The hematocrit (Ht or HCT), also known by several other names, is a blood test that measures the volume percentage (vol%) of red blood cells (RBC) in blood. The measurement depends on the number and size of red bloods cells. It is normally 40.7% to 50.3% for men and 36.1% to 44.3% for women. It is a part of a person's complete blood count results, along with hemoglobin concentration, white blood cell count, and platelet count. Because the purpose of red blood cells is to transfer oxygen from the lungs to body tissues, a blood sample's hematocrit—the red blood cell volume percentage—can become a point of reference of its capability of delivering oxygen. Hematocrit levels that are too high or too low can indicate a blood disorder, dehydration, or other medical conditions. An abnormally low hematocrit may suggest anemia, a decrease in the total amount of red blood cells, while an abnormally high hematocrit is called polycythemia. Both are potentially life-threatening disorders.

    Haematocrit is a test that measures the proportion of your blood that is made up of red blood cells (RBCs). Blood consists of RBCs, white blood cells (WBCs), and platelets suspended in a fluid portion called plasma. The haematocrit is a ratio of the volume of red blood cells to the volume of all these components together, called whole blood. The value is expressed as a percentage or fraction. For example, a haematocrit value of 40%...

    The haematocrit is routinely ordered as a part of the complete blood count (CBC). It may also be ordered by itself or with a haemoglobin level as part of a general health exam. These tests are often ordered when you have signs and symptoms of a condition affecting RBCs, such as anaemia and polycythemia.

    Some signs and symptoms of anaemia include:

    ·         Weakness or fatigue

    ·         Lack of energy

    ·         Fainting

    ·         Paleness (pallor)

    ·         Shortness of breath

    ·         Fast or irregular heartbeat

    ·         Cold hands or feet

    Some signs and symptoms of polycythemia include:

    ·         Disturbed vision

    ·         Dizziness

    ·         Headache

    ·         Flushing

    ·         Enlarged spleen

    A haematocrit may sometimes be ordered when you have signs and symptoms of serious dehydration, such as extreme thirst, dry mouth or mucous membranes, and lack of sweating or urination.

    This test may be performed several times or on a regular basis when you have been diagnosed with ongoing bleeding problems, anaemia, or polycythemia to determine the effectiveness of treatment. It may also be ordered routinely if you are undergoing treatment for cancer that is known to affect the bone marrow.

    Red blood cells (RBCs) typically make up roughly 37% to 49% of the volume of blood.

    Since a haematocrit is often performed as part of a complete blood count (CBC), results from other components, such as RBC count, haemoglobin, reticulocyte count, and/or red blood cell indices, are taken into consideration. Age, sex, and race are other factors to be considered. In general, the haematocrit mirrors the results of the RBC count and haemoglobin.

    A low haematocrit with low RBC count and low haemoglobin indicates anaemia. Some examples of causes include:

·         Excessive loss of blood from, for example, severe trauma, or chronic bleeding from sites such as the digestive tract (e.g., ulcers, polyps, colon cancer), the bladder or uterus (in women, heavy menstrual bleeding, for example)

·         Nutritional deficiencies such as iron, folate or B12 deficiency

·         Damage to the bone marrow from, for example, a toxin, radiation or chemotherapy, infection or drugs

·         Bone marrow disorders such as aplastic anaemia, myelodysplastic syndrome, or cancers such as leukaemia, lymphoma, multiple myeloma, or other cancers that spread to the marrow

·         Kidney failure—severe and chronic kidney diseases lead to decreased production of erythropoietin, a hormone produced by the kidneys that stimulate RBC production by the bone marrow.

·         Chronic inflammatory diseases or conditions

·         Decreased production of haemoglobin (e.g., thalassemia)

·         Excessive destruction of red blood cells, for example, haemolytic anaemia caused by autoimmunity or defects in the red blood cell itself; the defects could be hemoglobinopathy (e.g., sickle cell anaemia), abnormalities in the RBC membrane (e.g., hereditary spherocytosis) or RBC enzyme (e.g., G6PD deficiency)

    A high haematocrit with a high RBC count and high haemoglobin indicates polycythemia. Some examples of causes of a high haematocrit include:

·         Dehydration—this is the most common cause of a high haematocrit. As the volume of fluid in the blood drops, the RBCs per volume of fluid artificially rises; with adequate fluid intake, the haematocrit returns to normal.

·         Lung (pulmonary) disease—if you are unable to breathe in and absorb sufficient oxygen, the body tries to compensate by producing more red blood cells.

·         Congenital heart disease—in some forms, there is an abnormal connection between the two sides of the heart, leading to reduced oxygen levels in the blood. The body tries to compensate by producing more red blood cells.

·         Kidney tumour that produces excess erythropoietin

·         Smoking

·         Living at high altitudes (a compensation for decreased oxygen in the air)

·         Genetic causes (altered oxygen sensing, abnormality in haemoglobin oxygen release)

    Polycythemia vera—a rare disease in which the body produces excess RBCs inappropriately