What is the main cause of an increase in PCV?

• A. Dehydration
• B. Anemia
• C. Hypoxia
• D. Polycythemia

Answer: (A)

PCV reflects how much of a blood sample is made up of red blood cells. The most common way it goes up is through hemoconcentration—when plasma (the liquid part) is lost, such as with dehydration. With less plasma around, the red cells become more concentrated in the blood, so the PCV rises even though the actual number of red cells hasn’t changed.

Anemia would lower PCV because there are fewer red cells. Hypoxia can eventually drive more red cell production via erythropoietin, which can raise PCV, but this takes time and isn’t the immediate cause. Polycythemia (an actual increase in red cell mass) would raise PCV as well, but it’s a disease state rather than the common, acute cause seen with dehydration.