M00.161
BillablePneumococcal arthritis, right knee
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is M00.161 an HCC code?
Yes. M00.161 maps to Bone/Joint/Muscle/Severe Soft Tissue 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 M00.161
For M00.161to 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 M00.161 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
M00.161 is the ICD-10-CM diagnosis code for pneumococcal arthritis, right knee. A bacterial infection of the right knee joint caused by pneumococcus bacteria, resulting in joint inflammation and pain. M00.161 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering infectious arthropathies (m00-m02).
Under the CMS-HCC V28 risk adjustment model, M00.161 maps to Bone/Joint/Muscle/Severe Soft Tissue Infections/Necrosis (HCC 92) with a community, non-dual, aged base RAF weight of 0.479. Under the older CMS-HCC V24 model, M00.161 maps to Bone/Joint/Muscle Infections/Necrosis (HCC 39) with a community, non-dual, aged base RAF weight of 0.401. 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.
Verify right knee is documented; knee infections are common sites for pneumococcal arthritis. Because M00.161 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 M00.161 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Verify right knee is documented; knee infections are common sites for pneumococcal arthritis
- •Look for associated symptoms like swelling, warmth, and limited mobility in documentation
Clinical Significance
Septic arthritis of the right knee caused by Streptococcus pneumoniae (pneumococcal) organisms is a serious infectious condition requiring emergent treatment with intravenous antibiotics and often surgical intervention. Involvement of the right knee, a large weight-bearing or major joint, increases the risk of significant functional impairment and may require arthroscopic or open surgical drainage. This condition reflects high healthcare complexity and resource needs, making it significant for risk adjustment.
Documentation Requirements
- ✓Provider documentation explicitly stating pneumococcal septic arthritis or pyogenic arthritis with identified causative organism
- ✓Synovial fluid analysis results (cell count, Gram stain, crystal analysis to rule out gout/pseudogout)
- ✓Culture and sensitivity results identifying the specific pneumococcal organism, or clinical rationale if culture-negative
- ✓Clinical signs and symptoms of joint infection (acute onset joint pain, swelling, warmth, erythema, fever, limited range of motion)
- ✓Documentation confirming right-sided involvement with imaging or examination findings specific to that side
- ✓Treatment plan including antibiotic regimen, duration, and any surgical intervention (arthrocentesis, irrigation and debridement, arthroscopy)
Commonly Confused Codes
- •M00.2x (Other streptococcal arthritis) — Use M00.2x when the causative streptococcal organism is not Streptococcus pneumoniae; M00.1x is specifically for pneumococcal (Streptococcus pneumoniae) joint infections
- •M00.8x (Arthritis due to other bacteria) — Use M00.8x when the septic arthritis is caused by a non-streptococcal bacterium such as Staphylococcus aureus, Pseudomonas, or gram-negative rods
- •M00.0x (Staphylococcal arthritis) — Use M00.0x when culture identifies Staphylococcus species rather than Streptococcus pneumoniae