PCV reading: capillary tube fill.

• A. About three-quarters full.
• B. Full to the top.
• C. Half full.
• D. Quarter full.

Answer: (A)

In a microhematocrit (PCV) test, you measure the proportion of red blood cells by looking at the height of the packed red-cell layer after centrifugation relative to the total column in the capillary tube. Filling the tube to about three-quarters provides a good balance: it creates a substantial red-cell column that packs tightly without pushing into the sealed end, and it leaves enough space for the plasma on top so the interface between layers is clear.

If the tube is underfilled, the red-cell layer may not pack consistently and the relative heights become more sensitive to small errors, giving an inaccurate PCV. If it’s overfilled, sealing becomes difficult and the red-cell column can extend too close to the seal or distort the separation, also leading to an unreliable reading. Thus, about three-quarters full gives the most reliable, readable hematocrit.