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Question 294
Physiology → Haematology
Classification
Question
SSBAOptions
Explanation
Haematocrit can be calculated from the MCV and red-cell count. The calculated haematocrit is slightly lower than the centrifuged value because a small amount of plasma becomes trapped between the packed erythrocytes during centrifugation, increasing the apparent packed-cell volume.
Option Validity
A) Loss of erythrocytes during centrifugation does not explain the discrepancy. The higher centrifuged value results from plasma remaining between the packed red cells.
B) The buffy coat consists of white blood cells and platelets and is separately visible after centrifugation. It does not explain the difference between calculated and centrifuged haematocrit.
D) Reticulocyte destruction does not explain the discrepancy. Reticulocytes are the most immature erythrocytes normally found in the circulation.
E) Trapped plasma between packed erythrocytes, rather than sedimented plasma proteins, explains the higher centrifuged haematocrit.
Further Reading
Saxena. Textbook for MRCOG-1. Physiology - Haematology - Haematocrit Value, p.68.