indirect Coombs

**Principle:** Indirect = "in vitro" test for antibodies FREE in serum. (Direct Coombs = antibodies/complement already bound to the patient's RBCs in vivo.)

**Steps:** 1. Patient serum is incubated with reagent RBCs of known antigen type (e.g., Rh D-positive cells). 2. If the serum contains antibodies (usually IgG) to those antigens, they coat the reagent RBCs (sensitization) but IgG alone often cannot cause visible agglutination. 3. Wash cells, then add Coombs reagent (anti-human IgG +/- complement). 4. Anti-human globulin cross-links the IgG-coated cells, causing visible agglutination = POSITIVE.

**Mnemonic:** Indirect = Is there antibody in the serum? (tested in a tube/"indirectly"); Direct = antibodies already on the patient's cells.

**Clinical uses:** - **Prenatal screening:** Rh-negative mother screened at first prenatal visit and ~28 weeks for anti-D (and other alloantibodies) before/after RhoGAM (anti-D immunoglobulin). A positive result in an unsensitized mother who received RhoGAM can reflect passively acquired anti-D. A positive result with a true alloimmunized mother means the fetus is at risk for hemolytic disease of the fetus and newborn (HDFN); follow titers and monitor with middle cerebral artery Doppler for fetal anemia. - **Pretransfusion testing:** Antibody screen and crossmatch (the "minor/major crossmatch") to prevent hemolytic transfusion reactions. Major crossmatch = recipient serum + donor RBCs. - **Detection of alloantibodies** to minor antigens (Kell, Duffy, Kidd, etc.) after prior transfusion or pregnancy.

**Interpretation:** - Positive indirect Coombs: patient has serum antibodies against RBC antigens (alloantibodies, or autoantibodies in AIHA, which may also be detected in the serum when they are in excess). - Negative: no clinically significant antibodies detected against the screening cells.

**Contrast with Direct Coombs:** - Direct: patient RBCs + anti-human globulin. Positive in warm autoimmune hemolytic anemia (IgG; associated with SLE, CLL, drugs such as methyldopa, penicillin), cold agglutinin disease (C3 only; Mycoplasma pneumoniae, EBV), HDFN (ABO or Rh incompatibility; baby's cells coated with maternal IgG), and hemolytic transfusion reactions. - Indirect: patient serum + reagent RBCs.

**High-yield associations:** - Rh-negative mother + Rh-positive fetus: sensitization occurs at delivery/fetal-maternal hemorrhage; the first pregnancy is typically unaffected, subsequent pregnancies are at risk (IgG crosses the placenta). Prevention: anti-D immunoglobulin at ~28 weeks and within 72 hours of delivery of an Rh-positive infant (also after miscarriage, abortion, amniocentesis, or trauma). - ABO HDFN (mother type O, baby A or B) is usually milder, can occur in the first pregnancy, and often has a negative or weakly positive direct Coombs. - Mechanism of reaction: type II hypersensitivity (antibody-mediated). - Anti-D is IgG, so it is detected by the indirect Coombs; ABO natural antibodies are mostly IgM and agglutinate directly in saline without the Coombs reagent.