Medicare Prescription Drug Coverage Act of 1998 - Amends title XVIII (Medicare) of the Social Security Act (SSA) to: (1) provide for coverage of certain outpatient prescription drugs and home infusion drug therapy services; (2) outline payment rules and related requirements for such drugs and services; (3) provide for Medicare rebates for covered outpatient prescription drugs (or, in the alternative, for purchasing arrangement demonstrations which result in lower costs to the Medicare program and beneficiaries than under the rebate program); and (4) restructure payment for drugs, biologicals, and parenteral nutrients not paid on a cost or prospective payment basis as otherwise provided in Medicare part B (Supplementary Medical Insurance).

Expands the membership of the Medicare Payment Advisory Commission to include additional members, appointed by the Comptroller General, with expertise in pharmacology and prescription drug benefit programs.

Amends the Balanced Budget Act of 1997 to eliminate the report to the Congress on the average wholesale prices of drugs and biologicals.

Declares that any increase in the Medicare part B premium resulting from this Act is covered for qualified Medicare beneficiaries and for special low-income Medicare beneficiaries under respective programs of the Medicaid program under SSA title XIX.

Directs the Secretary of Health and Human Services to: (1) provide, in any statement of premiums, for a separate statement of the portion of such premiums attributable to this Act's amendments; and (2) establish a method under which such portion is waived for any Medicare part B enrollee who demonstrates that he or she has coverage actuarially equivalent to the Medicare part B coverage.