B26.85
BillableMumps arthritis
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is B26.85 an HCC code?
Yes. B26.85 maps to Bone/Joint/Muscle Infections/Necrosis under the CMS-HCC V28 risk adjustment model (and Bone/Joint/Muscle Infections/Necrosis under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for B26.85
For B26.85 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically — it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed B26.85 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
B26.85 is the ICD-10-CM diagnosis code for mumps arthritis. Inflammation of the joints (arthritis) caused by the mumps virus, occurring as a complication of mumps infection. B26.85 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering other viral diseases (b25-b34).
Under the CMS-HCC V28 risk adjustment model, B26.85 maps to Bone/Joint/Muscle Infections/Necrosis (HCC 92) with a community, non-dual, aged base RAF weight of 0.209. Under the older V24 model, B26.85 mapped to the same category but with a base RAF weight of 0.482 — V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
Document which joints are affected and whether symptoms are symmetric or asymmetric. Because B26.85 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for B26.85 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Document which joints are affected and whether symptoms are symmetric or asymmetric
- •Mumps arthritis is typically self-limited; specify if monoarticular or polyarticular involvement
Clinical Significance
Mumps arthritis is an uncommon but recognized complication of mumps virus infection, occurring in approximately 0.4% of mumps cases. It typically presents as a migratory polyarthritis affecting large joints and is usually self-limited, resolving within weeks. In the era of widespread MMR vaccination, this condition is rare but may be seen during outbreaks.
Documentation Requirements
- ✓Confirmed mumps infection (positive mumps IgM or viral PCR)
- ✓Joint involvement documented: specific joints affected, symmetry, duration
- ✓Timeline of arthritis relative to parotitis or other mumps symptoms
- ✓MMR vaccination history
- ✓Clinical course documentation: onset, peak, resolution