What are the causes of a positive direct Coombs test in patients, particularly those with a history of autoimmune disorders, such as Systemic Lupus Erythematosus (SLE) or Rheumatoid Arthritis (RA), or those who have received blood transfusions or have a history of hemolytic disease of the newborn?

Causes of Direct Coombs Test (DAT) Positivity

A positive Direct Coombs test indicates antibody or complement coating on red blood cells, most commonly caused by autoimmune disorders (particularly SLE, rheumatoid arthritis, and autoimmune hepatitis), lymphoproliferative diseases (especially chronic lymphocytic leukemia), drug exposures, infections (including CMV, Mycoplasma, and COVID-19), and transfusion reactions. 1, 2, 3

Primary Autoimmune Causes

Systemic lupus erythematosus is one of the most frequent causes of positive DAT, with 12.8% of SLE patients showing DAT positivity 4. Among these patients:

• 54.3% develop overt hemolytic anemia 4
• DAT positivity is independently associated with anti-RNP and anti-La antibodies 4
• Antibodies such as anti-Sm and anti-RNP are linked to hematological disorders including hemolytic anemia 5

Autoimmune hepatitis demonstrates significant association with positive DAT 5:

• Concurrent autoimmune hemolytic anemia occurs in AIH patients 5
• Type 1 AIH is commonly associated with autoimmune thyroid disease 5
• Type 2 AIH associates with type 1 diabetes, thyroid disease, and autoimmune skin conditions 5

Rheumatoid arthritis and other connective tissue diseases commonly produce positive DAT without overt hemolysis 1, 5

Lymphoproliferative Disorders

Chronic lymphocytic leukemia and non-Hodgkin lymphoma frequently cause positive DAT even without hemolysis 1, 3:

• These malignancies should be screened with bone marrow biopsy and flow cytometry when DAT is positive with unexplained cytopenias 3
• The American Society of Clinical Oncology recommends Coombs testing in CLL and NHL patients with anemia 3

Drug-Induced Causes

Medications represent a critical reversible cause requiring thorough drug exposure history 1, 3:

• Immunosuppressants: tacrolimus, cyclosporine, and sirolimus can cause drug-induced positive DAT without hemolysis 3
• Immune checkpoint inhibitors cause immune-related autoimmune hemolytic anemia, with 40% of cases showing DAT negativity despite clinical hemolysis 2, 3
• Sulfasalazine has been documented to cause Coombs-positive hemolytic anemia 6

Infectious Etiologies

Viral infections are increasingly recognized causes of DAT positivity:

• Cytomegalovirus can induce severe Coombs-positive hemolysis in immunocompetent young adults, with high CMV DNA titers detected by PCR 7
• Epstein-Barr virus may present with positive DAT and elevated IgM antibodies to viral capsid antigen 7
• COVID-19 shows significant association with positive DAT in 20% of hospitalized patients 8, likely due to cytokine storm-induced hyperinflammation, complement activation, and RBC alterations 8

Bacterial infections:

• Mycoplasma pneumoniae causes severe hemolytic anemia with strongly positive direct Coombs test, high cold agglutinin titers with anti-I specificity, and elevated antimycoplasma antibody titers 9

Inflammatory Bowel Disease

Ulcerative colitis associates with autoimmune hemolytic anemia in 1.7% of patients 6:

• Positive Coombs without hemolysis occurs in 2% of UC patients 6
• The complication occurs early in disease course (mean 17 months) and relates to disease activity and extent 6
• 77.7% of Coombs-positive UC patients have total colitis 6

Transfusion-Related Causes

Blood transfusions can cause DAT positivity through:

• Alloimmunization to red blood cell antigens, requiring maternal blood type, Rh(D) status, and indirect Coombs testing 5
• Hemolytic disease evaluation requires Kleihauer-Betke stain for fetal hemoglobin 5

Critical Diagnostic Pitfall

Do not assume positive DAT always indicates active hemolysis 3. Confirm with hemolysis markers:

• LDH, haptoglobin, indirect bilirubin, and reticulocyte count 1, 2, 3
• Low reticulocyte count with positive DAT indicates antibody sensitization without active hemolysis 1
• Normal peripheral smear excludes schistocytes or spherocytes suggesting destruction 1

Essential Workup for Positive DAT

When DAT is positive, systematically evaluate:

Hemolysis confirmation: LDH, haptoglobin, indirect bilirubin, reticulocyte count 1, 2

Autoimmune screening: ANA, RF, HLA-B27, anti-RNP, anti-Sm, anti-La 1, 2, 3

Infectious workup: CMV PCR, EBV serology, Mycoplasma antibodies, COVID-19 testing 1, 2, 7, 9

Medication review: Complete drug history including recent additions, particularly immunosuppressants and checkpoint inhibitors 2, 3

Lymphoproliferative evaluation: CBC with differential, peripheral smear, consider bone marrow biopsy if cytopenias present 2, 3