Antiglobulin Test - Part 1

Anti-Globulin Test (Coombs Test)

  • Overview of the Anti-Globulin Test

    • The anti-globulin test is also known as the Coombs test.

    • Discovered when researchers prepared an antibody that reacted with human globulins, a family of human proteins.

    • This reagent is the anti-human globulin (AHG) which detects antibodies to IgG and complement proteins attached to red blood cells (RBCs) in both in vivo (within the body) and in vitro (test tube).

  • Historical Context

    • Previously only IgM antibodies were detected.

    • The invention of the anti-globulin test enabled the discovery of more blood group systems.

Immunological Principles Related to Blood Banking

  • Overview of Immunology in Blood Bank Context

    • The body generates antibodies in response to foreign antigens; in blood banking, these are the antigens present on RBCs.

    • Antibodies bind to foreign antigens, leading to clearance by the reticuloendothelial system (RES), primarily the spleen or liver.

    • Cleared antigens can activate the complement system, leading to hemolysis of red blood cells.

  • Antibodies Involved in Blood Banking

    • IgG Antibodies

    • Typically produced in response to most immunological challenges.

    • Considered incomplete antibodies and are too small to agglutinate sensitized red blood cells directly.

    • Require incubation to enhance binding for agglutination.

    • IgM Antibodies

    • Larger in size and can be detected through methods such as immediate spin or centrifuge readout.

  • Immune Hemolysis

    • Antibodies attached to RBCs trigger splenic macrophages to bind through Fc receptors, clearing the antigen-antibody complex.

    • Damage to RBCs can lead to the formation of spherocytes, which is one of the morphologies indicative of immune hemolysis.

Direct and Indirect Anti-Globulin Tests (DAT and IAT)

  • Definition and Purpose

    • Direct Anti-Globulin Test (DAT)

    • Detects RBCs coated with IgG antibodies or complement in vivo.

    • A single-stage technique.

    • Indirect Anti-Globulin Test (IAT)

    • Assesses the reaction of antibodies against RBCs to determine if antibodies are present in serum/plasma.

    • A two-stage technique performed in vitro.

  • Sample Preparation

    • It is crucial to wash samples to remove unbound molecules before adding the AHG reagent to prevent neutralization of the reagent.

Applications of Anti-Globulin Tests

  • Tests and Procedures Associated with the Anti-Globulin Test

    • Tests include antibody screens, antibody identification (ID), crossmatching, blood group phenotyping, and weak D testing.

    • Indirect tests often employ anti-human globulin reagent (AHG).

  • Reagents Used

    • Anti-Human Globulin (AHG): Prepared by injecting rabbits with human IgG and complement protein to stimulate antibody production.

    • Most blood banks commonly use polyspecific AHG, containing both anti-IgG and anti-C3d components.

    • If polyspecific is positive, monospecific tests are performed (specific anti-IgG and anti-C3b/D).

  • Coombs Control Cells

    • After centrifuging the AHG tube, control cells (Rh-positive cells coated with anti-B, typically anti-D) are added to verify test validity.

    • A negative reaction with check cells indicates testing invalidity, often due to inadequate washing of samples.

Factor Influences on Indirect Anti-Globulin Tests

  • Various Factors can Impact Results

    • Serum to cell ratio: Typical use of 2 drops of serum for every 1 drop of red cells to prevent prozone/postzone effects.

    • Incubation temperature: Ideally kept at 37 degrees Celsius to facilitate IgG behavior and complement activation.

    • Incubation time: Clinically significant antibodies often detectable within 30 minutes but can also be identified using enhancement media within 10-15 minutes.

    • pH and saline quality: Fresh buffered saline at pH 7.2-7.4 is necessary.

Potentiators Used in Testing

  • Enhancement of Antibody Reaction

    • Potentiators reduce the zeta potential (charge around cells) to promote agglutination:

    • 22% Albumin: Reduces zeta potential but may miss clinically significant antibodies.

    • Low Ionic Strength Solution (LIS): Commonly used to enhance antibody uptake and decrease incubation time.

    • Polyethylene Glycol (PEG): Concentrates antibodies for better detection of significant antibodies.

Interpretations of Results

  • Positive Direct Anti-Globulin Test (DAT)

    • Indicates immune-mediated destruction of red cells.

    • RBCs normally lack IgG/complement coating; presence indicates a disease process.

  • Potential Causes for Positive DAT Results

    • Hemolytic transfusion reactions (alloantibodies attaching to donor RBCs).

    • Hemolytic disease of the fetus and newborn (maternal antibodies cross the placenta and attach to fetal RBCs).

    • Autoimmune hemolytic anemia (patient autoantibodies coating self-RBCs).

    • Drug-induced hemolytic anemia from drug-antibody complexes.

  • Transfusion Reactions

    • Can be acute (due to ABO incompatibility) or delayed (extravascular hemolysis following transfusion).

    • Acute reactions can result in intravascular hemolysis and require immediate attention.

Hemolytic Disease of Fetus and Newborn (HDFN)

  • Mechanism and Identification

    • Maternal IgG antibodies target fetal erythrocytes with corresponding antigens.

    • Positive DAT is the primary diagnostic test for HDFN, while a negative result indicates absence of the disorder.

    • Conditions can elevate bilirubin levels, leading to complications like kernicterus if untreated.

Immune Hemolytic Anemia

  • Overview of Immune Disorders

    • Autoantibodies may coat RBCs leading to destruction in various diseases (e.g., Systemic Lupus Erythematosus).

    • Differentiation is important for diagnosing conditions related to drug reactions or autoimmune responses.

  • Drug-Induced Hemolytic Anemia Types

    • Type 1 (Hapten-dependent): Drug binds to RBC membranes and stimulates antibody formation (e.g., penicillin).

    • Type 2 (Autoantibody): Directly induced by drugs like methyldopa, leading to autoantibody formation against RBCs.

    • Type 3: Drugs induce antibodies that bind RBCs only in their presence; knowledge on this is limited.