A high red blood cell count indicates there are too many red blood cells circulating in your bloodstream. Red blood cells (RBCs), also called erythrocytes, are produced in your bone marrow and transport oxygen from your lungs to tissues throughout your body. A high red blood cell count is also called erythrocytosis (uh-rith-roh-sie-TOE-sis).
The threshold for a high red blood cell count is somewhat different from one medical practice to another. A high red blood cell count is generally defined as more than 5.72 million red blood cells per microliter (mcL) of blood for men and 5.03 million per mcL for women. In children, the threshold for high red blood cell count varies with age and sex.
Polycythemia Primary Causes
In primary polycythemia, inherent or acquired problems with red blood cell production lead to polycythemia. Two main conditions that belong to this category are polycythemia vera (PV or polycythemia rubra vera [PRV]) and primary familial and congenital polycythemia (PFCP).
- Polycythemia vera (PV) is related to a genetic mutation in the JAK2 gene, which is thought to increase the sensitivity of bone marrow cells to Epo, resulting in increased red blood cell production. Levels of other types of blood cells (white blood cells and platelets) are also often increased in this condition.
- Primary familial and congenital polycythemia (PFCP) is a condition related to a mutation in the EPOR gene and causes increased production of red blood cells in response to Epo.
What are the symptoms of a raised red cell count?
There are many symptoms that can be a feature of increased blood thickness but these are often rather vague. They include:
- a ruddy complexion
- blurred vision or patchy loss of vision
- in extreme cases, stroke or coma.
The risk of clotting (thrombosis) increases if you have other risk factors for blood vessel disease such as previous clot, high blood pressure, raised cholesterol or diabetes.
Types of Polycythaemia
- Primary polycythaemia: In primary polycythaemia the increase in red blood cells is due to inherent problems in their production.
- Secondary polycythaemia: Secondary polycythaemia generally occurs as a response to other factors or underlying conditions that promote red blood cell production.
- Red cell production (erythropoiesis) takes place in the bone marrow through a complex sequence of tightly regulated steps. The main regulator of the red cell production is the hormone erythropoietin (EPO). This hormone is largely secreted by the kidneys, although, about 10% may be produced and secreted by the liver.
Erythropoietin secretion is up-regulated in response to low oxygen levels (hypoxia) in the blood. More oxygen can be carried to tissues when erythropoietin stimulates red blood cell production in the bone marrow to compensate for the hypoxia.
Neonatal ( newborn) polycythaemia can be seen in 1% to 5% of newborns. The most common causes may be related to transfusion of blood, transfer of placental blood to the infant after delivery, or chronic inadequate oxygenation of the foetus (intrauterine hypoxia) due to placental insufficiency.
How is a raised red cell count diagnosed?
A battery of different tools is used to decide if and why a patient has an erythrocytosis. An increase in red cells can first show in a blood test result as:
- an increase in red cell numbers
- a rise in haemoglobin
- a rise in the packed cell volume (haematocrit). This is a laboratory measure of the volume of red cells in the blood.
The initial blood test should be repeated to confirm the abnormal result. You will normally then be asked about your medical history, symptoms, medications and smoking habits.
A physical examination will also be performed to look for signs of possible underlying disorders.
Unless there is an obvious cause or the haematocrit is clearly very abnormal, a red cell mass study will be performed. This is often carried out in a hospital’s nuclear medicine department.
The principle is to attach a very weak radioactive dye to the red blood cells and toa protein in the plasma (the liquid part of the blood). This involves mixing the dyes with a sample of your blood and then returning it to your body. The dyed cells distribute themselves among your red cells, making it possible to calculate what the total mass of red cells must be. This calculation can also be done using special radiosensitive cameras.
This red cell mass study determines if there is:
- absolute erythrocytosis (raised red cell mass, normal plasma volume)
- apparent erythrocytosis (normal red cell mass, reduced plasma volume).
Other diagnostic tests that may be performed include:
- blood tests of kidney and liver function
- measurement of iron, folic acid and vitamin B12, which are all important in red cell production
- measurement of oxygen levels in the blood – usually with a probe clipped onto your finger
- urine test for the presence of blood, sugar or other abnormality
- chest X-ray to check that the lungs and heart appear normal
- ultrasound of the abdomen to check the kidneys, liver and for any increase in the size of the spleen or fibroids in the womb.
Depending on the results of these tests, you may need further tests that are more specific to the likely underlying cause of your raised red cell count. These include:
- a bone marrow sample (performed under local anaesthetic)
- tests of lung function
- an echocardiogram to examine the structure of the heart
- genetic testing for mutations in JAK2 (common in polycythaemia vera), the erythropoietin receptor, von Hippel Lindau protein and proline dehydroxylase enzymes
- measurement of how tightly haemoglobin ‘holds on to oxygen’ (oxygen dissociation curve)
- a sleep study will be requested if a condition called sleep apnoea is suspected; this will involve an overnight stay in a special laboratory to see if prolonged breath-holding attacks occur when you sleep.
If an inherited condition is suspected, family members may be asked to have blood tests.