Agglutination occurs when blood cells or bacteria clump together, and it is often a response to a wound or injury. Blood cell clumps, called agglutinates, are visible, making them a reliable test for detecting antibodies in a blood sample. For example, agglutination occurs if a person undergoes a blood transfusion with a blood type that doesn't match his own.
Various agglutination tests are used to determine if an antibody is present in a blood sample, including direct agglutination, indirect agglutination, hemagglutination and Rh factor testing.
Direct agglutination tests for the presence of antibodies to a particular antigen in a blood sample. Indirect agglutination, also called passive agglutination, occurs when an artificial agent is the carrier of the antigen being tested. Hemagglutination occurs when red blood cells agglutinate due to an antibody, virus or bacteria coating the blood cells.
Rh factor testing is used to determine a person's blood type and is an essential test for pregnant women. There are four blood types: A, B, AB and O. The positive or negative designation of those blood types is the Rh factor. The Rh factor is a protein carried on the red blood cells. Approximately 15 percent of the population is Rh negative and doesn't carry the protein. Rh negative and Rh positive conditions are both normal. Because a problem arises if an Rh-positive mother is carrying an Rh-negative fetus, Rh testing is always performed in pregnancy so that treatment can be given if necessary.