Role of Coombs Test in Hyperbilirubinemia
The Direct Antiglobulin Test (DAT/Coombs test) is essential in hyperbilirubinemia workup when immune-mediated hemolysis is suspected, particularly in neonates with jaundice, patients with hematologic malignancies (CLL, NHL), those with autoimmune disease history, or when conjugated hyperbilirubinemia occurs with anemia. 1
Neonatal Hyperbilirubinemia
In newborns presenting with jaundice, the direct Coombs test is mandatory when the mother is Rh negative or has unknown blood type, as this identifies ABO or Rh incompatibility causing isoimmune hemolytic disease. 2, 3 The test should be performed on cord blood or infant blood samples. 3
When to Order DAT in Neonates:
- Mother is Rh negative or lacks prenatal blood typing - DAT is strongly recommended 2
- Mother is blood group O, Rh positive - DAT is optional but close monitoring required 2
- Total serum bilirubin (TSB) rising despite intensive phototherapy - suggests ongoing hemolysis requiring DAT 1
- TSB ≥25 mg/dL (428 μmol/L) or ≥20 mg/dL in sick/premature infants - urgent DAT needed 1
Complete Laboratory Panel for Positive DAT:
When DAT is positive, obtain: 1, 3
- TSB and direct bilirubin levels
- Blood type (ABO, Rh) of mother and infant
- Complete blood count with differential and peripheral smear
- Reticulocyte count
- Serum albumin
- G6PD testing (if ethnically indicated or poor phototherapy response)
Adult Hyperbilirubinemia
Specific Populations Requiring DAT:
Patients with chronic lymphocytic leukemia (CLL), non-Hodgkin's lymphoma, or autoimmune disease history should have Coombs testing when anemia develops, as these populations have increased risk of immune-mediated hemolysis. 1
Diagnostic Algorithm for Conjugated Hyperbilirubinemia:
First, determine if hyperbilirubinemia is conjugated (direct) or unconjugated (indirect) 1
For isolated unconjugated hyperbilirubinemia in asymptomatic adults:
For conjugated hyperbilirubinemia with anemia:
Critical Interpretation Points
DAT Should NOT Be Used as Screening Tool:
The DAT should only be ordered when clinical evidence suggests in vivo hemolysis - ordering without indication leads to high false-positive rates. 4, 5 Clinical indicators include:
- Anemia with elevated indirect bilirubin
- Low haptoglobin
- Elevated reticulocyte count
- Peripheral smear showing spherocytes or RBC fragments 6
Important Caveats:
Normal LDH does not exclude AIHA - approximately 25% of AIHA cases present with normal LDH levels despite active hemolysis. 6 Do not rely solely on LDH elevation to trigger DAT ordering.
False-negative DAT results occur when low levels of RBC autoantibodies are present. 5, 7 If clinical suspicion for AIHA remains high despite negative DAT, consider flow cytometry for RBC-bound IgG detection, which has superior sensitivity. 7
Eluate testing increases specificity when DAT is positive, helping determine autoantibody specificity and confirming true immune-mediated hemolysis versus false-positive results. 4, 5
Management Implications of Positive DAT
Neonatal Isoimmune Hemolytic Disease:
- Initiate intensive phototherapy immediately at lower bilirubin thresholds than non-hemolytic jaundice 1, 3
- Administer IVIG 0.5-1 g/kg over 2 hours if TSB rises despite phototherapy or approaches within 2-3 mg/dL of exchange transfusion threshold 1, 3
- Prepare for exchange transfusion if TSB ≥25 mg/dL or continues rising after 6 hours of intensive phototherapy 3
- Monitor TSB every 2-3 hours initially 3