MS-DRG · FY 2026 v43.0
MS-DRG 866 – Viral Illness without MCC
A Medical DRG in MDC 18 (Infectious & Parasitic Diseases, Systemic or Unspecified Sites). Inpatient cases group to DRG 866 when the ICD-10-CM principal diagnosis is one of the 141 codes listed below and no secondary diagnosis on the MCC list is present. Its FY 2026 relative weight is 0.8696 with a geometric mean length of stay of 2.7 days.
Medical DRG – assigned by principal diagnosis✓ Official CMS v43.0 dataset
0.8696
Relative Weight
2.7
Geometric Mean LOS (days)
3.5
Arithmetic Mean LOS (days)
141
Principal Diagnosis Codes
Severity family
DRG 865with MCCWeight 1.4983 · GMLOS 4.0 daysDRG 866without MCCWeight 0.8696 · GMLOS 2.7 days
How a case is assigned to DRG 866
The grouper first assigns the case to MDC 18 from the principal diagnosis and finds no qualifying operating-room procedure (this is a Medical DRG; ICD-10-PCS codes do not drive assignment here), then applies the severity split from secondary diagnoses: with MCC → 865; without MCC → 866.
ICD-10-CM
Principal Diagnosis Codes 141 codes · 33 categories
141 of 141 shown
A70 Chlamydia psittaci infections1 code
- A70 Chlamydia psittaci infections
A74 Other diseases caused by chlamydiae3 codes
A80 Acute poliomyelitis1 code
- A80.4 Acute nonparalytic poliomyelitis
A90 Dengue fever [classical dengue]1 code
- A90 Dengue fever [classical dengue]
A91 Dengue hemorrhagic fever1 code
- A91 Dengue hemorrhagic fever
A92 Other mosquito-borne viral fevers10 codes
- A92.0 Chikungunya virus disease
- A92.1 O'nyong-nyong fever
- A92.30 West Nile virus infection, unspecified
- A92.31 West Nile virus infection with encephalitis
- A92.32 West Nile virus infection with other neurologic manifestation
- A92.39 West Nile virus infection with other complications
- A92.4 Rift Valley fever
- A92.5 Zika virus disease
- A92.8 Other specified mosquito-borne viral fevers
- A92.9 Mosquito-borne viral fever, unspecified
A93 Other arthropod-borne viral fevers, not elsewhere classified4 codes
A94 Unspecified arthropod-borne viral fever1 code
- A94 Unspecified arthropod-borne viral fever
A95 Yellow fever3 codes
A96 Arenaviral hemorrhagic fever5 codes
A98 Other viral hemorrhagic fevers, not elsewhere classified6 codes
A99 Unspecified viral hemorrhagic fever1 code
- A99 Unspecified viral hemorrhagic fever
B00 Herpesviral [herpes simplex] infections1 code
- B00.89 Other herpesviral infection
B01 Varicella [chickenpox]4 codes
B03 Smallpox1 code
- B03 Smallpox
B04 Monkeypox1 code
- B04 Monkeypox
B05 Measles4 codes
B06 Rubella [German measles]3 codes
B08 Other viral infections characterized by skin and mucous membrane lesions, not elsewhere classified18 codes
- B08.010 Cowpox
- B08.011 Vaccinia not from vaccine
- B08.04 Paravaccinia, unspecified
- B08.09 Other orthopoxvirus infections
- B08.20 Exanthema subitum [sixth disease], unspecified
- B08.21 Exanthema subitum [sixth disease] due to human herpesvirus 6
- B08.22 Exanthema subitum [sixth disease] due to human herpesvirus 7
- B08.3 Erythema infectiosum [fifth disease]
- B08.4 Enteroviral vesicular stomatitis with exanthem
- B08.60 Parapoxvirus infection, unspecified
- B08.61 Bovine stomatitis
- B08.62 Sealpox
- B08.69 Other parapoxvirus infections
- B08.70 Yatapoxvirus infection, unspecified
- B08.71 Tanapox virus disease
- B08.72 Yaba pox virus disease
- B08.79 Other yatapoxvirus infections
- B08.8 Other specified viral infections characterized by skin and mucous membrane lesions
B09 Unspecified viral infection characterized by skin and mucous membrane lesions1 code
- B09 Unspecified viral infection characterized by skin and mucous membrane lesions
B25 Cytomegaloviral disease2 codes
B26 Mumps5 codes
B27 Infectious mononucleosis16 codes
- B27.00 Gammaherpesviral mononucleosis without complication
- B27.01 Gammaherpesviral mononucleosis with polyneuropathy
- B27.02 Gammaherpesviral mononucleosis with meningitis
- B27.09 Gammaherpesviral mononucleosis with other complications
- B27.10 Cytomegaloviral mononucleosis without complications
- B27.11 Cytomegaloviral mononucleosis with polyneuropathy
- B27.12 Cytomegaloviral mononucleosis with meningitis
- B27.19 Cytomegaloviral mononucleosis with other complication
- B27.80 Other infectious mononucleosis without complication
- B27.81 Other infectious mononucleosis with polyneuropathy
- B27.82 Other infectious mononucleosis with meningitis
- B27.89 Other infectious mononucleosis with other complication
- B27.90 Infectious mononucleosis, unspecified without complication
- B27.91 Infectious mononucleosis, unspecified with polyneuropathy
- B27.92 Infectious mononucleosis, unspecified with meningitis
