Objective: Idiopathic inflammatory myopathies (IIMs) have heterogeneous presentations and require multiple lines of testing, some of which are invasive, to diagnose. Additionally, treatment is chronic and can be costly. Delays in diagnosis are known to occur and can lead to significant morbidity. This study aims to investigate the influence of socioeconomic status (SES) on time to diagnosis, treatment, and improvement in disease status of IIMs.

Methods: This is a retrospective study patients from 2011-2021 from a university hospital and a county safety-net hospital. Salient clinicopathologic and demographic data including race, ethnicity, sex, zip code, median household income, and insurance data were abstracted. Patients with ICD codes for polymyositis, dermatomyositis, inclusion body myositis, other inflammatory or immune-mediated myositis were included. The modeling of time-to-event and binary outcomes incorporated covariates such as median household income, race, ethnicity, sex, insurance status (Charity vs. Medicare/Medicaid vs. Other), and inpatient status (Inpatient = 1, Outpatient = 0). Time-to-event data were analyzed using Cox proportional hazards models through the survival package in R.

Results: Our study included 2,258 subjects, of which 1,734 were from the academic hospital and 524 were from the safety-net hospital. After filtering for patients with the proper diagnosis, there were 46 from the academic and 143 from the safety-net hospital. There is no significant difference in time from first abnormal muscle enzyme level (creatine kinase, CK) to diagnosis between race, median household income, ethnicity, and initial presentation in the inpatient vs. outpatient setting. However, those receiving charity had a 4.58-fold increase in the instantaneous probability of being diagnosed after first positive CK after 230 days (p = 0.009). Those on Medicare/Medicaid had a 62% decrease in the instantaneous probability of being diagnosed after first positive CK within the first 94 days (p = 0.02) but a 4.60-fold increase in the instantaneous probability of being diagnosed after first positive CK after 230 days (p = 0.006). There is no significant difference in the probability of a muscle biopsy being order or the time to disease improvement between races, ethnicities, sexes, income, or insurance status.

Conclusions: Our findings showed that there could be delays in diagnosis for those with Medicare/Medicaid or those receiving charity to finance their care. There was no impact detected from race, ethnicities, or sexes. While income did not clearly correlate with insurance status, income is limited by zip-code level data rather than individual data. Our study sheds light on populations that may have delays in diagnosis and treatment, which could lead to increased disease morbidity. Unfortunately, increased morbidity in the population identified will only perpetuate the cycle of poverty and add greater burden to the healthcare system. Future studies will seek to examine impact of SES and disease outcomes.