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The additional 3.8% Medicare tax will not apply unless your adjusted gross income (AGI) exceeds: (1) $200,000 if you're unmarried, (2) $250,000 if you're a married joint-filer, or (3) $125,000 if you use married filing separate status.You will find that most items in taxation deal with ADJUSTED GROSS INCOME.
The new tax is on investment income.
Currently, the government levies a Medicare payroll tax on all wages, with half paid by the individual and half by the employer.
It applies to traditional investment income derived from dividends, real estate investments, interest, or profits from trading financial products such as stocks and bonds.
Another expert here:
The 3.8% medicare tax applies to the lesser of your net unearned investment income or the amount of your income in excess of $200,000 ($250,000 married joint filer) AGI. Investment income for these purposes consists of items such as dividends, interest, capital gains, etc. For example, if your filing status is married joint filing and your AGI is $300,000 but you have no investment income then the tax does not apply. However, if your investment income is more than the excess then the tax is on the excess. For example, if married filing jointly and your AGI is $300,000 which includes $60,000 of investment income, then the tax would apply to $50,000 (the amount in excess of $250,000) because this is less than the amount of your investment income.