Primary Care Enhancement Act of 2018 This bill amends the Internal Revenue Code to: (1) allow individuals who are eligible for a health savings account (HSA) to participate in a direct primary care service arrangement without losing their eligibility due to the arrangement, and (2) permit an HSA to be used to pay for a direct primary care service arrangement. Under a direct primary care service arrangement, an individual receives medical care consisting solely of primary care services provided by primary care practitioners in exchange for a fixed periodic fee. The fees are subject to limits specified in the bill. The primary care services covered by the arrangement may not include procedures that require the use of general anesthesia, prescription drugs (other than vaccines), and laboratory services not typically administered in an ambulatory primary care setting.