Polycythaemia is a condition characterized by an abnormal increase in the concentration of red blood cells in the bloodstream. This elevation is typically measured by increased levels of hemoglobin and hematocrit in a routine blood test. The term describes a blood composition where the solid components, particularly red blood cells, are disproportionately high relative to the liquid portion, or plasma. This increase can significantly affect the body’s circulatory system and is generally an indication of an underlying issue.
What Polycythaemia Is
Polycythaemia essentially means the blood has become “thicker” due to the excess volume of red blood cells, a phenomenon known as hyperviscosity. This thickening impedes the normal flow of blood, requiring the heart to work harder to pump the more viscous fluid through the circulatory system. The primary function of red blood cells is to transport oxygen throughout the body using the iron-rich protein hemoglobin.
A diagnosis is often indicated by elevated laboratory values, particularly hematocrit, which is the percentage of total blood volume made up of red blood cells. Polycythaemia is generally considered when hematocrit levels exceed 48% in women and 52% in men. This increased cell mass can lead to sluggish blood flow, which impairs oxygen delivery to tissues and organs despite the higher total number of oxygen carriers.
Primary and Secondary Classifications
The condition is broadly categorized into two types based on the origin of the excess red blood cell production. The distinction between primary and secondary polycythaemia is foundational because it dictates the entire treatment strategy. This classification centers on whether the cause is intrinsic to the bone marrow or a reaction to an external stimulus.
Primary polycythaemia, known as Polycythaemia Vera (PV), is an acquired disorder originating in the bone marrow. It is classified as a myeloproliferative neoplasm, meaning the bone marrow stem cells produce too many red blood cells, and often an excess of white blood cells and platelets, independently of the body’s needs. The production in PV is autonomous, often driven by a somatic mutation in the JAK2 gene, which is present in over 95% of cases. This genetic change causes the cells to proliferate excessively even when the hormone that normally controls red cell production, erythropoietin (EPO), is low.
Secondary polycythaemia is a physiological response to an underlying external condition, making it an adaptive state. The most common trigger is chronic hypoxia, or persistently low oxygen levels in the blood, which the body attempts to compensate for by producing more red blood cells. Conditions such as severe chronic lung disease, obstructive sleep apnea, or living at a high altitude can cause chronic hypoxia. In these cases, the kidneys sense the low oxygen and respond by releasing high or normal levels of EPO, which then stimulates the bone marrow to increase red blood cell production.
Symptoms and Associated Health Risks
The symptoms of polycythaemia are largely a direct result of the blood’s increased viscosity and the resulting poor circulation. Patients often report non-specific complaints such as chronic fatigue, persistent headaches, and dizziness. A common symptom is pruritus, or severe itching, especially after exposure to warm water like a hot bath or shower.
The thickening of the blood can also manifest as visual disturbances like blurred vision or temporary loss of vision, as well as a feeling of fullness in the head. Some people experience erythromelalgia, which is a burning pain, redness, and tingling sensation in the hands or feet, caused by microvascular clotting. Paradoxically, some patients with PV may also experience abnormal bleeding, such as frequent nosebleeds or easy bruising, due to dysfunctional platelets.
The most serious health risk associated with polycythaemia is the increased propensity for thrombosis, or the formation of blood clots. The sluggish, thick blood is more likely to clot in both arteries and veins. This heightened risk can lead to life-threatening events, including a heart attack, a stroke, or a deep vein thrombosis (DVT), which can then travel to the lungs as a pulmonary embolism.
Testing and Managing the Condition
Diagnosis begins with a Complete Blood Count (CBC) to measure hemoglobin and hematocrit levels. If these values are elevated, further testing is necessary to differentiate between the primary and secondary classifications. A serum erythropoietin (EPO) level test is a differentiating marker; a low EPO level suggests PV because the autonomous bone marrow production suppresses the hormone.
Genetic testing is performed to check for the JAK2 V617F mutation, which is the hallmark of Polycythaemia Vera. A bone marrow biopsy may be required to confirm PV, as it reveals the hypercellularity and increased production of blood cell lines characteristic of the disease. For secondary polycythaemia, diagnostic efforts focus on identifying the underlying cause, such as chronic lung disease or sleep apnea.
Management strategies are tailored to the specific type of polycythaemia. For all PV patients, the primary treatment is therapeutic phlebotomy. This involves periodically removing a unit of blood to physically reduce the red blood cell mass and lower the hematocrit level to a target below 45%. Low-dose aspirin is also recommended for nearly all PV patients to reduce the risk of blood clots by inhibiting platelet function.
Patients considered high-risk, typically those over 60 or with a history of clotting, may receive cytoreductive therapy, such as the medication hydroxyurea, to suppress the overproduction of blood cells in the bone marrow. For secondary polycythaemia, the goal is to treat the underlying cause, such as improving oxygenation with continuous positive airway pressure (CPAP) for sleep apnea or managing chronic obstructive pulmonary disease. Addressing the root cause can often normalize the red blood cell count without requiring cytoreductive drugs.