If a PCV is 39%, what is the approximate red cell count?

If a PCV is 39%, what is the approximate red cell count?

• A. 6.5x10^6/cu mm
• B. 2.0x10^6/cu mm
• C. 10.0x10^6/cu mm
• D. 20.0x10^6/cu mm

Answer: (A)

Hematocrit (PCV) reflects both how many red cells there are and how large they are, via the relation that hematocrit is roughly the product of red cell count and mean cell volume (MCV). To estimate the red cell count from PCV, you use: RBC count ≈ Hct fraction × 10^9 / MCV, with RBC in millions per microliter and MCV in femtoliters. A PCV of 0.39 and assuming an MCV around 60 fL (a microcytic, smaller cell) gives RBC count ≈ 0.39 × 10^9 / 60 ≈ 6.5 × 10^6/µL. That aligns with the given answer. If you used a larger MCV (around 90 fL, more typical of normocytic cells), the estimate would drop to about 4.3–4.9 × 10^6/µL, illustrating why PCV alone needs MCV for precise RBC counting. The other numeric values would require implausibly small or large MCVs for a simple approximate calculation.

Hematocrit (PCV) reflects both how many red cells there are and how large they are, via the relation that hematocrit is roughly the product of red cell count and mean cell volume (MCV). To estimate the red cell count from PCV, you use: RBC count ≈ Hct fraction × 10^9 / MCV, with RBC in millions per microliter and MCV in femtoliters. A PCV of 0.39 and assuming an MCV around 60 fL (a microcytic, smaller cell) gives RBC count ≈ 0.39 × 10^9 / 60 ≈ 6.5 × 10^6/µL. That aligns with the given answer. If you used a larger MCV (around 90 fL, more typical of normocytic cells), the estimate would drop to about 4.3–4.9 × 10^6/µL, illustrating why PCV alone needs MCV for precise RBC counting. The other numeric values would require implausibly small or large MCVs for a simple approximate calculation.