CPT Modifier 91

Repeat clinical diagnostic laboratory test.

CPT modifier 91 must be submitted with services that meet these criteria, regardless of whether the test is also submitted with HCPCS modifier AY.  
  • This modifier may not be submitted when tests are rerun to confirm initial results. This is due to testing problems with specimens or equipment or for any other reason when a normal, one-time reportable result is all that is required.
  • This modifier may not be used when other codes describe a series of test results (e.g., glucose tolerance tests)
  • For clinical laboratory tests ordered by an ESRD facility, these tests must be submitted with CPT modifier 91 if any single service (same CPT code) is ordered for the same patient, the specimen is collected more than once in a single day, and the service is medically necessary  

Reference: IOM Pub 100-4, Chapter 16, Section 100.5.1.

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