Key facts for DRG 166
- FY2027 relative weight
- 3.8730
- Higher than 85% of all MS-DRGs
- Change vs FY2026
- +3.6%
- FY2026 weight 3.7380
- Geometric mean LOS
- 7.6 days
- Arithmetic mean 10.5 days
- MDC
- 04
- Assignment of Diagnosis Codes
- Severity level
- with MCC (major complication or comorbidity)
- Transfer policy
- Post-acute transfer DRG
TL;DR
MS-DRG 166 is a surgical group in MDC 04 (Assignment of Diagnosis Codes) at the highest-severity level of its family. CMS assigns it a FY2027 relative weight of 3.8730 with a geometric mean length of stay of 7.6 days and an arithmetic mean of 10.5. Its weight moved up 3.6% from FY2026 (3.7380). That weight is higher than 85% of all surgical and medical MS-DRGs. Its CC/MCC family (DRG 167, DRG 168) spans weights 1.3890 to 3.8730. The v44 Definitions Manual assigns it through 1996 operating room procedures codes.
What changed from FY2026 to FY2027
CMS recalibrates every MS-DRG weight annually from the prior year's MedPAR claims. For DRG 166 the FY2027 relative weight is 3.8730 against 3.7380 in FY2026, a rise of 3.61%, which exceeds the 2% threshold worth re-checking in contract models.
| Metric | FY2026 | FY2027 | Change |
|---|---|---|---|
| Relative weight | 3.7380 | 3.8730 | +0.1350 |
| Geometric mean LOS (days) | 7.6 | 7.6 | +0.0 |
| Arithmetic mean LOS (days) | 10.5 | 10.5 | +0.0 |
CC and MCC family
DRG 166 shares its base definition with 2 other MS-DRGs split by severity. The family's weights span 1.3890 to 3.8730, a 2.79× spread, which is the payment effect of documenting qualifying complications and comorbidities.
Grouper logic (v44 Definitions Manual)
Operating Room Procedures: 1996 ICD-10 codes drive assignment to this group; the first 12 are shown.
| ICD-10 code | Description |
|---|---|
| 008Q0ZZ | Division of Vagus Nerve, Open Approach |
| 008Q3ZZ | Division of Vagus Nerve, Percutaneous Approach |
| 008Q4ZZ | Division of Vagus Nerve, Percutaneous Endoscopic Approach |
| 021P08A | Bypass Pulmonary Trunk from Innominate Artery with Zooplastic Tissue, Open Approach |
| 021P08B | Bypass Pulmonary Trunk from Subclavian with Zooplastic Tissue, Open Approach |
| 021P08D | Bypass Pulmonary Trunk from Carotid with Zooplastic Tissue, Open Approach |
| 021P09A | Bypass Pulmonary Trunk from Innominate Artery with Autologous Venous Tissue, Open Approach |
| 021P09B | Bypass Pulmonary Trunk from Subclavian with Autologous Venous Tissue, Open Approach |
| 021P09D | Bypass Pulmonary Trunk from Carotid with Autologous Venous Tissue, Open Approach |
| 021P0AA | Bypass Pulmonary Trunk from Innominate Artery with Autologous Arterial Tissue, Open Approach |
| 021P0AB | Bypass Pulmonary Trunk from Subclavian with Autologous Arterial Tissue, Open Approach |
| 021P0AD | Bypass Pulmonary Trunk from Carotid with Autologous Arterial Tissue, Open Approach |
Documentation and denial exposure
As a surgical DRG, assignment depends on a qualifying operating-room procedure being coded from the operative report; a missing or non-OR procedure code drops the stay into a medical DRG in MDC 04 with a lower weight. Because this family splits by severity, auditors target CC and MCC capture: each secondary diagnosis must be documented as present, monitored, evaluated or treated during the stay to survive a DRG-validation review. Typical inpatient denial codes for this group: CARC 16 (claim lacks information needed to group the stay), CARC 50 (the admission or procedure is judged not medically necessary), CARC 97 (a service is bundled into the DRG payment).
