Cold agglutinin disease is a rare form of autoimmune hemolytic anemia in which the body produces antibodies that attack its own red blood cells, triggering their destruction through a process driven by the complement system. The antibodies involved bind to red blood cells at cool temperatures, which is where the name comes from. With roughly 1 case per 100,000 people diagnosed each year in the United States and a strong tilt toward older adults, it is uncommon enough that many physicians encounter it only a handful of times in their careers, yet consequential enough that delayed recognition can lead to severe anemia, blood clots, and significant impairment in daily life.
What Happens Inside the Body
The core problem in cold agglutinin disease is an antibody called a cold agglutinin, almost always of the IgM type. These antibodies latch onto a specific sugar-based marker, usually the “I antigen,” on the surface of red blood cells. They do this most efficiently at temperatures around 0–4 °C, but in many patients the binding can occur at temperatures closer to normal body temperature, especially in the fingers, toes, ears, and nose where blood cools as it circulates near the skin.1Iran Red Crescent Medical Journal. Cold Agglutinin Disease; A Laboratory Challenge
Once these antibodies attach, they activate a chain reaction in the immune system called the classical complement pathway. Complement proteins punch holes in the red blood cell membrane or tag them for destruction by the liver. The result is hemolysis, the premature breakdown of red blood cells, which leads to anemia. The antibodies themselves also cause red blood cells to clump together (agglutinate), which contributes to circulatory symptoms in the extremities.2PubMed. Sutimlimab in Cold Agglutinin Disease
Primary Versus Secondary Forms
When physicians talk about cold agglutinin disease, they usually mean the primary form, which is now understood to be its own specific condition driven by an abnormal population of B cells in the bone marrow. These clonal B cells produce the harmful IgM antibodies. The bone marrow in primary cold agglutinin disease typically shows small clusters of abnormal B cells, though the overall burden is usually modest, with a median infiltration of about 10% of marrow cells.3PubMed Central. Primary cold agglutinin-associated lymphoproliferative disease: a B-cell lymphoma of the bone marrow distinct from lymphoplasmacytic lymphoma This underlying bone marrow disorder is classified as a distinct type of low-grade B-cell lymphoproliferative disease, separate from other lymphomas that can look superficially similar.4PubMed Central. Cold agglutinin disease
A secondary form, sometimes called cold agglutinin syndrome, can develop temporarily in connection with infections or other underlying conditions. Epstein-Barr virus (the virus behind infectious mononucleosis) is one of the classic triggers.5PubMed Central. Cold autoimmune haemolytic anaemia secondary to Epstein Barr virus infection presenting with peripheral gangrene; case report Mycoplasma pneumoniae infections can also cause it. The key difference is that the secondary form tends to be driven by polyclonal antibodies (produced by many different B-cell populations) rather than a single clone, and it often resolves once the underlying infection or condition is treated. Primary cold agglutinin disease, by contrast, is chronic and requires ongoing management.6PubMed Central. Autoimmune Hemolytic Anemias: Classifications, Pathophysiology, Diagnoses and Management
Who Gets It and How Common It Is
Cold agglutinin disease overwhelmingly affects older adults, with a peak around age 70, and occurs slightly more often in women than in men.7Iran Red Crescent Medical Journal. Cold Agglutinin Disease; A Laboratory Challenge U.S. data from multiple databases show annual incidence in the range of roughly 0.6 to 1.2 per 100,000 people, with point prevalence (the number of people living with the disease at a given time) ranging from about 1.6 to 3.3 per 100,000, depending on the database and year studied. Both incidence and prevalence were highest among people over 65.8PLoS One. Prevalence and incidence of primary autoimmune hemolytic anemia and cold agglutinin disease in the United States, 2016–2023
Geography matters in a way you might expect for a cold-triggered disease. A multinational study of 232 patients found roughly fourfold differences between colder and warmer climates: prevalence was about 20 per million in cold-climate regions versus 5 per million in warmer ones, and incidence was about 1.9 per million per year versus 0.48.9PubMed. Cold agglutinin disease revisited: a multinational, observational study of 232 patients This doesn’t mean the cold itself causes the disease, but cold exposure worsens symptoms and may push more people toward diagnosis.
What the Symptoms Feel Like
The hallmark symptom is anemia and the fatigue that comes with it. Because red blood cells are constantly being destroyed, many patients live with chronically low hemoglobin levels. The fatigue can be profound and is one of the symptoms patients rate as most disabling.
