"After rigorous exploration of this data, we have not found any reasons that we believe could justify this difference, leading to concerns that it is the result of HCA corporate efforts to increase admissions without medical need," SEIU wrote in the report.

Because inpatient admissions receive higher Medicare reimbursement rates, the union is concerned that higher volumes are driven by "corporate efforts to boost their profits" and "without respect to medical need."

The union outlined six proposals to address its allegations.

• With 40% of HCA's revenues coming from taxpayer-funded Medicare and Medicaid programs, Congress should convene a thorough, robust investigation of HCA's business practices.
• The U.S. Department of Health and Human Services should conduct its own investigation. Investors should request information from the company regarding its compliance protocols.
• Because the HCA practices examined may not be limited to traditional Medicare, all non-Medicare payors should review claims data and scrutinize reimbursement contracts to determine whether significant numbers of patients might have been admitted unnecessarily.
• For the same reason, self-insured employers in HCA's markets should undertake similar reviews.
• State regulators and policymakers should investigate HCA's specific practices and operations in their states.

"The opportunity to secure meaningful reform of HCA's systemic over-admissions of Medicare patients cannot be overstated," the SEIU report concluded. "As the nation's largest for-profit hospital corporation, HCA Healthcare is the industry leader, setting standards for care that are echoed by thousands of smaller chains across the country. Meaningful reform of HCA's decade-long emergency department admissions policies would disrupt an unethical business model for an industry giant and improve patient care for the millions of Americans who enter HCA medical facilities every year."