Boston Medical Center may have its credit rating lowered by Standard & Poor's after it lost public funding as a result of the state's universal healthcare law. The nonprofit hospital is getting less money from the state to cover the cost of uninsured patients because of the 2007 law, said Jennifer Soule, a Standard & Poor's analyst in Boston. The credit rating company put Boston Medical's $378 million in tax-exempt bonds, rated A-, under review for a downgrade.