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MS-DRG 866 · MDC 18 · Medical

MS-DRG 866: Viral Illness without MCC

Reviewed by QuickIntell RCM Editorial Team · Last reviewed

Data effective
Data currency: IPPS Table 5 FY2027 final rule (effective October 1, 2026); Definitions Manual v44 (effective October 1, 2026). Next CMS release: April 1, 2027 (v44.1 mid-year update), then the FY2028 final rule in August 2027.

Key facts for DRG 866

FY2027 relative weight
0.8690
Higher than 15% of all MS-DRGs
Change vs FY2026
-0.1%
FY2026 weight 0.8700
Geometric mean LOS
2.7 days
Arithmetic mean 3.5 days
MDC
18
Infectious and Parasitic Diseases, Systemic or Unspecified Sites
Severity level
without MCC
Transfer policy
Not a transfer DRG

TL;DR

MS-DRG 866 is a medical group in MDC 18 (Infectious and Parasitic Diseases, Systemic or Unspecified Sites) at the base severity level of its family. CMS assigns it a FY2027 relative weight of 0.8690 with a geometric mean length of stay of 2.7 days and an arithmetic mean of 3.5. Its weight moved down 0.1% from FY2026 (0.8700). That weight is higher than 15% of all medical and surgical MS-DRGs. Its CC/MCC family (DRG 865) spans weights 0.8690 to 1.3490. The v44 Definitions Manual assigns it through 141 principal diagnosis codes.

What changed from FY2026 to FY2027

CMS recalibrates every MS-DRG weight annually from the prior year's MedPAR claims. For DRG 866 the FY2027 relative weight is 0.8690 against 0.8700 in FY2026, a fall of 0.11%.

FY2026 versus FY2027 payment factors for MS-DRG 866
MetricFY2026FY2027Change
Relative weight0.87000.8690-0.0010
Geometric mean LOS (days)2.72.7+0.0
Arithmetic mean LOS (days)3.53.5+0.0
Weights shown are the FY2027 final values after the 10% cap on year-over-year reductions where applicable.

CC and MCC family

DRG 866 shares its base definition with 1 other MS-DRG split by severity. The family's weights span 0.8690 to 1.3490, a 1.55× spread, which is the payment effect of documenting qualifying complications and comorbidities.

MS-DRGs in the same base group as DRG 866
DRGTitleFY2027 weightGMLOS
865Viral Illness with MCC1.34903.7
866Viral Illness without MCC0.86902.7

Grouper logic (v44 Definitions Manual)

Principal Diagnosis: 141 ICD-10 codes drive assignment to this group; the first 12 are shown.

Principal Diagnosis codes assigned to DRG 866 (sample)
ICD-10 codeDescription
A70Chlamydia psittaci infections
A7481Chlamydial peritonitis
A7489Other chlamydial diseases
A749Chlamydial infection, unspecified
A804Acute nonparalytic poliomyelitis
A90Dengue fever [classical dengue]
A91Dengue hemorrhagic fever
A920Chikungunya virus disease
A921O'nyong-nyong fever
A9230West Nile virus infection, unspecified
A9231West Nile virus infection with encephalitis
A9232West Nile virus infection with other neurologic manifestation

Documentation and denial exposure

As a medical DRG, assignment depends on the principal diagnosis sequenced from the attending's documentation; a secondary condition sequenced first, or a symptom code in place of the confirmed diagnosis, changes the MDC or the DRG. 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 866 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.

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Frequently asked questions — DRG 866

What is MS-DRG 866?

MS-DRG 866 is "Viral Illness without MCC", a medical Medicare Severity Diagnosis-Related Group in MDC 18, Infectious and Parasitic Diseases, Systemic or Unspecified Sites. 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 866?

The FY2027 relative weight is 0.8690 (IPPS final rule Table 5, effective October 1, 2026). In FY2026 it was 0.8700, a change of -0.1%.

What is the average length of stay for DRG 866?

CMS reports a geometric mean length of stay of 2.7 days and an arithmetic mean of 3.5 days for FY2027. The geometric mean is used for transfer-payment calculations.

Which DRGs are in the same CC/MCC family as 866?

DRG 865 (Viral Illness with MCC, weight 1.3490). The family's weights range from 0.8690 to 1.3490, so documented complications and comorbidities change payment materially.

What documentation supports the severity level of DRG 866?

DRG 866 is the base-severity assignment: no secondary diagnosis on the CC or MCC lists was coded, or the only ones present are excluded for this principal diagnosis. Revenue-integrity review here looks for undercapture, meaning conditions treated during the stay (electrolyte disorders, specified anemia, acute blood loss, pressure injuries present on admission) that were never documented to the specificity the lists require. A compliant query to the attending, not a coder assumption, is the only way to move a stay to the CC or MCC sibling.

Which codes group to DRG 866?

The v44 Definitions Manual lists 141 principal diagnosis codes for this group. Examples from the principal diagnosis list: A70 (Chlamydia psittaci infections); A7481 (Chlamydial peritonitis); A7489 (Other chlamydial diseases); A749 (Chlamydial infection, unspecified).

Is DRG 866 a post-acute transfer DRG?

No. DRG 866 is not on the FY2027 post-acute care transfer list, so the transfer per-diem policy does not apply to it.

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.

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.