305 ILCS 5/5-1.3
To the extent permissible under federal law, the State may pay for medical services only after payment from all other sources of payment have been exhausted, or after the Department has determined that pursuit of such payment is economically unfeasible. Applicants for, and recipients of, medical assistance under this Code shall disclose to the State all insurance coverage they have. To the extent permissible under federal law, the State shall require vendors of medical services to bill third-party payers for services that may be covered by those third-party payers prior to submission of a request for payment to the State. The Department shall, to the extent permissible under federal law, reject a request for payment of a medical service that should first have been submitted to a third-party payer.
305 ILCS 5/5-1.3