Polycythemia is a condition that results in an increased level of circulating red blood cells in the bloodstream. People with polycythemia have an increase in hematocrit, hemoglobin, or red blood cell count above the normal limits.
Polycythemia is normally reported in terms of increased hematocrit (hematocrit is the ratio of the volume of red blood cells to the total volume of blood) or hemoglobin concentration (hemoglobin is a protein responsible for transporting oxygen in the blood).
- Hematocrit (HCT): Polycythemia is considered when the hematocrit is greater than 48% in women and 52% in men.
- Hemoglobin (HGB): Polycythemia is considered when a hemoglobin level of greater than 16.5g/dL in women or hemoglobin level greater than18.5 g/dL in men.
Polycythemia can be divided into two categories; primary and secondary.
- Primary polycythemia: In primary polycythemia the increase in red blood cells is due to inherent problems in the process of red blood cell production.
- Secondary polycythemia: Secondary polycythemia 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) polycythemia 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 fetus (intrauterine hypoxia) due to placental insufficiency.
Why Will Your Doctor Order the Test
According to the National Institutes of Health, the test is almost always performed as part of a complete blood cell (CBC) test. (NIH) This measures the amount of all types of components in the blood, including:
- red blood cells
- white blood cells
Your doctor may perform the test if he or she suspects you have a condition that affects your RBCs, or if there is any sign that you have low blood oxygen.
Such signs might be anemia, bruising, general fatigue, and/or nutritional deficiencies.
Often, a CBC test will be part of a routine physical examination, since it is a good indicator of your overall health. It may also be performed before a surgery.
If you have been previously diagnosed with a blood condition that may affect RBC count or you are taking any medications that affect your RBCs, your doctor may order the test to monitor your condition or treatment.
How Is the Test Performed?
An RBC count is a simple blood test performed by a healthcare practitioner at your doctor’s office. Blood will be drawn from a vein, usually at the inside of your elbow. The steps involved in the blood draw typically are:
- The puncture site will be cleaned with an antiseptic.
- A healthcare provider will wrap an elastic band around the upper arm in order to make your vein swell with blood.
- A needle will be gently inserted into the vein, and blood will collect in an attached vial or tube.
- The elastic band will be removed from your arm.
- The blood sample will be sent to a laboratory for analysis.
What are the causes of primary polycythemia?
Primary polycythemias are due to acquired or inherited genetic mutations causing abnormally high levels of red blood cell precursors.
Primary familial and congenital polycythemia (PFCP) and polycythemia vera (PV) are in this category.
Polycythemia vera is a rare condition. One study indicated that there were 65,243 patients with PV in the United States in 2003.
Polycythemia vera is typically associated with an elevated white blood cell count (leukocytosis) and platelet count (thrombocytosis).
An enlarged spleen (splenomegaly) and low erythropoietin levels are other clinical features of polycythemia vera.
Until recently, the exact mechanism of polycythemia vera was not well understood. In 2005, genetic mutations of the JAK2 gene were found to be responsible for most cases of polycythemia vera.
These mutations are thought to possibly increase the sensitivity of the red blood cell precursors to erythropoietin, thereby, increasing red blood cell production.
Primary familial and congenital polycythemia (PFCP)
Primary familial and congenital polycythemia (PFCP) is also thought to be caused by genetic mutations resulting in increased responsiveness to normal levels of erythropoietin.
Most cases are caused by different mutations to the EPOR gene.
How Should You Prepare for the Test?
There is no special preparation required for this test. However, you should tell your doctor if you are taking any medications, including over-the-counter drugs or supplements.
Talk to your doctor to find out if any other preparation is required.
What Are the Risks of Taking the Test?
As with any blood test, there is a risk of bleeding, bruising, and/or infection at the puncture site. You may feel moderate pain or a sharp pricking sensation when the needle is first inserted into your arm.
Normal Results: Standard Ranges of RBCs | Normal Range
According to the Mayo Clinic, the general normal ranges by gender are as follows (Mayo Clinic). However, these may vary slightly depending on the laboratory or doctor:
Males: 4.32 to 5.72 million cells per microliter
Females: 3.9 to 5.03 million cells per microliter
What Does an Abnormal Result Mean?
Higher Than Normal:
If your RBC is higher than normal, it is called erythrocytosis. This may be caused by:
- cigarette smoking
- congenitalheart disease
- renal cell carcinoma (kidney cancer)
- pulmonary fibrosis
- polycythemia vera (a bone marrow disease that causes overproduction of RBCs and is associated with a genetic mutation)
When you move to a higher altitude, your RBC count may increase for several weeks, since there is less oxygen present in the air.
Certain drugs, such as gentamicin and methyldopa, can also increase RBC count. Be sure to tell your doctor about any medications you may take.
Lower Than Normal:
Below-average numbers of RBCs may be caused by:
- bone marrow failure
- erythropoietin deficiency (the primary cause of anemia in patients with chronic kidney disease)
- hemolysis (or RBC destruction) due to transfusion, blood vessel injury, or other causes
- leukemia (cancer of the blood cells)
- multiple myeloma (cancer of the plasma cells in bone marrow)
- nutritional deficiencies (including iron, copper, folate, and vitamins B12 and B6)
Certain drugs can also lower your RBC count, especially chemotherapy drugs, chloramphenicol, hydantoins and quinidine.
What If I Have Abnormal Results?
Your doctor will discuss any abnormalities in your results with you. A high or low RBC count may help identify the cause of your symptoms.
Depending on your results and your situation, your doctor will order the additional tests or treatments necessary to return you to health.
What is the outlook (prognosis) for polycythemia?
The outlook on polycythemia depends on the underlying cause. Overall the general outlook is favorable for people with this condition, especially those with secondary causes.
The outlook for primary polycythemia is fair. While it is typically incurable and long-standing, for many people, it is controllable and treatable.
For example, untreated, polycythemia vera (PV) was initially thought to have a poor prognosis with a life expectancy of one to two years from the time of diagnosis.
However, polycythemia vera prognosis is now greatly improved to 10-15 years survival after diagnosis with treatment by phlebotomy alone.
The addition of medications, such as, hydroxyurea or aspirin may improve survival even more.