In which of the following species is rouleaux formation common?

• A. Horses
• B. Rats
• C. Dogs
• D. Pigs

Answer: (A)

Rouleaux formation is the stacking of red blood cells on a smear, caused when plasma proteins—especially fibrinogen and globulins—are elevated or when the forces keeping cells apart (zeta potential) are reduced. This makes the cells stick together in long chains or clumps that resemble a stack of coins. Horses commonly show rouleaux on routine smears because their plasma protein levels can be higher, particularly during inflammatory states, so this pattern is frequently seen and can even be considered a normal-appearing finding in equine samples. In contrast, other species less often display rouleaux under normal conditions; when it does occur in them, it often points to marked hyperproteinemia or inflammation. To distinguish rouleaux from true agglutination caused by antibodies, a saline dilution test can be used—the rouleaux pattern disperses with saline, while true agglutination remains clumped.