Guide · Inpatient Rehabilitation
IRF-PAI & CMG Reference
How the IRF-PAI assessment becomes a case-mix group, what the CMG code encodes, and where the relative weights live. The estimator takes the weight; this explains where it comes from.
No PHIReferenceInformational reference — paraphrased from CMS rules and manuals, not billing or legal advice; verify against the current IRF Grouper version and CMS Table 2.
From IRF-PAI to CMG
GrouperEvery IRF stay has an IRF-PAI assessment. The IRF Grouper software reads it and assigns the case-mix group from these inputs:
| Impairment group | the rehabilitation impairment category (the reason for the admission) |
| Functional status | motor function (and, historically, cognitive) from the assessment items |
| Age | used to split certain groups |
| Comorbidities | ICD-10-CM codes (IRF-PAI Item 24) that map to a comorbidity tier |
The grouper is versioned annually. The FY 2026 version (5.50) applies to IRF-PAI assessments with a discharge date on or after October 1, 2025 — confirm the version in effect for your dates.
The CMG / HIPPS code
5 characters| Position 1 | comorbidity-tier letter — A (no qualifying comorbidity) or B / C / D for the tiers |
| Positions 2–5 | the CMG number (impairment category and functional level) |
A qualifying comorbidity raises the tier and the payment. The same case maps to one CMG and one tier; the combined code is what appears on the Revenue Code 0024 line.
Relative weights and average length of stay for each CMG/tier are published in CMS Table 2 of the final rule and are not reproduced here. The estimator asks you to enter the weight for your CMG/tier.
