P55.1
BillableABO isoimmunization of newborn
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is P55.1 an HCC code?
Yes. P55.1 maps to Hemolytic and Aplastic Anemias under the CMS-HCC V28 risk adjustment model.
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 P55.1
For P55.1 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 P55.1 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
P55.1 is the ICD-10-CM diagnosis code for abo isoimmunization of newborn. A condition where the newborn's red blood cells are destroyed by antibodies from the mother due to ABO blood type incompatibility. P55.1 sits in the ICD-10-CM chapter for certain conditions originating in the perinatal period (p00-p96), within the section covering hemorrhagic and hematological disorders of newborn (p50-p61).
Under the CMS-HCC V28 risk adjustment model, P55.1 maps to Hemolytic and Aplastic Anemias (HCC 109) with a community, non-dual, aged base RAF weight of 0.291. P55.1 was not retained as a payment HCC under the older V24 model, so V28 introduced or recategorized it during the 2024–2026 phase-in. 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.
ABO incompatibility is more common than Rh but typically causes milder disease. Because P55.1 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 P55.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •ABO incompatibility is more common than Rh but typically causes milder disease
- •Document maternal and infant blood types and antibody titers when available
Clinical Significance
ABO isoimmunization represents maternal-fetal blood group incompatibility leading to hemolysis and potential severe anemia in newborns, requiring immediate monitoring and treatment. This condition can progress to kernicterus if untreated and may require exchange transfusion, making accurate diagnosis and coding essential for care planning.
Documentation Requirements
- ✓Documentation of ABO blood group incompatibility between mother and baby
- ✓Evidence of hemolysis with elevated bilirubin levels
- ✓Maternal antibody testing (anti-A or anti-B antibodies)
- ✓Newborn blood type and direct Coombs test results
- ✓Assessment of anemia severity and hemoglobin levels
- ✓Monitoring for hyperbilirubinemia and kernicterus risk
- ✓Treatment interventions including phototherapy or exchange transfusion
- ✓Serial bilirubin monitoring and response to treatment