When a surgeon performs more than one procedure on a patient during the same operative session, Medicare will not pay 100% for all of them. This is where the Modifier 51 (Multiple Procedures) rule comes in.
How the 50% Reduction Works
Medicare uses a strict ranking system based on Relative Value Units (RVUs). The primary procedure (the one with the highest RVU) is paid at 100% of the fee schedule.
Every subsequent procedure billed with Modifier 51 is automatically reduced and paid at exactly 50% of the fee schedule. Commercial payers often apply even steeper reductions for the 3rd and 4th procedures.
When NOT to Use Modifier 51
Never append Modifier 51 to an Add-On Code (codes with a + symbol in the CPT book, like 22552 for an additional spinal level). Add-on codes are already priced with the multiple procedure reduction factored in. Appending Modifier 51 will cause you to get penalized twice!
NCCI Edits and Modifier 59
Modifier 51 is a pricing modifier, not an unbundling modifier. If two procedures hit an NCCI edit (they are mutually exclusive), you must use Modifier 59/XS, not 51, to get them paid.