Absolute versus relative polycythaemia
Polycythaemia is a raised haematocrit (the fraction of blood volume taken up by red cells), either from more red cells or from a reduced plasma volume.
Remember
Polycythaemia is a raised haematocrit. It is absolute when there really are more red cells, and relative when the plasma volume has fallen, usually from dehydration.
Memory aid
relative means the same red cells in less water; absolute means more red cells
Use it to sort a raised haematocrit into a relative rise or an absolute rise.
The haematocrit tells you how crowded the blood is, not why. It can be high for two reasons, and the cause decides what you investigate and how you treat.
In an absolute rise there really are more red cells, a larger red cell mass (the volume of circulating red cells). In a relative rise the red cell number is normal but the plasma volume has fallen, usually from dehydration, so the cells are more concentrated.
An absolute rise has primary and secondary causes. In a primary cause the bone marrow itself is abnormal, as in polycythaemia vera, a bone marrow disorder. In a secondary cause the marrow is driven by erythropoietin, the kidney hormone that stimulates red cell production, for example at high altitude, in chronic lung disease or with a tumour that makes erythropoietin. Extra red cells received rather than made, for example through a transfusion, can also raise the red cell mass, though only in specific circumstances.
Think relative first, then secondary, then primary. A relative polycythaemia is the commonest cause of a raised haematocrit, and it usually corrects with fluids.
More red cells make the blood thicker (more viscous), which may bring headaches and dizziness. Removing blood, called therapeutic phlebotomy, can be used to lower the haematocrit in an absolute rise.
Key facts
- Raised haematocrit
- polycythaemia is a laboratory state where the haematocrit is above 52% in males and above 48% in females.
- Absolute rise
- the total red cell mass is truly increased, unlike a relative rise where only the plasma volume has fallen.
- Relative rise
- the red cell number is normal but the plasma volume has fallen, usually from dehydration.
- Absolute causes
- primary (polycythaemia vera) or secondary (raised erythropoietin).
- Order of thinking
- relative first, then secondary, then primary.
- Direct test
- the red cell mass can be measured directly in specialist centres.
Exam traps
- Dehydration does not give an absolute polycythaemia: the plasma volume falls, so the haematocrit rises while the red cell mass stays normal, which makes it a relative rise.
- A very high white cell count can give a falsely high haematocrit, but the red cell mass is not truly increased, so it does not support an absolute polycythaemia.
- A spleen that contracts and empties its pool only redistributes red cells already in the body, so it does not create the sustained rise in red cell mass of an absolute polycythaemia.
Read the full notes
Tested in 7 SimpleMed+ questions
Practice it in Haematology, with the answer and a full explanation after every question.
Practice these questions