Which situation requires the use of Modifier 91?

Modifier 91 is specifically utilized to indicate that the same laboratory test was performed multiple times on the same day. This modifier is important in situations where it is clinically necessary to repeat a laboratory test to obtain additional information or clarify a diagnosis. For instance, a healthcare provider may order a particular test again during the same day to monitor a condition closely or due to inconclusive initial results. The use of Modifier 91 ensures that the payer understands that the multiple tests are not duplicative but rather serve a legitimate clinical purpose.

In contrast, situations that do not require the use of Modifier 91 include cases where further testing is indicated (like the first option), since that does not imply the same test is repeated but rather new or additional tests might be necessary. Tests performed by different laboratories (the second option) might involve separate billing and do not involve the same test being repeated within the same day. Finally, tests ordered on separate days (the fourth option) also do not require this modifier, as they do not fall under the category of repeated testing on the same day. Thus, the correct answer accurately reflects the specific use of Modifier 91 in the context of repeated tests.