Cold-triggered circulatory symptoms are the other major feature. Acrocyanosis, a bluish discoloration of the fingers, toes, ears, or nose when exposed to cool temperatures, is common. Some patients experience Raynaud’s phenomenon, in which the fingers cycle through pallor, blue discoloration, and redness with cold exposure or stress.10PubMed Central. A middle-aged woman with the initial symptom of acrocyanosis diagnosed with cold agglutinin disease: A case report In rare and severe cases, the agglutination can be extensive enough to block small blood vessels and lead to gangrene of the fingertips or toes.11PubMed Central. Cold autoimmune haemolytic anaemia secondary to Epstein Barr virus infection presenting with peripheral gangrene; case report
Signs of hemolysis also show up in blood tests: elevated bilirubin (a breakdown product of hemoglobin that can cause jaundice), low haptoglobin, elevated lactate dehydrogenase, and an elevated reticulocyte count as the bone marrow tries to compensate by making new red blood cells faster. Symptoms tend to worsen in winter months or any situation that drops skin temperature.
The Blood Clot Risk That Often Gets Overlooked
One of the less-discussed consequences of cold agglutinin disease is a meaningfully higher risk of blood clots. A ten-year retrospective analysis found that about 30% of patients with cold agglutinin disease experienced at least one thrombotic event, compared with about 18% in a matched group without the disease. The overall risk was roughly doubled, with an adjusted hazard ratio of 1.94. Patients with the fewest other medical conditions showed an even greater relative increase, suggesting the disease itself is a significant independent driver of clotting risk.12PubMed Central. Increased risk of thrombotic events in cold agglutinin disease: A 10‐year retrospective analysis
A Danish study found that about 7% of patients with cold agglutinin disease had a thrombotic event within the first year after diagnosis, compared with about 2% in a comparison group. The gap persisted out to five years.13Blood Advances. Occurrence, thromboembolic risk, and mortality in Danish patients with cold agglutinin disease Both venous clots (deep vein thrombosis, pulmonary embolism) and arterial events (strokes, heart attacks) were elevated. This is something patients and physicians need to keep on their radar, even if the patient’s anemia seems well controlled.
Diagnosis and the Lab Pitfalls
Diagnosis rests on a few key findings. The direct antiglobulin test (often called a Coombs test) is the starting point: in cold agglutinin disease, this test is positive for complement component C3d on the surface of red blood cells but typically negative for IgG, which helps distinguish it from warm autoimmune hemolytic anemia.14PubMed Central. Cold Agglutinin Disease Alongside this, a cold agglutinin titer is measured, and the thermal amplitude (the highest temperature at which the antibodies can still bind red blood cells) is assessed. A high titer with a thermal amplitude approaching 37 °C points to clinically significant disease.
Cold agglutinins create a particularly frustrating problem in the lab. When a blood sample cools to room temperature, the antibodies start clumping the red blood cells inside the collection tube. This can wreak havoc on routine blood counts: automated analyzers may falsely report a very low red blood cell count, a sky-high mean corpuscular volume, or bizarre hemoglobin values. The fix is simple but easily overlooked. Warming the sample to 37 °C before running it through the analyzer disperses the clumps and gives accurate results.15PubMed Central. Effect of cold agglutinins on red blood cell parameters in a trauma patient: a case report Similarly, serological testing for cold agglutinins is more reliable when blood is kept warm from the moment it is drawn through plasma separation, rather than being rewarmed after the fact, though studies show reheating also produces acceptable results.16PubMed Central. Unifying serological testing for cold agglutinins
Blood crossmatching before transfusion presents a similar headache. Cold agglutinins can make donor blood appear incompatible when it actually is not. Warming the crossmatch sample to 37 °C resolves the false incompatibility in the vast majority of cases. In one study of 30 patients with crossmatch problems, warming reversed the agglutination in 28 of them, meaning only 2 had a true blood group incompatibility.17PubMed Central. Resolving Incompatible Blood Cross-Matching: The Role of 37°C Water Bath in Transfusion Safety
Telling It Apart From Related Conditions
Cold agglutinin disease sits within a family of autoimmune hemolytic anemias, and sorting one type from another matters because the treatments are different. Warm autoimmune hemolytic anemia, the most common type overall, is driven by IgG antibodies that work at body temperature and responds well to steroids and splenectomy, neither of which is particularly effective in cold agglutinin disease. Paroxysmal cold hemoglobinuria is another cold-reactive condition, but it is caused by an IgG antibody (rather than IgM) that binds in the cold and activates complement when the blood rewarms. It shows up mostly in children after viral infections.18PubMed Central. Autoimmune Hemolytic Anemias: Classifications, Pathophysiology, Diagnoses and Management
The distinction between primary cold agglutinin disease and the secondary cold agglutinin syndrome is also clinically important, as covered earlier. Primary disease has a monoclonal antibody from a clonal bone marrow disorder. Secondary syndrome has polyclonal antibodies linked to an identifiable trigger. Treatment of the secondary form targets the underlying cause, while the primary form requires disease-specific therapy.
