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These erythrocytes are grouped in “rouleaux,” a “stack-of-coins” pattern most often associated with elevated levels of immunoglobulin or fibrinogen. These positively-charged plasma proteins coat the negatively-charged surface of the erythrocytes, resulting in neutralization of the cells’ surface charge and enabling them to come together to form rouleaux.

Rouleaux formation should not be confused with agglutination, which occurs when erythrocytes are “clumped” together by circulating autoantibodies. A drop of saline in the slide will disrupt rouleaux, whereas agglutinated areas remain intact.

This blood smear was obtained from a patient with a relapse of multiple myeloma.