Final answer:
For venipuncture, the blood volume drawn should be about 2 to 2.5 times the volume of serum or plasma needed, to account for the cellular components of blood. Serum is plasma without clotting factors, and hematocrit affects plasma volume calculations.
Step-by-step explanation:
The amount of blood drawn for a venipuncture should be proportional to the volume of serum or plasma that is required. Typically, the blood volume drawn is about 2 to 2.5 times the required serum or plasma volume. This factor accounts for the cellular components of the blood and ensures that enough fluid remains following clotting (for serum) or separation (for plasma) to provide the needed amount of serum or plasma for testing.
Plasma is the liquid component of blood in which the blood cells are suspended, while serum is the fluid that remains after the blood has clotted and the clotting factors have been removed. Therefore, if plasma is needed, the calculation should consider the person's hematocrit level. If a person has a hematocrit of 45, meaning 45% of their blood volume is occupied by red blood cells, the remaining 55% is plasma. Consequently, if you require a certain volume of plasma for testing, you must draw nearly twice that volume to account for the cellular components.
As for the critical thinking question, if a patient's hematocrit is 42 percent, approximately 58 percent of the patient's blood is plasma, since the plasma volume is the complement of the hematocrit percentage.