Haematocrit is the ratio of:
**Core Concept**
Haematocrit is a critical parameter in haematology, representing the proportion of blood volume occupied by red blood cells (RBCs). It is an essential indicator of various haematological conditions, such as anaemia, polycythaemia, and haemodilution.
**Why the Correct Answer is Right**
The correct answer is the ratio of the volume of red blood cells (RBCs) to the total blood volume. This ratio is expressed as a percentage and is calculated by centrifuging a blood sample and measuring the volume of the packed RBCs. The haematocrit value is influenced by factors such as the concentration of RBCs, plasma proteins, and blood cell distribution width. In clinical practice, haematocrit is used to diagnose and monitor conditions like polycythaemia vera, where there is an overproduction of RBCs, and haemorrhagic shock, where there is a significant loss of blood volume.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the haematocrit ratio specifically refers to the proportion of RBCs in the blood, not the total blood cell count.
**Option B:** This is incorrect because the mean corpuscular volume (MCV) measures the average size of RBCs, not the proportion of RBCs in the blood.
**Option C:** This is incorrect because the mean corpuscular haemoglobin concentration (MCHC) measures the concentration of haemoglobin in RBCs, not the proportion of RBCs in the blood.
**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that a low haematocrit value can indicate haemodilution, which can occur in conditions such as heart failure, liver cirrhosis, or severe burns, where there is an increase in plasma volume relative to RBCs.
**Correct Answer:** C. Volume of red blood cells to total blood volume.