Avoiding the Cold and Other Supportive Measures
The most basic intervention is also the one patients have the most control over: avoiding cold exposure. This means keeping the home warm, dressing in layers with particular attention to the hands, feet, and face, and being cautious about cold food and drinks. In a hospital setting, intravenous fluids and blood products should be warmed before administration, because even room-temperature fluids can trigger agglutination in sensitive patients.19PubMed Central. Cold agglutinin-induced hemolytic anemia during room temperature fluid resuscitation: a case report Some patients find that relocating to a warmer climate meaningfully reduces their symptom burden, though this is obviously not practical for everyone.
Folic acid supplementation is commonly recommended because the bone marrow’s constant effort to replace destroyed red blood cells increases the demand for folate. Transfusions are used when anemia becomes severe, though they are a stopgap, not a treatment for the underlying disease.
Drug Treatments That Target the Underlying Problem
For decades, cold agglutinin disease was frustratingly resistant to the drugs that worked in warm autoimmune hemolytic anemia. Corticosteroids are largely ineffective, and splenectomy does not help because the red blood cell destruction in cold agglutinin disease happens mainly in the liver, not the spleen. Two therapeutic strategies have changed the landscape: targeting the abnormal B cells that produce the antibodies, and blocking the complement pathway that the antibodies activate.
B-Cell Directed Therapy
Rituximab, an antibody that depletes B cells, was the first drug to show meaningful responses. The combination of rituximab with bendamustine (a chemotherapy agent) has become one of the more effective B-cell directed options, producing high response rates, frequent complete responses, and response durations that can last years. Treatment is given as a limited course rather than ongoing infusions.20PubMed. Current Treatment Options in Cold Agglutinin Disease: B-Cell Directed or Complement Directed Therapy? The tradeoff is that these drugs suppress the immune system, so the risk of infections during and after treatment needs to be weighed against the benefit.
Complement Inhibition With Sutimlimab
Sutimlimab represents a fundamentally different approach. Rather than going after the B cells making the antibodies, it blocks the complement cascade that the antibodies set off. Specifically, it targets C1s, a protein at the very start of the classical complement pathway. By shutting down this step, sutimlimab prevents the cascade from ever getting going, and red blood cell destruction drops quickly.21PubMed. Sutimlimab in Cold Agglutinin Disease
In a first-in-human trial, seven of ten patients responded, with a median best hemoglobin increase of about 4 g/dL after six weeks of treatment. Four patients saw their hemoglobin normalize completely. Responses were seen even in patients who had failed rituximab-based regimens.22Blood. Inhibition of complement C1s improves severe hemolytic anemia in cold agglutinin disease: a first-in-human trial The larger phase III CARDINAL trial confirmed these results: 54% of patients met the primary endpoint of reaching a hemoglobin of at least 12 g/dL or increasing by at least 2 g/dL. The placebo-controlled CADENZA trial showed an even clearer separation, with 73% of sutimlimab-treated patients meeting a composite endpoint versus 15% on placebo.23PubMed. Sutimlimab: A Complement C1s Inhibitor for the Management of Cold Agglutinin Disease-Associated Hemolysis
Beyond the blood numbers, sutimlimab produced rapid and sustained improvements in fatigue and quality of life. Patients in the CARDINAL study started with fatigue scores indicating severe impairment, and meaningful improvement appeared within the first week and persisted over two years of follow-up.24PubMed Central. Sutimlimab improves quality of life in patients with cold agglutinin disease: results of patient-reported outcomes from the CARDINAL study However, when treatment was stopped after a nine-week washout, fatigue scores drifted back toward where they started.25Blood Advances. Long-term sutimlimab improves quality of life for patients with cold agglutinin disease: CARDINAL 2-year follow-up This underscores a key practical reality: sutimlimab controls the disease but does not cure it. Patients need ongoing infusions, typically every two weeks, to maintain the benefit.
