cell count. hemoglobin, measured after the red blood cells are lysed, is usually reported in units of grams per litre (g/l) or grams per decilitre (g/dl). assuming that the red blood cells are normal, there is a constant relationship between hemoglobin and hematocrit: the hematocrit percentage is approximately three times greater than the hemoglobin value in g/dl, plus or minus three. this relationship, called the rule of three, can be used to confirm that cbc results are correct. two other measurements are calculated from the red blood cell count, the hemoglobin concentration, and the hematocrit: the mean corpuscular hemoglobin and the mean corpuscular hemoglobin concentration. these parameters describe the hemoglobin content of each red blood cell. the mch and mchc can be confusing; in essence the mch is a measure of the average amount of hemoglobin per red blood cell. the mchc gives the average proportion of the cell that is hemoglobin. the mch does not take into account the size of the red blood cells whereas the mchc does. collectively, the mcv, mch, and mchc are referred to as the red blood cell indices. changes in these indices are visible on the blood smear: red blood cells that are abnormally large or small can be identified by comparison to the sizes of white blood cells, and cells with a low hemoglobin concentration appear pale. another parameter is calculated from the initial measurements of red blood cells: the red blood cell distribution width or rdw, which reflects the degree of variation in the cells' size. blood smear from a person with iron deficiency anemia, displaying characteristic red blood cell morphology. the red blood cells are abnormally small (microcytosis), have large areas of central pallor (hypochromia), and vary greatly in size (anisocytosis). an abnormally low hemoglobin, hematocrit, or red blood cell count indicates anemia. anemia is not a diagnosis on its own, but it points to an underlying condition affecting the person's red blood cells. general causes of anemia include blood loss, production of defective red blood cells (ineffective erythropoeisis), decreased production of red blood cells (insufficient erythropoeisis), and increased destruction of red blood cells (hemolytic anemia). anemia reduces the blood's ability to carry oxygen, causing symptoms like tiredness and shortness of breath. if the hemoglobin level falls below thresholds based on the person's clinical condition, a blood transfusion may be necessary. an increased number of red blood cells, leading to an increase in the hemoglobin and hematocrit, is called polycythemia. dehydration or use of diuretics can cause a "relative" polycythemia by decreasing the amount of plasma compared to red cells. a true increase in the number of red blood cells, called absolute polycythemia, can occur when the body produces more red blood cells to compensate for chronically low oxygen levels in conditions like lung or heart disease, or when a person has abnormally high levels of erythropoietin, a hormone that stimulates production of red blood cells. in polycythemia vera, the bone marrow produces red cells and other blood cells at an excessively high rate. evaluation of red blood cell indices is helpful in determining the cause of anemia. if the mcv is low, the anemia is termed microcytic, while anemia with a high mcv is called macrocytic anemia. anemia with a low mchc is called hypochromic anemia. if anemia is present but the red blood cell indices are normal, the anemia is considered normochromic and normocytic. the term hyperchromia, referring to a high mchc, is generally not used. elevation of the mchc above the upper reference value is rare, mainly occurring in conditions such as spherocytosis, sickle cell disease and hemoglobin c disease. an elevated mchc can also be a false result from conditions like red blood cell agglutination (which causes a false decrease in the red blood cell count, elevating the mchc) or highly elevated amounts of lipids in the blood (which causes a false increase in the hemoglobin result). microcytic anemia is typically associated with iron deficiency, thalassemia, and anemia of chronic disease, while macrocytic anemia is associated with alcoholism, folate and b12 deficiency, use of some drugs, and some bone marrow diseases. acute blood loss, hemolytic anemia, bone marrow disorders, and various chronic diseases can result in anemia with a normocytic blood picture. the mcv serves an additional purpose in laboratory quality control. it is relatively stable over time compared to other cbc parameters, so a large change in mcv may indicate that the sample was drawn from the wrong patient. a low rdw has no clinical significance, but an elevated rdw represents increased variation in red blood cell size, a condition known as anisocytosis. anisocytosis is common in nutritional anemias such as iron deficiency anemia and anemia due to vitamin b12 or folate deficiency, while people with thalassemia may have a normal rdw. based on the cbc results, further steps can be taken to investigate anemia, such as a ferritin test to confirm the presence of iron deficiency, or hemoglobin electrophoresis to diagnose a hemoglobinopathy such as thalassemia or sickle cell disease. white blood cells main articles: white blood cell and white blood cell differential sample cbc in chronic myeloid leukemia analyte result white cell count 98.8 x 109/l hemoglobin 116 g/l hematocrit 0.349 l/l mcv 89.0 fl platelet count 1070 x 109/l analyte result neutrophils 48% lymphocytes 3% monocytes 4% eosinophils 3% basophils 21% band neutrophils 8% metamyelocytes 3% myelocytes 8% blast cells 2% the white blood cell and platelet counts are markedly increased, and anemia is present. the differential count shows basophilia and the presence of band neutrophils, immature granulocytes and blast cells. white blood cells defend against infections and are involved in the inflammatory response. a high white blood cell count, which is called leukocytosis, often occurs in infections, inflammation, and states of physiologic stress. it can also be caused by diseases that involve abnormal production of blood cells, such as myeloproliferative and lymphoproliferative disorders. a decreased white blood cell count, termed leukopenia, can lead to an increased risk of acquiring infections, and occurs in treatments like chemotherapy and radiation therapy and many conditions that inhibit the production of blood cells. sepsis is associated with both leukocytosis and leukopenia. the total white blood cell count is usually reported in cells per microlitre