Editorial Pitfalls in the diagnosis of autoimmune haemolytic anaemia Wilma Barcellini Onco-Haematology Unit, Service of Pathophysiology of Anaemias, IRCCS Ca' Granda Foundation, "Maggiore" Hospital, Milan, Italy
The diagnosis of autoimmune haemolytic anaemia (AIHA) is usually simple and based on the presence of haemolytic anaemia and serological evidence of anti-erythrocyte autoantibodies, detected by the direct antiglobulin test (DAT)1. There are several DAT methods with different sensitivities and specificities, levels of automation and diffusion among laboratories: the DAT-tube method is the traditional agglutination technique, usually performed with broad-spectrum Coombs' reagents. Clinically, it is important to perform the test with monospecific antisera, to distinguish "warm", "cold", and "mixed" forms. The first (~70% of all cases) are mainly due to immunoglobulin (Ig) G which generally react at 37 °C, are usually directed against epitopes of the Rhesus (Rh) system, and primarily determine extravascular haemolysis. Cold forms (∼20% of all cases) are due to IgM, which are able to fix complement more efficiently than other isotypes, have an optimal temperature of reaction at 4 °C, are directed against the I/i system, and prevalently cause intravascular haemolysis; cold IgM autoantibodies can easily be detected by the spontaneous agglutination of red blood cells (RBC) at 20 °C. Of note, the amount of erythrocyte destruction by intravascular haemolysis has been calculated as 200 mL of RBC in 1 hour, whereas the destruction by extravascular haemolysis is 10-fold less. According to DAT results and to the thermal characteristics of the autoantibody, AIHA are usually classified into warm forms (DAT+ for IgG only or IgG plus C3d), cold agglutinin disease (DAT+ for C3d only, with cold agglutinins of I specificity), and mixed forms (DAT+ for IgG and C3d, with coexistence of warm autoantibodies and high titre cold agglutinins). It is worth mentioning the Donath-Landsteiner autoantibody, a bithermic haemolysin able to fix complement at cold temperatures and to cause RBC lysis at 37 °C, directed against the erythrocyte P antigen, and responsible for paroxysmal cold haemoglobinuria, a rare disease mainly observed as an acute form in children (
The diagnosis of autoimmune haemolytic anaemia (AIHA) is usually simple and based on the presence of haemolytic anaemia and serological evidence of anti-erythrocyte autoantibodies, detected by the direct antiglobulin test (DAT)1. There are several DAT methods with different sensitivities and specificities, levels of automation and diffusion among laboratories: the DAT-tube method is the traditional agglutination technique, usually performed with broad-spectrum Coombs' reagents. Clinically, it is important to perform the test with monospecific antisera, to distinguish "warm", "cold", and "mixed" forms. The first (~70% of all cases) are mainly due to immunoglobulin (Ig) G which generally react at 37 °C, are usually directed against epitopes of the Rhesus (Rh) system, and primarily determine extravascular haemolysis. Cold forms (∼20% of all cases) are due to IgM, which are able to fix complement more efficiently than other isotypes, have an optimal temperature of reaction at 4 °C, are directed against the I/i system, and prevalently cause intravascular haemolysis; cold IgM autoantibodies can easily be detected by the spontaneous agglutination of red blood cells (RBC) at 20 °C. Of note, the amount of erythrocyte destruction by intravascular haemolysis has been calculated as 200 mL of RBC in 1 hour, whereas the destruction by extravascular haemolysis is 10-fold less. According to DAT results and to the thermal characteristics of the autoantibody, AIHA are usually classified into warm forms (DAT+ for IgG only or IgG plus C3d), cold agglutinin disease (DAT+ for C3d only, with cold agglutinins of I specificity), and mixed forms (DAT+ for IgG and C3d, with coexistence of warm autoantibodies and high titre cold agglutinins). It is worth mentioning the Donath-Landsteiner autoantibody, a bithermic haemolysin able to fix complement at cold temperatures and to cause RBC lysis at 37 °C, directed against the erythrocyte P antigen, and responsible for paroxysmal cold haemoglobinuria, a rare disease mainly observed as an acute form in children (