Surgery and Anesthesia Considerations
Cold agglutinin disease poses specific challenges when a patient needs surgery, particularly any operation involving cardiopulmonary bypass (the heart-lung machine). Standard cardiac surgery protocols often cool the patient’s blood to protect organs during the procedure, but cooling blood in someone with cold agglutinins can trigger massive agglutination and potentially catastrophic hemolysis.
A review of patients with cold agglutinins undergoing cardiac surgery at the Mayo Clinic found that with appropriate precautions, all patients came through safely without major complications. The key measures were maintaining normothermic bypass at 37 °C and using warm cardioplegia (the solution that stops the heart during surgery) rather than the cold cardioplegia that is standard in many centers. Only one patient in the series had postoperative hemolysis requiring transfusion, and that resolved with active warming. Patients known to have clinically active disease should have their cold agglutinin titers and thermal amplitude tested and receive a hematology consultation before any surgery involving bypass.26PubMed. Cold agglutinins in patients undergoing cardiac surgery requiring cardiopulmonary bypass
Outside of cardiac surgery, the general principle holds: keep the patient warm. Operating rooms are notoriously cold, and anesthesia impairs the body’s ability to regulate temperature. Warming blankets, heated IV fluids, and keeping the room temperature up are all standard precautions for patients with this disease.
The Bone Marrow Disorder Behind the Scenes
For years, the bone marrow process driving primary cold agglutinin disease was lumped in with other low-grade lymphomas, particularly lymphoplasmacytic lymphoma (the disease associated with Waldenström macroglobulinemia). More recent work has established that the two are distinct entities. In cold agglutinin disease, the bone marrow infiltrate consists of small monotonous B cells forming nodular clusters within the marrow space, not hugging the bone trabeculae the way follicular lymphoma or lymphoplasmacytic lymphoma does. The cells carry a characteristic surface marker profile and, critically, lack the MYD88 L265P mutation that is a hallmark of lymphoplasmacytic lymphoma.27PubMed Central. Primary cold agglutinin-associated lymphoproliferative disease: a B-cell lymphoma of the bone marrow distinct from lymphoplasmacytic lymphoma
Over 80% of bone marrow biopsies in cold agglutinin disease show these characteristic nodular B-cell aggregates.28American Journal of Clinical Pathology. The Histopathology of Cold Agglutinin Disease–Associated B-Cell Lymphoproliferative Disease The infiltrate is generally low-grade and indolent. Most patients do not develop aggressive lymphoma, though they do need monitoring. This distinction matters practically: it means the disease requires treatment strategies tailored to its own biology rather than borrowed from the lymphoplasmacytic lymphoma playbook.
The Burden on Daily Life and the Healthcare System
Even with emerging treatments, cold agglutinin disease places a heavy load on patients and the medical system. An analysis of a large U.S. electronic health record database found that about a third of patients were hospitalized at least once within 12 months of diagnosis, and over half were hospitalized during longer follow-up. Healthcare utilization was markedly worse in patients with severe anemia compared with those whose anemia was milder.29PubMed Central. An observational analysis of disease burden in patients with cold agglutinin disease: Results from a large US electronic health record database
Compared with matched individuals without the disease, patients with cold agglutinin disease had roughly three times as many inpatient hospitalizations, more than double the outpatient visits, and dramatically more transfusion days in the year after diagnosis.30PubMed. Healthcare resource utilization among commercially insured patients with cold agglutinin disease in the United States The fatigue alone can be disabling. In the CARDINAL trial, patients entered with fatigue scores indicating severe impairment, and the improvements with sutimlimab, while substantial, underscore just how poor baseline quality of life can be.31PubMed Central. Sutimlimab improves quality of life in patients with cold agglutinin disease: results of patient-reported outcomes from the CARDINAL study Patients often describe rearranging their entire lives around temperature avoidance, limiting travel, outdoor activities, and even social engagements during colder months. For a disease sometimes dismissed as “just anemia,” the real-world impact can be surprisingly pervasive.