- B27.99 Infectious mononucleosis, unspecified with other complication
B33 Other viral diseases, not elsewhere classified4 codes
B34 Viral infection of unspecified site7 codes
B97 Viral agents as the cause of diseases classified elsewhere20 codes
- B97.0 Adenovirus as the cause of diseases classified elsewhere
- B97.10 Unspecified enterovirus as the cause of diseases classified elsewhere
- B97.11 Coxsackievirus as the cause of diseases classified elsewhere
- B97.12 Echovirus as the cause of diseases classified elsewhere
- B97.19 Other enterovirus as the cause of diseases classified elsewhere
- B97.21 SARS-associated coronavirus as the cause of diseases classified elsewhere
- B97.29 Other coronavirus as the cause of diseases classified elsewhere
- B97.30 Unspecified retrovirus as the cause of diseases classified elsewhere
- B97.31 Lentivirus as the cause of diseases classified elsewhere
- B97.32 Oncovirus as the cause of diseases classified elsewhere
- B97.33 Human T-cell lymphotrophic virus, type I [HTLV-I] as the cause of diseases classified elsewhere
- B97.34 Human T-cell lymphotrophic virus, type II [HTLV-II] as the cause of diseases classified elsewhere
- B97.35 Human immunodeficiency virus, type 2 [HIV 2] as the cause of diseases classified elsewhere
- B97.39 Other retrovirus as the cause of diseases classified elsewhere
- B97.4 Respiratory syncytial virus as the cause of diseases classified elsewhere
- B97.5 Reovirus as the cause of diseases classified elsewhere
- B97.6 Parvovirus as the cause of diseases classified elsewhere
- B97.7 Papillomavirus as the cause of diseases classified elsewhere
- B97.81 Human metapneumovirus as the cause of diseases classified elsewhere
- B97.89 Other viral agents as the cause of diseases classified elsewhere
J09 Influenza due to certain identified influenza viruses2 codes
J10 Influenza due to other identified influenza virus5 codes
- J10.2 Influenza due to other identified influenza virus with gastrointestinal manifestations
- J10.81 Influenza due to other identified influenza virus with encephalopathy
- J10.82 Influenza due to other identified influenza virus with myocarditis
- J10.83 Influenza due to other identified influenza virus with otitis media
- J10.89 Influenza due to other identified influenza virus with other manifestations
J11 Influenza due to unidentified influenza virus5 codes
- J11.2 Influenza due to unidentified influenza virus with gastrointestinal manifestations
- J11.81 Influenza due to unidentified influenza virus with encephalopathy
- J11.82 Influenza due to unidentified influenza virus with myocarditis
- J11.83 Influenza due to unidentified influenza virus with otitis media
- J11.89 Influenza due to unidentified influenza virus with other manifestations
L44 Other papulosquamous disorders1 code
- L44.4 Infantile papular acrodermatitis [Gianotti-Crosti]
T88 Other complications of surgical and medical care, not elsewhere classified1 code
- T88.1XXA Other complications following immunization, not elsewhere classified, initial encounter
Z21 Asymptomatic human immunodeficiency virus [HIV] infection status1 code
- Z21 Asymptomatic human immunodeficiency virus [HIV] infection status
ICD-10-PCS
Procedure Codes
Procedure codes do not drive assignment for this DRG. DRG 866 is a Medical DRG, assigned from the principal diagnosis above.
Frequently Asked Questions
What is MS-DRG 866?
MS-DRG 866 (Viral Illness without MCC) is a medical DRG in MDC 18 (Infectious & Parasitic Diseases, Systemic or Unspecified Sites). Its FY 2026 relative weight is 0.8696 with a geometric mean length of stay of 2.7 days.
What is the difference between DRG 865, 866?
All 2 share the same base category; the severity of secondary diagnoses separates them: 865 (with MCC), 866 (without MCC). A major complication or comorbidity (MCC) places the case in the highest-weighted DRG of the family; a CC in the middle; neither in the lowest.
How many diagnosis codes group to DRG 866?
141 ICD-10-CM principal diagnosis codes group to this DRG, spanning 33 three-character categories.
What is the relative weight of DRG 866?
The FY 2026 relative weight of this DRG is 0.8696, meaning reimbursement of roughly 0.87 times the average Medicare inpatient case. Multiply by a hospital's blended base rate for the approximate payment.
← PreviousDRG 865 – Viral Illness with MCCNext →DRG 867 – Other Infectious and Parasitic Diseases Diagnoses with MCC
Source: CMS ICD-10 MS-DRG v43.0 Definitions Manual and the FY 2026 IPPS Final Rule (Table 5), effective October 1, 2025 through September 30, 2026. We publish the DRG assignments as CMS defines them and add the navigation, search, and code links.
