Laboratory Workup for Cold Agglutinin Disease
When you suspect cold agglutinin disease, immediately order a C3d-specific direct antiglobulin test (DAT), cold agglutinin titer at 4°C, complete blood count with peripheral smear, and reticulocyte count—all samples must be maintained at 37°C from collection through analysis to avoid spurious results.
Essential Diagnostic Tests
Core Immunohematologic Testing
Direct Antiglobulin Test (DAT) with monospecific antisera: Must show C3d positivity while being negative for IgG. This pattern is pathognomonic for CAD and distinguishes it from warm autoimmune hemolytic anemia 1, 2, 3.
Cold agglutinin titer: Must be ≥1:64 at 4°C, though most CAD patients demonstrate titers >1:1,000 4, 2. The titer alone does not predict severity—thermal amplitude (the highest temperature at which agglutination occurs) is a better predictor of clinical severity 3.
Cold agglutinin immunoglobulin class: Approximately 90% are IgM class 2. This should be confirmed as part of the workup.
Hemolysis Markers
Complete blood count: Expect normocytic or macrocytic anemia. Critical caveat: Cold agglutinins cause spuriously low RBC counts and falsely elevated MCH/MCHC on automated analyzers if samples are not kept warm 5.
Reticulocyte count: Elevated, indicating hemolytic process 2.
Lactate dehydrogenase (LDH): Elevated 6.
Indirect bilirubin: Elevated 6.
Haptoglobin: Decreased (though predominantly extravascular hemolysis occurs in liver) 3.
Bone Marrow Evaluation
Unilateral bone marrow aspirate and biopsy: Required to document clonal lymphoplasmacytic cell population 4. CAD is now recognized as a distinct lymphoproliferative disorder by WHO 2022 classification 3.
Flow cytometry and immunohistochemistry: To characterize the B-cell clone 4.
MYD88 (L265P) mutation testing: Recommended via allele-specific PCR, though absence does not exclude diagnosis 4.
Critical Pre-Analytical Requirements
All blood samples must be collected and maintained at 37°C until analysis is complete 1. This is non-negotiable—failure to do so produces:
- Falsely low RBC counts
- Spuriously elevated MCH/MCHC (>100 g/dL should trigger suspicion) 5
- Inaccurate hemoglobin measurements
- Invalid cold agglutinin titers
Use a Bain-Marie water bath at 37°C if necessary to repeat testing 5.
Additional Testing Under Specific Circumstances
When IgM Paraprotein is Present
Serum protein electrophoresis with immunofixation: To quantify monoclonal IgM 4.
Serum viscosity: If hyperviscosity symptoms present (though <10% of CAD patients develop this) 4.
Cryoglobulin testing (cryocrit): Up to 20% of patients have cryoglobulins, though only 5% are symptomatic. Cryoglobulins can falsely lower IgM measurements—keep samples warm 4.
Complement Levels
Imaging
- CT chest/abdomen/pelvis or PET-CT: To assess for lymphadenopathy, splenomegaly, or extramedullary disease associated with the underlying lymphoproliferative disorder 4.
Differential Diagnosis Considerations
A negative C3d DAT should immediately redirect you toward warm AIHA, not CAD 2. This distinction is critical because corticosteroids—effective in warm AIHA—are not effective in CAD and should not be used 2, 3.
If Ham test (acidified-serum lysis test) is positive, consider paroxysmal nocturnal hemoglobinuria in the differential, though CAD patients can occasionally show weak Ham positivity 6. PNH clone detection by flow cytometry differentiates these entities 6.
Common Pitfalls to Avoid
Sample handling errors: The most frequent cause of diagnostic confusion. Always maintain 37°C 1, 5.
Relying on titer alone: A titer of 1:64 meets diagnostic criteria, but thermal amplitude better predicts clinical severity 3.
Missing the underlying lymphoproliferative disorder: CAD is not just an antibody problem—it requires bone marrow evaluation 4, 3.
Confusing CAD with secondary cold agglutinin syndrome (CAS): CAS occurs with infections (especially Mycoplasma pneumoniae) or lymphomas and resolves with treatment of the underlying cause 7, 3. Primary CAD requires clone-directed therapy.