Which condition may cause polycythemia, characterized by an elevated hematocrit?

Polycythemia, which is an increase in the concentration of red blood cells in the blood, can indeed be characterized by an elevated hematocrit. Dehydration is a common cause of relative polycythemia. When an animal is dehydrated, the volume of plasma decreases, which results in a relative increase in the concentration of red blood cells, thus raising the hematocrit level.

In this context, dehydration leads to a reduction in the fluid portion of blood, presenting a falsely elevated hematocrit even though the actual number of red blood cells may not be increased. This condition is temporary and usually resolves with rehydration.

Understanding the conditions that lead to polycythemia can help distinguish between absolute and relative increases in red blood cells. Anemia, blood loss, and immune-mediated hemolytic anemia (IMHA) typically result in a decrease in red blood cell counts rather than an increase, thereby failing to account for elevated hematocrit.