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Question 357
Physiology → Haematology
Classification
Question
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The haematocrit of the same blood sample is determined once by centrifugation and once by calculation from mean cell volume and red-cell count.
Question Stem
Why is the calculated value expected to be slightly lower than the centrifuged value?
Options
A
Centrifugation removes haemoglobin from erythrocytes.
B
Calculation includes the buffy coat within the red-cell fraction.
C
Mean cell volume falls during centrifugation.
D
A small amount of plasma remains trapped between erythrocytes during centrifugation.
E
Centrifugation causes erythrocytes to become nucleated.
Explanation
Haematocrit can be calculated by multiplying mean cell volume by red-cell count. This gives a slightly lower value than centrifugation because a small amount of plasma becomes trapped between the packed cells during centrifugation.
Option Validity
A) The difference is not attributed to removal of haemoglobin during centrifugation.
B) The buffy coat consists of white cells and platelets, but its inclusion in a calculated red-cell fraction is not the stated reason for the difference.
C) A change in mean cell volume during centrifugation is not given as the explanation for the higher centrifuged value.
E) Mature human erythrocytes are non-nucleated; centrifugation does not cause them to acquire nuclei.
Further Reading
Saxena. Textbook for MRCOG-1. Physiology - Haematology - Haematocrit Value, p.68.