Medicare Part B provides coverage for doctor visits and other "outpatient" costs such as medical equipment and physical therapy. It also covers some preventive costs such as diabetes testing, glaucoma screening, and colon and prostate cancer screening.
Part B comes with a monthly premium you must pay; in 2016, the base premium is $121.80, But if your adjusted gross income is above $85,000 (or $170,000 for married couples filing jointly), you'll be hit with a supplemental premium that will add at least 40% to your cost.
