# Cold agglutinin disease
**Cold agglutinin disease** (CAD) is a rare form of autoimmune hemolytic anemia in which a clonal population of bone marrow B cells produces monoclonal IgM-κ autoantibodies that bind red blood cells at cool temperatures and destroy them through activation of the classical complement pathway. It accounts for approximately 15% to 25% of autoimmune hemolytic anemia cases and combines two features in one disorder: a low-grade lymphoproliferative clone and complement-mediated hemolysis.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
| Key fact | Detail |
|---|---|
| Frequency | About 15%–25% of autoimmune hemolytic anemia cases; estimated prevalence 5–20 per million population, annual incidence roughly 0.5–1.9 per million[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) |
| Who it affects | Predominantly older adults, median age at diagnosis about 67–72 years, with a female predominance[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) |
| Mechanism | Monoclonal IgM-κ binds the I antigen on erythrocytes below core temperature, activating the classical complement pathway; C3b-opsonized cells are cleared mainly by Kupffer cells in the liver[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) |
| Diagnosis | Chronic hemolysis, monospecific direct antiglobulin test strongly positive for C3d and negative for IgG, cold agglutinin titer ≥1:64 at 4 °C, and no overt malignant lymphoma[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) |
| Ineffective treatments | Corticosteroids and splenectomy, mainstays of warm autoimmune hemolytic anemia, are generally ineffective in CAD and not recommended[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) |
| First approved drug | Sutimlimab, a humanized anti-C1s monoclonal antibody, received US approval in February 2022[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) |
| Survival | Median survival of approximately 16 years from diagnosis, close to the ~18.4-year life expectancy of the age-matched general population[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) |
## Pathophysiology
CAD follows a two-hit process. The first hit is a clonal B-cell expansion in the bone marrow, a low-grade lymphoproliferative disorder that secretes a monoclonal IgM-κ autoantibody. The clone characteristically shows IGHV4-34 gene rearrangement and lacks the MYD88 L265P mutation, which distinguishes it from lymphoplasmacytic lymphoma and [Waldenström macroglobulinemia](https://www.edgepedia.org/waldenstrom-macroglobulinemia).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) [Bone marrow](https://www.edgepedia.org/bone-marrow) infiltration is mostly under 10% and may be tiny, so the clone can be difficult to detect.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC13462152/)
The second hit is complement activation. In the cooler peripheral circulation, the IgM binds the I antigen on the red blood cell surface and activates the classical pathway in the sequence C1 → C4 → C2 → C3. C3b-opsonized erythrocytes are cleared by Kupffer cells in the liver, producing predominantly extravascular hemolysis; progression to the membrane attack complex (C5b-9) causes intravascular hemolysis, chiefly during acute exacerbations.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
The 2022 World Health Organization classification recognized CAD-associated lymphoproliferative disease as a distinct entity, separate from lymphoplasmacytic lymphoma and Waldenström macroglobulinemia, based on its IGHV4-34 restriction, absent MYD88 L265P, minimal plasmacytic differentiation, and nodular CD20-positive marrow infiltration without perinodular mast cells.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
## Clinical features
[Hemolytic anemia](https://www.edgepedia.org/hemolytic-anemia) is the cardinal symptom. In a large observational study, the median hemoglobin level was 9.2 g/dL; severe anemia (below 8.0 g/dL) was present in 27% of patients, moderate anemia (8.0–10.0 g/dL) in 37%, mild anemia in 24%, and 12% had compensated hemolysis.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC13462152/) More than half of patients receive red blood cell transfusions at some point, and hemolysis often worsens seasonally in colder months and during febrile illnesses.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
**Cold-induced circulatory symptoms** arise from agglutination in acral vessels, where cooling lets IgM bind erythrocytes and impede capillary passage. Acrocyanosis (bluish discoloration of hands and feet from poor circulation) occurs in 50% to 90% of patients, often with Raynaud-like phenomena; in the observational study 52% had acrocyanosis or Raynaud-like symptoms.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC13462152/) These symptoms are not complement-mediated and are therefore not addressed by complement inhibitors.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) Patients also report profound fatigue, and CAD carries an increased thrombotic risk: the reported hazard ratio for venous thromboembolism is approximately 3.3 relative to the general population, and arterial thrombotic events are also more frequent.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
## Diagnosis
The work-up proceeds in steps. First, hemolysis is confirmed by elevated lactate dehydrogenase and indirect bilirubin, reticulocytosis, and reduced haptoglobin, with the blood smear possibly showing erythrocyte agglutination. Second, a monospecific direct antiglobulin test (DAT) shows the characteristic pattern: strong C3d positivity with a negative or only weakly positive IgG result. Third, a cold agglutinin titer of at least 1:64 at 4 °C confirms cold-reactive autoimmunity. Finally, secondary cold agglutinin syndrome is excluded by evaluating for infection ([Mycoplasma pneumoniae](https://www.edgepedia.org/mycoplasma-pneumoniae), [Epstein–Barr virus](https://www.edgepedia.org/epstein-barr-virus), cytomegalovirus), autoimmune disease, and lymphoma; bone marrow examination confirms the clonal process but is not mandatory for every patient.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
