Are CMS G-Code Functional Limitation Severity Modifiers Responsive to Change Across an Episode of Outpatient Rehabilitation?

Background The Centers for Medicare & Medicaid Services has mandated rehabilitation professionals to document patients' impairment levels. There is no evidence of responsiveness to change of functional limitation severity modifier codes.

Objective The purpose of this study was to assess the validity of G-code functional limitation severity modifier codes in determining change in function.

Design This was a retrospective observational study.

Methods Patients completed the Activity Measure for Post-Acute Care (AM-PAC) and were assigned G-codes, with severity modifiers based on AM-PAC scores at initial and follow-up visits. Patients were classified as having AM-PAC scores in the upper or lower range for each severity modifier, and sensitivity, specificity, and positive and negative predictive values for change in severity modifier level and odds of changing by one severity modifier level using a change in AM-PAC score of at least 1 minimal detectable change at the 95% confidence interval (MDC₉₅) as the standard were determined.

Results Sensitivity and specificity of change in severity modifier in determining change in function were dependent on patients' initial AM-PAC scores. Improvement in severity modifier level was 2.2 to 4.5 times more likely with scores at the higher end of the range within a severity modifier level than with scores in the lower end of the range. Decline in severity modifier level was 2.7 to 4.8 times more likely with scores at the lower end of the range within a severity modifier than with scores in the higher end of the range.

Limitations Data were from one health care system, and most patients had orthopedic conditions. The MDC₉₅ for AM-PAC tool may not be the best standard for defining functional change.

Conclusions The G-code functional limitation severity modifier system may not be valid for determining change in function and is not recommended for determining if patients have changed over the course of outpatient therapy.