The Internal Revenue Service (IRS) has issued guidance on the additional 0.9 percent Medicare tax on highly compensated individuals that commences on Jan. 1, 2013. The tax is part of the Patient Protection and Affordable Care Act (PPACA), or what is commonly referred to as Obamacare.

The guidance indicates that the additional tax kicks in once an individual's income, including commission and other fringe benefits, reaches $200,000. However, depending upon how affected individuals file their income tax, the additional Medicare liability may rise or fall.

The 0.9 percent tax, which is in addition to the normal 1.45 percent Medicare tax that all workers pay, takes effect at $125,000 for married individuals who file separately, while it kicks in at $250,000 for married couples filing jointly. Individual filers face the tax at $200,000 of taxable income.

The employer share of the Medicare tax, which is also 1.45 percent, is not affected by the PPACA rule.