The high prevalence of multimorbidity among middle-aged and older adults has become a serious issue needing to be addressed by China's healthcare system. The number of chronic diseases is related to health service utilization and medical costs, but there is still a lack of relevant national surveys in China.

To understand the prevalence and features of multimorbidity and to examine its associations with health service utilization and medical costs among middle-aged and older Chinese adults.

Data were collected from the 2018 wave of China Health and Retirement Longitudinal Study (CHARLS) during April to October 2021, involving 16 674 Chinese adults (≥45 years old) . Multimorbidity was defined as the coexistence of two or more of the self-reported 14 chronic conditions. Health service utilization was measured using inpatient service utilization in the past year and outpatient service utilization in the past month. Medical costs were measured using total inpatient cost and out-of-pocket (OOP) cost for inpatient care in the past one year, total outpatient costs and OOP cost for outpatient care in the past one month. Logistic regression was used to estimate the association between the number of chronic diseases and health service utilization. Quantile regression was adopted to estimate the association between the number of chronic diseases and medical costs.

Of all participants, 9 561 (57.34%) had multimorbidity. 2 624 (15.74%) had utilized inpatient services in the past year, and 2 588 (15.52%) used outpatient services in the past one month. Inpatient service utilization, outpatient service utilization, total inpatient cost, OOP cost for inpatient care, total outpatient cost, and OOP cost for outpatient care varied significantly by the number of chronic diseases (P<0.05) . Multivariate Logistic regression analysis indicated that the number of chronic diseases was associated with inpatient service utilization (P<0.05) . Suffering from 1, 2, 3, 4 and ≥5 chronic diseases was associated with 1.882 times〔95%CI (1.547, 2.290) 〕, 2.939 times〔95%CI (2.429, 3.555) 〕, 4.231 times〔95%CI (3.490, 5.130) 〕, 5.723 times〔95%CI (4.680, 7.000) 〕, and 8.671 times〔95%CI (7.173, 10.482) 〕 higher rate of inpatient service utilization, respectively. Having 1, 2, 3, 4 and ≥5 chronic diseases was associated with 1.684 times〔95%CI (1.421, 1.995) 〕, 2.481 times〔95%CI (2.101, 2.931) 〕, 3.691 times〔95%CI (3.115, 4.374) 〕, 3.774 times〔95%CI (3.134, 4.544) 〕, and 5.577 times〔95%CI (4.698, 6.620) 〕 higher rate of outpatient service utilization, respectively. Each increased chronic disease was associated with an increase in both total inpatient costs and OOP for inpatient care at the upper and middle (50, 75 and 90 percentiles) percentile levels, with larger effects on the upper percentile 〔90th percentile Coeff (95%CI) =1 248.43 (219.20, 2 277.66) for total hospital costs; 90th percentile Coeff (95%CI) =706.36 (266.87, 1 145.86) for OOP for inpatient care〕. Each increased chronic disease was also associated with an increase in both total outpatient costs and OOP for outpatient care, and the effects on the upper percentiles were larger〔90th percentile Coeff (95%CI) =196.33 (31.06, 361.61) for total outpatient costs; 90th percentile Coeff (95%CI) =128.56 (26.83, 230.28) for OOP for outpatient care〕.

In middle-aged and older Chinese adults, multimorbidity was highly prevalent, and the increase in the number of coexisted chronic diseases was associated with higher rate of health service utilization and medical costs. The government should pay more attention on primary care to manage the demand for health services and medical costs associated with multimorbidity.