Ancillary services are medical services provided in a hospital while a patient is an inpatient, but paid by Medicare Part B (outpatient care) when the Part A (hospitalization) claim is denied because Medicare believes that it was unreasonable or unnecessary for the person to be admitted as an inpatient. Ancillary services include things like diagnostic x-rays and lab tests, prosthetic devices, physical therapy, and various screening tests, among others.