What are rouleaux formations?

Red blood cells usually don't stick together because they have
proteins in their membranes that are negatively charged, and the negative charges repel cells apart. Sometimes these negative charges become coated with other proteins, which neutralizes the negative charge allowing cells to stack one on top of another, forming so-called rouleaux. Such findings can also be seen in blood smears that are not prepared correctly, so there is some variability. Rouleaux can be seen in anyone with myeloma, and may suggest in some cases a worsening of disease, although that is not always the case.