Abstract
Rouleaux is a phenomenon that commonly occurs in patients who have an increased number of circulating protein macromolecules. It is a benign, in vitro reaction that appears microscopically as red blood cells (RBCs) line up against each other; many liken the RBC aggregation to “stacked coins.” This unexpected reactivity may cause confusion in direct agglutination testing such as reverse blood typing and crossmatching. Saline replacement is the established method to resolve rouleaux. True agglutination will remain when plasma is replaced with saline for resuspension of the RBC button. Rouleaux will no longer be seen when the plasma proteins are removed. Immunohematology 2018;34:91–92.