patients with asthma.

Our findings showed that female sex and younger age were associated with CRN, which were consistent with previous studies.9–11 15 Our findings add to evidence that the gender disparity remains an issue in CRN among patients with asthma. Although younger patients might have fewer medical problems and be actively employed,19 they still had nearly two times the odds of experiencing CRN. Possible explanations could be lower earnings, making it harder to cover healthcare costs, or because they are less likely to be in a marriage or partnership where health responsibilities are shared. Adults aged 30–44 are more likely to be enrolled in high deductible health plans, which require individuals to pay more out of pocket for healthcare needs until the deductible is met.20 These health plans are often associated with greater cost-related non-adherence.21 We found that low-income or uninsured patients with asthma were at increased risk for CRN. In such vulnerable populations, financial hardships could contribute to non-adherence. They might try to skip or reduce medication doses to save money.22 Without health insurance coverage, the cost of prescribed medications could result in higher out-of-pocket expenses for them, making it more challenging to afford necessary healthcare utilisation. Notably, patients with 100%–200% of FPL had the greatest CRN. This near-poor population often does not qualify for Medicaid in the USA, which might increase barriers to access healthcare.23 24 For example, a US national report showed that 23.9% of near-poor individuals were uninsured, compared with 26.2% of the poor population.23 Decreased health insurance coverage might make near-poor patients more vulnerable to CRN.23 Therefore, targeted policies to support this near-poor population with asthma are warranted to ensure that they are not left behind.

In our study, we found patients residing in the South and Midwest were associated with increased odds of experiencing CRN. Additionally, individuals from the South exhibited the highest CRN prevalence in most study years and the smallest decline in prevalence, which might be explained by differences in access to healthcare across geographical regions.25 26 People living in the South, particularly in rural areas, might face reduced healthcare utilisation due to financial burdens.26 Furthermore, residence in the South was associated with an increased risk of uncontrolled asthma,25 which is possibly reflected by the increased CRN rates observed among Southern residents. Therefore, targeted policies or interventions to reduce CRN may consider prioritising those residing in the South. Patients with asthma who had lower educational levels reported CRN more frequently than those with higher educational levels. The level of education is associated with a number of factors that impact health outcomes, including health literacy.27 Patients with a low health literacy might not be aware of the importance of routine medication use for their asthma control. This finding also underscores the need for healthcare providers to actively encourage their patients to take prescribed medications regularly for better asthma control. In this study, we demonstrated that asthma patients with comorbidities were more likely to experience CRN than those without comorbidities. This finding aligns with previous research on patients with other chronic diseases.9–11 Patients with comorbidities often experience a high pill-burden.28 29 This problem not only causes high medication expenses but also discourages them from taking medications on time,28 ultimately leading to medication non-adherence due to costs. These findings underscore the need for health policy interventions to alleviate financial constraints due to medication expenses. Future studies can also investigate the financial burden of patients with asthma who experienced CRN.

Our study demonstrated that patients with asthma who experienced CRN are at increased risk of asthma-related adverse events, including asthma attacks and ER visits. This finding was consistent with previous studies.30 31 Williams et al showed that 24% of asthma exacerbations were attributable to non-adherence to inhaled corticosteroids.30 Focusing on cost considerations, our results revealed that non-adherence was still an issue in poor asthma control. For healthcare providers, it is imperative to monitor patient’s adherence to medications to prevent asthma exacerbations, especially when treating patients with financial concerns. As the continuous use of asthma medications is associated with substantial costs in the long run,2 32 understanding patient’s economic conditions and preferences is important. For example, shared decision-making with patients before initiating treatments can serve as a valuable approach.33 This collaborative process allows patients to choose medications that are affordable, potentially improving adherence and achieving better asthma control. Moreover, employing real-time prescription benefit tools to provide prescribers with patient-specific drug pricing and alternatives at the point of prescribing could be beneficial.34 These tools may help improve medication adherence and reduce prescription drug costs,34 ultimately leading to a decrease in CRN.

Our findings also carry important policy implications. First, healthcare policies related to frequently monitor CRN rates are necessary, particularly for underserved populations with asthma, such as those without health insurance or living in the South. Furthermore, collaboration with public health experts can help establish targeted CRN prevalence goals to be achieved through policy interventions. Reducing CRN may not only improve asthma control but also alleviate the financial burden on the healthcare system.35 In 2002, the annual cost of non-adherence among patients with asthma to the US healthcare system was estimated at $300 billion.35 A US study further demonstrated that a 25% improvement in adherence was associated with a 10% reduction in the odds of asthma-related ER visits or hospitalisations.36 Therefore, ongoing efforts to enhance medication adherence are essential to reduce its economic impact on patients with asthma. In the USA, several state governments have implemented laws which cap the cost of asthma inhalers for patients at between $25 and $50 starting in 2025,37 while some pharmaceutical manufacturers have introduced caps on out-of-pocket spending for asthma inhalers for certain patients.38

There are several strengths of this study. First, our study was conducted with a nationally representative survey dataset, which enhanced our generalisability to the overall US population. Furthermore, by using data from 2011 to 2022, we provided a comprehensive view of the trend and prevalence in CRN among US adults with asthma over the past 12 years, as well observations of public policy that expanded health insurance access during that time. However, there are some limitations to our study. First, the NHIS relies on self-reporting by respondents, and recall bias might exist. Without medical records or diagnostic tests, individuals’ disease status might be misclassified. Future research using data sources that can be linked to clinical healthcare records may mitigate disease misclassification. Second, due to the cross-sectional design, the results can only be interpreted as associations rather than causal relationships. Third, we were unable to evaluate the effect of asthma severity due to limited measures in the NHIS. It is possible that the severity of asthma might affect a patient’s financial determinants of health, and in turn lead to impacts on CRN. Fourth, due to limited measures in the NHIS, we could not include variables reflecting economic indicators, such as the consumer price index.39 Future studies may consider adjusting for factors that account for changes in medication prices when assessing CRN. Fifth, as joinpoint regression analysis was primarily designed to evaluate the direction of trends,16 we could not control for covariates in the trend analysis. Therefore, further validation using different methods for trend analysis is necessary to avoid potential overestimations or underestimations of trends in CRN. Sixth, questions regarding CRN in the NHIS were broad and based on the overall medications taken by respondents. It is possible that the cost of asthma medication might not be the driver for CRN, but rather a costlier non-asthma comorbidity. However, we adjusted for the number of comorbidities to account for the effects of non-asthma medications. Further investigation into CRN specific to asthma medications is required. Finally, NHIS questions about CRN did not specify whether respondents with CRN shared or took other’s medications to overcome their cost constraints.40 Patients engaging in such behaviours might still have good asthma control due to continued medication use,40 which could potentially underestimate the impact of CRN on asthma control.

In conclusion, we observed a declining trend in the prevalence of CRN over the past 12 years, during which time nearly one in six US adults with asthma was non-adherent to medications due to costs. Several factors, including demographics and comorbidities, were found to be determinants of CRN. Patients with asthma who had CRN were at an elevated risk of experiencing asthma-related adverse events. Since asthma is one of the leading chronic diseases, the burden in cost-related non-adherence to medications highlights the need for appropriate policies and social support to address such problems.

Data availability statement

No data are available.

Ethics statementsPatient consent for publication

Not applicable.

Ethics approval

This study was reviewed and an exemption was obtained from the Taipei Medical University Joint Institutional Review Board (ID: N202402009).