Senior Citizen Protection Act of 1998 - Amends title XIX (Medicaid) of the Social Security Act to extend the authority of State Medicaid fraud control units, upon the approval of the Inspector General (IG) of the relevant Federal agency in a particular case or investigation, to investigate and prosecute: (1) fraud in connection with other Federal health care programs (if the suspected fraud or law violation in such case or investigation is primarily related to the State Medicaid plan); and (2) abuse of residents of non-Medicaid board and care facilities, if their assistance is requested by State and local agencies. Provides that when the IG grants such approval, he or she retains continuing authority to join the case or investigation, or after consultation with the unit, to replace it as the primary agency assigned to the case or investigation.

Provides that overpayments collected by a State Medicaid fraud control unit shall be credited exclusively to, and be available for expenditure under, the Federal health care program that was subject to the activity that was the basis for the collection.