Two practical points improve accuracy. Blood samples should be collected and transported at 37 °C, because in vitro cooling causes agglutination that can spuriously raise the measured mean corpuscular volume and lower the red blood cell count; these artifacts resolve on rewarming. Thermal amplitude, the highest temperature at which cold agglutinins react with the antigen, predicts clinical severity better than titer alone: a thermal amplitude of roughly 28 °C to 30 °C or higher marks a pathogenic threshold, while a high titer with a low thermal amplitude may be clinically silent.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9088174/)
**Distinguishing CAD from related entities** matters for treatment. Cold agglutinin syndrome (CAS) is a secondary condition in which cold agglutinins arise from infection, lymphoma, or other disorders; infection-associated CAS is typically polyclonal and transient, resolving with the underlying cause, and management is directed at that cause. [Warm autoimmune hemolytic anemia](https://www.edgepedia.org/autoimmune-hemolytic-anemia-warm-type), by contrast, is IgG-mediated and [Fc receptor](https://www.edgepedia.org/fc-receptor)-dependent, responds to corticosteroids and splenectomy, and shows an IgG-positive DAT pattern.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
## Treatment
Not all patients need drug therapy; treatment is indicated for clinically significant anemia (for example, hemoglobin below 10 g/dL or a significant decline), transfusion dependence, disabling circulatory symptoms, or anticipated cold exposure or surgery. General measures for all patients include thermal protection, folic acid supplementation, thromboprophylaxis in high-risk settings, and mandatory use of a blood warmer during transfusion.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
**B-cell-directed therapy** targets the underlying clone and can produce durable, treatment-free remission. Rituximab monotherapy (375 mg/m² weekly for four weeks) yields overall response rates of about 50% to 60% with complete responses under 5% and a median response duration of about 11 months; it suits frail or elderly patients and can be repeated at relapse. For many otherwise healthy patients, bendamustine combined with rituximab is a favored initial treatment, producing overall response rates of 71% to 78% and complete response rates of 40% to 53%; according to the Nordic prospective experience, remission was still sustained after five years in about 77% of patients, though the regimen causes myelosuppression, with roughly a third of patients experiencing grade 3–4 neutropenia.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
**Complement inhibition with sutimlimab** transformed the field. Sutimlimab is a humanized monoclonal antibody against C1s that selectively blocks the classical pathway while sparing the lectin and alternative pathways, preserving much of innate immune surveillance.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) Among 24 patients requiring transfusions who enrolled in CARDINAL, a phase 3 study without blinding, sutimlimab achieved its composite primary endpoint for 54% of participants: mean hemoglobin climbed from 8.6 to 11.2 g/dL, 71% no longer needed transfusions, and fatigue improved by 10.9 points on the FACIT-Fatigue scale, with effects appearing within a week.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) The randomized, double-blind, placebo-controlled CADENZA trial in 42 non-transfusion-dependent patients confirmed these findings: the primary endpoint was met by 72.7% of sutimlimab patients versus 15% of placebo patients (odds ratio 15.9; p < 0.001), and hemoglobin reached at least 12 g/dL in 50% versus 7%.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
Long-term data reinforce sustained benefit but also its limits. In CADENZA Part B, 32 of 39 patients completed up to one further year of sutimlimab (median treatment duration 99 weeks); hemoglobin improved from 9.3 g/dL at baseline to at least 11.0 g/dL on treatment and bilirubin fell from 35.0 μmol/L to at most 20.0 μmol/L, with no systemic lupus erythematosus or meningococcal infections observed. After a 9-week washout, classical pathway inhibition was reversed and hemolytic markers approached baseline levels, showing that disease activity reoccurs when treatment stops.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC11293518/) Because sutimlimab does not affect the underlying clone, chronic treatment is necessary, and combining it with B-cell-directed therapy is attractive when a fixed-duration approach is desired.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/) [Vaccination](https://www.edgepedia.org/vaccination) against encapsulated bacteria should be completed or updated at least two weeks before the first dose.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
**Other and emerging agents** fill remaining gaps. Terminal C5 inhibition with eculizumab is generally unsuitable because hemolysis is largely C3b-mediated and extravascular, upstream of C5. Riliprubart, a second-generation anti-C1s antibody directed at active C1s, showed hemoglobin above 11 g/dL from day 29 sustained about 15 weeks after a single dose in a phase 1b study of 12 patients, suggesting possible dosing every 12 weeks. Iptacopan, an oral factor B inhibitor, met its primary endpoint in 50% of a 10-patient CAD cohort with a mean hemoglobin rise of 2.2 g/dL at week 12, and pegcetacoplan, a subcutaneous C3 inhibitor, raised median hemoglobin by 2.4 g/dL at week 48 in a phase 2 study of 13 patients, though it has not advanced to pivotal development for CAD.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
## References
1. [Choi, Dae-Ho (2026). Cold agglutinin disease: pathophysiology, diagnosis, and the evolving therapeutic landscape. Blood Research.](https://pmc.ncbi.nlm.nih.gov/articles/PMC13545198/)
2. [Cold agglutinin disease (clinical features review), PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC13462152/)
3. [Long-term efficacy and safety of continued complement C1s inhibition with sutimlimab in cold agglutinin disease: CADENZA study Part B, PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC11293518/)
4. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC9088174/)
---
*Topic: Encyclopedia › Life and health › Human health and medicine › Diseases and injuries › Cardiovascular and blood conditions › Blood disorders (hematologic conditions) › Anemias › Hemolytic anemias › Cold agglutinin disease (cold antibody hemolytic anemia)*
*Initially written Sep 17, 2026 · Reviewed: Sep 17, 2026; Sep 30, 2026; Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license