At present, China is under the realistic background of huge differences in the utilization of medical services and the policy background of the government constantly promoting the equalization of medical services [29]. Deepening the research on the unequal utilization of medical services is conducive to formulate more reasonable health policies. Based on Roemer's framework of equality of opportunity, we think that the imbalanced utilization caused by individual health needs is reasonable. Actually, there is unreasonable inequality in health service utilization, but the extent of inequality and its contributors are less discussed. Therefore, using national representative survey data from CHARLS, we investigated the related factors of the utilization of outpatient, inpatient, and self-treatment services among Chinese middle-aged and elderly people; quantified the absolute IOp and relative IOp of the utilization of these 3 kinds of health services; and estimated the contributors of IOp. This study innovatively expanded measures of IOp and its decomposition into the area of health service utilization among middle-aged and elderly adults and provided new evidence for improving health service equity in China.

Here, we found that the utilization of outpatient, inpatient, and self-treatment services among Chinese middle-aged and elderly people improved from 2013 to 2018, but differences in health service utilization among individuals still existed. The empirical results showed that residence area, OOP ratio, medical distance, work status in circumstance factors and physical examination in effort factors are important explanatory variables of the 3 types of health service utilization among middle-aged and elderly people (Table 3), which is mirrored by previous studies [32, 50, 51].

The IOp of outpatient care, inpatient care, and self-treatment utilization among the middle-aged and elderly overall declined over time (Table 4): the IOp ratio of outpatient care decreased from 36.85% to 34.06%, that of inpatient care decreased from 50.18% to 36.33%, and that of self-treatment decreased from 38.73% to 36.86%, respectively. This trend may be due to the great progress made in China's health care reform in recent years, such as the construction of a 3-tier health service network and the expansion of basic medical insurance coverage, which have increased the availability and affordability of health services. However, the inequality of opportunity is still serious. The absolute IOp of self-treatment is the highest among the 3 medical services. Self-treatment is usually considered a fast, accessible, and more convenient method of health service utilization among middle-aged and elderly people. However, the cost of self-treatment is usually not covered by medical insurance in China, which may reduce the willingness of middle-aged and elderly people to utilize self-treatment. Self-treatment is more sensitive to circumstance factors, such as social health insurance and income, which may lead to significant IOp. Therefore, medical insurance policies should further expand their coverage and increase the reimbursement rate for self-treatment utilization so as to reduce the IOp. The highest ratio of IOp in total inequality is in inpatient services, consistent with the results of Zhang et al. [28]. As we all know, inpatient service may have the highest cost of all the health services utilization, while groups with poor insurance benefits or low income may be prevented from using such health care, which indicating that hospitalization may have more unreasonable inequalities than the other 2 health service types, and it is more difficult to improve through self-efforts. Therefore, policy measures should pay more attention to the circumstance factors of inpatient care to alleviate the IOp of health service utilization.

Shapley decomposition revealed that the contribution of circumstance factors to the unequal utilization of various health services is gradually expanding; however, the contribution of individual effort factors is gradually weakening. Among circumstance factors, OOP ratio contributed the most to the IOp of outpatient care. This may indicate that differences in individual co-payment levels may account for much of the inequality in outpatient care, which is also confirmed by prior literature [52, 53]. At present, China's basic medical insurance system still does not provide a high reimbursement level for outpatient care. It is universally accepted that a reasonable OOP proportion is 30%–40% for health expenditures [54], but, as of 2018, the OOP proportion of outpatient care for middle-aged and elderly people in China was 86.79% according to this study. Additionally, there are still dramatical discrepancies in security level between different insurance insurances in China—for example, the reimbursement level of resident medical insurance is lower than that of employee medical insurance [35, 38], which may lead to different IOp of health service utilization for middle-aged and elderly people having different medical insurances. Hence, the government should further optimize medical insurance policies to increase the security level of outpatient service utilization and narrow the reimbursement gap between different medical insurances.

In contrast, some socioeconomic variables in circumstance factors, such as work status and income, have limited contributions to the IOp of outpatient and self-treatment but a great impact on the IOp of inpatient service, respectively. This suggests that occupational and income factors play an important role in inpatient utilization, and these differences in socioeconomic status greatly contribute to the IOp of hospitalization utilization. Inpatient care utilization is sensitive to individuals' socioeconomic factors due to the higher costs associated with it. The middle-aged and elderly with informal jobs or lower incomes are always at risk of being prevented from taking advantage of inpatient utilization. Fortunately, the contribution of work status and income to the IOp of inpatient care utilization is decreasing according to our findings. Policy efforts should continue to focus on increasing the income level or providing medical subsidies for middle-aged and elderly people with low socioeconomic status to reduce the inequality of opportunities [33].

The change of medical distance's contribution to IOp of the three types of health service utilization also deserves our attention. Medical distance's contribution to the IOp of hospitalization and self-treatment decreased from 2013 to 2018, which may reflect the recent implementation effect of the new medical reform policy in China, such as timely settlement of medical insurance in non-local treatment [55], which makes it more convenient for residents to obtain inpatient care and self-treatment services across regions. It is also worth noting that residents' medical distance is getting farther and farther from 2013 to 2018, and the medical distance has aggravated the IOp in outpatient service utilization. One possible explanation is that health resources in China are still scarce and scattered, which reflects the inequality of the geographical distribution of health resources in China, and similar findings also exist in the research of other scholars [56]. Therefore, it is necessary to increase the investment in areas with weak health resources according to distance, demographic structure, transportation convenience, and medical needs to ensure equitable access to health services [57].

Our study also confirms the dramatical influence of residence area on the unequal utilization of health services, which is consistent with existing studies [34, 51, 52], suggesting that there is still a considerable difference in health service utilization between urban and rural middle-aged and elderly people. The possible reason for this phenomenon is that, due to the unbalanced social and economic development between urban and rural areas in China, the socioeconomic status of middle-aged and elderly people in rural areas is more vulnerable than that of those in urban areas, and high medical expenditures are more likely to cause them to become impoverished, resulting in IOp of health service utilization. Although China implemented the policy of universal coverage of basic medical insurance in 2016, there is still a big gap between urban and rural areas in terms of medical service coverage, medical insurance financing level, resource allocation and policy implementation effect in China at present, resulting in differentiated medical supply between urban and rural areas [52]. Therefore, policies should still be tilted towards rural middle-aged and elderly people, such as optimizing the medical security system and regional medical resource allocation level [58] to improve the accessibility of rural medical services. In addition, measures such as strengthening health education and improving employment to narrow the gap between the social and economic status of middle-aged and elderly people in urban and rural areas should also be taken, so as to reduce the IOp of health service utilization caused by residential areas.

There are still some limitations that need to be acknowledged. We only used expenditure variables to measure personal health service utilization, without addressing more detailed health care demands or health-service quality variables, which may underestimate health service utilization to some extent. Besides, the expenditure data we used were self-reported by respondents, which may have recall bias and may not reflect the true health service utilization. Furthermore, there may be more circumstance factors leading to the IOp of health service utilization than those listed in this study. However, we could not include all related factors due to questionnaire limitations. Further studies will further explore the inequality caused by price or supply factors of medical services.