Laboratory Tests for the Diagnosis of Cold Agglutinin Disease
Biochemical tests usually detect hemolysis and require a blood sample that is drawn by inserting a needle into a vein in the arm. This type of testing includes lactate dehydrogenase test, aspartate aminotransferase test, total-direct bilirubin test, and haptoglobin test.
Some tests detect the levels of cold agglutinins and the number of red blood cells for CAD diagnosis. Blood smear analysis provides additional visual confirmation of the clumping (agglutination) of red blood cells that leads to their disintegration (lysis). However, this test alone is not sufficient to diagnose CAD, and it cannot differentiate between other disorders that may cause red blood cell clumping.
Cold Agglutinin Titer
The cold agglutinin (CA) titer refers to the number of dilutions after which the antibodies can still cause the agglutination of red blood cells. It is a measure of autoantibody concentration and red blood cell-binding strength. Cold agglutinins are present at very low concentrations in all people. They may also be detected in individuals with pneumonia. CA titers exceeding 1:64 dilution can be used as an indication of CAD.
Complete Blood Count
A complete blood count is a count of each cell type in the blood, including white blood cells, red blood cells, and platelets. A complete blood count also may be used as a measure of hemoglobin — the protein in red blood cells that carries oxygen. The complete blood count of a patient with CAD will have fewer red blood cells than normal.
The Coomb’s test can detect cold agglutinin antibodies in the blood. The test comes in two types: direct and indirect. The direct Coomb’s test detects antibodies and other immune system-related proteins that are attached to red blood cells. The indirect Coomb’s test detects free-floating autoantibodies such as cold-agglutinins in the serum, the liquid component of blood.
Thermal Amplitude Test
The thermal amplitude test measures the temperatures at which red blood cells clump together when combined with a blood sample from a CAD patient. This clumping is visible both under a microscope and with the naked eye. Normal plasma should not cause clumping during the thermal amplitude test. The plasma of people with CAD will cause clumping of blood cells at temperatures below 98.6 F.