A second type of irregular arrangement is erythrocyte agglutination. Agglutination is recognized by irregularly sized and spaced clumps of several to many erythrocytes. It may be impossible to visualize individual cells if a high degree of agglutination is present. Autoagglutination may indicate the presence of a cold reacting antibody in the patient sample, which reacts with erythrocyte antigens during slide preparation. In this case, warming the blood sample to 37oC before making a repeat slide may eliminate the problem. Agglutination is observable at a magnification of 400X.
True rouleaux and agglutination may each be observed throughout all areas of the blood smear, from feathered edge to the start of the smear.