Hi - Mod 53 can be payer- and setting-specific, but Medicare includes a row for G0121 with modifier 53 in the Physician Fee Schedule with code status A.
https://www.cms.gov/medicaremedicar...sicianfeeschedpfs-relative-value-files/rvu22a
In Medicare Claims Processing Manual, Chapter 12, Section 30.1.B, there is this info:
B. Incomplete Colonoscopies (Codes 44388, 45378, G0105 and G0121)
An incomplete colonoscopy, e.g., the inability to advance the colonoscope to the cecum or
colon-small intestine anastomosis due to unforeseen circumstances, is billed and paid using
colonoscopy through stoma code 44388, colonoscopy code 45378, and screening colonoscopy
codes G0105 and G0121 with modifier “-53.” (Code 44388 is valid with modifier 53
beginning January 1, 2016.) The Medicare physician fee schedule database has specific values
for codes 44388-53, 45378-53, G0105-53 and G0121-53. An incomplete colonoscopy
performed prior to January 1, 2016, is paid at the same rate as a sigmoidoscopy. Beginning
January 1, 2016, Medicare will pay for the interrupted colonoscopy at a rate that is calculated
using one-half the value of the inputs for the codes.
Hope that helps!