Undocumented immigrants (UDIs) in the United States are an understudied population with unique health needs. Half are uninsured, with state-to-state variation. Creative methodologies have been used to approximate UDIs in health data by geography and scope. However, no review exists of UDI health coverage or methods used to study this population. Authors conducted a narrative review defining health coverage options by state and research methodologies for UDIs. California Health Interview Survey (CHIS) was included in the datasets studied. 

Findings: UDI health coverage was determined for all 50 states and Washington, DC: three states and Washington, DC. were considered “available,” 28 were “limited,” providing care to special UDI populations, and 19 were “restricted” with no coverage options. Thirty-seven articles on UDI health coverage were included in this study. Methodologies to study UDI patients were unstandardized. Most studies were single- center, retrospective, or qualitative. Creative methods were used to approximate UDI health data, including linking social services datasets and using Emergency Medicaid claims. Large-scale data set studies were rare, but California's restricted Medi-Cal demonstrated successful use of claims data for UDI research. Included research articles were categorized by state coverage and an average number of studies produced per state in that category: available, limited, and restricted.