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Widal test

An agglutination test based on the agglutination of Salmonella enterica serovar Typhi cells by antibodies in a patient's serum.

Positive Widal test

Indicates that the patient's serum contains antibodies that react with antigens of S. Typhi.

False-positive Widal test

Can occur due to antibodies from previous infections or vaccinations that may react with S. Typhi antigens.

False-negative Widal test

Can occur if the patient has not produced enough antibodies for a detectable agglutination reaction.

Titer in agglutination assay

The reciprocal of the highest dilution of serum that still produces visible agglutination.

Antibody titer when agglutination visible at 1:64 dilution

64.

Serial dilutions in agglutination tests

Used to determine the highest dilution producing a positive reaction, allowing calculation of the antibody titer.

Higher antibody titer

Generally indicates a greater concentration of specific antibodies in the serum.

Passive agglutination

An assay where a soluble antigen is attached to a particle, like a latex bead, enabling visible clumping by antibodies.

Reverse passive agglutination

An assay where antibodies are attached to particles to detect a soluble antigen in a patient's sample.

Difference between passive and reverse passive agglutination

Passive: antigen attached to particles to detect antibodies. Reverse passive: antibody attached to particles to detect antigen.

Usefulness of latex beads in passive agglutination

They provide a large, visible surface that makes antibody-antigen binding easier to observe as clumping.

Hemagglutination

Agglutination involving red blood cells (RBCs).

Determining blood type using hemagglutination

Mixing a person's RBCs with antibodies against specific blood-group antigens to visually identify present antigens.

Agglutination with anti-A serum

Indicates the RBCs have A antigen on their surface.

Agglutination with anti-B serum

Indicates the RBCs have B antigen on their surface.

Agglutination with anti-D serum

Indicates the RBCs possess the Rh(D) antigen, meaning the person is Rh-positive.

Blood type with both A and B antigens

Type AB.

Blood type with neither A nor B antigens

Type O.

Blood typing as a direct agglutination test

The RBCs themselves carry the antigens that react directly with the testing antibodies.

ABO blood group system

Based on the presence or absence of A and B antigens on red blood cells.

Antibodies in type A blood plasma

Anti-B antibodies.

Antibodies in type B blood plasma

Anti-A antibodies.

Antibodies in type O blood plasma

Both anti-A and anti-B antibodies.

Antibodies in type AB blood plasma

Neither anti-A nor anti-B antibodies.

Importance of matching blood transfusions

Incompatible RBC antigens and antibodies can cause agglutination and destruction of transfused RBCs.

Cross-matching

Testing donor and recipient blood together to determine compatibility for transfusion.

Risks of transfusing incompatible blood

Antibodies can bind to transfused RBCs, causing agglutination and hemolysis.

Purpose of agglutination assays in microbiology

Rapidly identify microorganisms, microbial antigens, or antibodies against specific pathogens.

Major advantage of agglutination assays

They are generally rapid, simple, and inexpensive, producing easily visible results.