How QuickIntell uses DRG 166 data
QuickCode groups inpatient encounters against the current v44 logic, flags CC and MCC candidates that lack supporting documentation, and shows the weight difference across the family before the claim drops. QuickScribe captures the attending's language that supports severity, and QuickRCM works DRG-validation denials with the grouper evidence attached.
Frequently asked questions — DRG 166
What is MS-DRG 166?
MS-DRG 166 is "Other Respiratory System O.R. Procedures with MCC", a surgical Medicare Severity Diagnosis-Related Group in MDC 04, Assignment of Diagnosis Codes. Medicare pays inpatient stays grouped here at the hospital's base rate multiplied by the DRG relative weight.
What is the FY2027 relative weight for DRG 166?
The FY2027 relative weight is 3.8730 (IPPS final rule Table 5, effective October 1, 2026). In FY2026 it was 3.7380, a change of +3.6%.
What is the average length of stay for DRG 166?
CMS reports a geometric mean length of stay of 7.6 days and an arithmetic mean of 10.5 days for FY2027. The geometric mean is used for transfer-payment calculations.
Which DRGs are in the same CC/MCC family as 166?
DRG 167 (Other Respiratory System O.R. Procedures with CC, weight 1.8240); DRG 168 (Other Respiratory System O.R. Procedures without CC/MCC, weight 1.3890). The family's weights range from 1.3890 to 3.8730, so documented complications and comorbidities change payment materially.
What documentation supports the severity level of DRG 166?
DRG 166 requires at least one secondary diagnosis on the CMS Major Complication or Comorbidity list, documented as present and clinically addressed during the stay (monitored, evaluated, treated or extending the stay). Conditions such as acute respiratory failure, severe sepsis or acute kidney injury with specified cause qualify only when the attending's note states the diagnosis itself, not just the lab values. Recovery auditors downgrade this group to the CC or base level when the MCC rests on an unconfirmed query or a resolved historical condition, so the query response and the discharge summary must agree.
Which codes group to DRG 166?
The v44 Definitions Manual lists 1996 operating room procedures codes for this group. Examples from the operating room procedures list: 008Q0ZZ (Division of Vagus Nerve, Open Approach); 008Q3ZZ (Division of Vagus Nerve, Percutaneous Approach); 008Q4ZZ (Division of Vagus Nerve, Percutaneous Endoscopic Approach); 021P08A (Bypass Pulmonary Trunk from Innominate Artery with Zooplastic Tissue, Open Approach).
Is DRG 166 a post-acute transfer DRG?
Yes. CMS flags DRG 166 under the post-acute care transfer policy, so a discharge to a qualifying post-acute setting before the geometric mean length of stay is paid a per-diem amount rather than the full DRG payment.
Sources
Every figure on this page is taken from the CMS publications below, as released by the Centers for Medicare & Medicaid Services. Projection built 2026-10-01. Verify against the primary file before billing or contracting decisions.
- IPPS FY2027 Final Rule Table 5 (CMS-1849-F)Version v44 FY2027 · effective 2026-10-01 · file CMS-1849-F Table 5.txtSHA-256 01003dd571c1e2f5…
- IPPS FY2026 Final Rule Table 5 (CMS-1833-F)Version v43 FY2026 · effective 2025-10-01 · file CMS-1833-F Table 5.txtSHA-256 bf8c390d14b3cd3e…
- ICD-10 MS-DRG Definitions Manual v44 (text)Version v44 · effective 2026-10-01 · file fy2027-fr-icd10-ms-drg-definitions-manual-files-v44.zipSHA-256 ae4f6c11727fe91f…
Disclaimer
This page is an operational reference compiled from the CMS IPPS FY2027 final rule (Table 5) and the v44 ICD-10 MS-DRG Definitions Manual. Relative weights are national factors; actual payment depends on each hospital's wage-adjusted base rate, add-ons, outliers and transfer adjustments. Nothing here is legal, clinical or billing advice; verify grouping against your encoder and the published CMS files.