of blood (/μl) or 109 cells per litre (× 109/l). in the white blood cell differential, the different types of white blood cells are identified and counted. the results are reported as a percentage and as an absolute number per unit volume. five types of white blood cells—neutrophils, lymphocytes, monocytes, eosinophils, and basophils—are typically measured. some instruments report the number of immature granulocytes, which is a classification consisting of precursors of neutrophils; specifically, promyelocytes, myelocytes and metamyelocytes. other cell types are reported if they are identified in the manual differential. differential results are useful in diagnosing and monitoring many medical conditions. for example, an elevated neutrophil count (neutrophilia) is associated with bacterial infection, inflammation, and myeloproliferative disorders, while a decreased count (neutropenia) may occur in individuals who are undergoing chemotherapy or taking certain drugs, or who have diseases affecting the bone marrow. neutropenia can also be caused by some congenital disorders and may occur transiently after viral or bacterial infections in children. people with severe neutropenia and clinical signs of infection are treated with antibiotics to prevent potentially life-threatening disease. blood film from a person with chronic myeloid leukemia: many immature and abnormal white blood cells are visible. an increased number of band neutrophils—young neutrophils that lack segmented nuclei—or immature granulocytes is termed left shift and occurs in sepsis and some blood disorders, but is normal in pregnancy. an elevated lymphocyte count (lymphocytosis) is associated with viral infection and lymphoproliferative disorders like chronic lymphocytic leukemia; elevated monocyte counts (monocytosis) are associated with chronic inflammatory states; and the eosinophil count is often increased (eosinophilia) in parasitic infections and allergic conditions. an increased number of basophils, termed basophilia, can occur in myeloproliferative disorders like chronic myeloid leukemia and polycythemia vera. the presence of some types of abnormal cells, such as blast cells or lymphocytes with neoplastic features, is suggestive of a hematologic malignancy. platelets main articles: platelet and mean platelet volume blood film of essential thrombocythemia. platelets are visible as small purple structures. platelets play an essential role in clotting. when the wall of a blood vessel is damaged, platelets adhere to the exposed surface at the site of injury and plug the gap. simultaneous activation of the coagulation cascade results in the formation of fibrin, which reinforces the platelet plug to create a stable clot. a low platelet count, known as thrombocytopenia, may cause bleeding if severe. it can occur in individuals who are undergoing treatments that suppress the bone marrow, such as chemotherapy or radiation therapy, or taking certain drugs, such as heparin, that can induce the immune system to destroy platelets. thrombocytopenia is a feature of many blood disorders, like acute leukemia and aplastic anemia, as well as some autoimmune diseases. if the platelet count is extremely low, a platelet transfusion may be performed. thrombocytosis, meaning a high platelet count, may occur in states of inflammation or trauma, as well as in iron deficiency, and the platelet count may reach exceptionally high levels in people with essential thrombocythemia, a rare blood disease. the platelet count can be reported in units of cells per microlitre of blood (/μl), 103 cells per microlitre (× 103/μl), or 109 cells per litre (× 109/l). the mean platelet volume (mpv) measures the average size of platelets in femtolitres. it can aid in determining the cause of thrombocytopenia; an elevated mpv may occur when young platelets are released into the bloodstream to compensate for increased destruction of platelets, while decreased production of platelets due to dysfunction of the bone marrow can result in a low mpv. the mpv is also useful for differentiating between congenital diseases that cause thrombocytopenia. the immature platelet fraction (ipf) or reticulated platelet count is reported by some analyzers and provides information about the rate of platelet production by measuring the number of immature platelets in the blood. reticulocyte count main article: reticulocyte red blood cells stained with new methylene blue: the cells containing dark blue structures are reticulocytes. reticulocytes are immature red blood cells, which, unlike the mature cells, contain rna. a reticulocyte count is sometimes performed as part of a complete blood count, usually to investigate the cause of a person's anemia or evaluate their response to treatment. anemia with a high reticulocyte count can indicate that the bone marrow is producing red blood cells at a higher rate to compensate for blood loss or hemolysis, while anemia with a low reticulocyte count may suggest that the person has a condition that reduces the body's ability to produce red blood cells. when people with nutritional anemia are given nutrient supplementation, an increase in the reticulocyte count indicates that their body is responding to the treatment by producing more red blood cells. hematology analyzers perform reticulocyte counts by staining red blood cells with a dye that binds to rna and measuring the number of reticulocytes through light scattering or fluorescence analysis. the test can be performed manually by staining the blood with new methylene blue and counting the percentage of red blood cells containing rna under the microscope. the reticulocyte count is expressed as an absolute number or as a percentage of red blood cells. some instruments measure the average amount of hemoglobin in each reticulocyte; a parameter that has been studied as an indicator of iron deficiency in people who have conditions that interfere with standard tests. the immature reticulocyte fraction (irf) is another measurement produced by some analyzers which quantifies the maturity of reticulocytes: cells that are less mature contain more rna and thus produce a stronger fluorescent signal. this information can be useful in diagnosing anemias and evaluating red blood cell production following anemia treatment or bone marrow transplantation. nucleated red blood cells blood smear from a newborn baby, showing a few nucleated red cells main article: nucleated red blood cell during their formation in bone marrow, and in the liver and spleen in fetuses, red blood cells contain a cell nucleus, which is usually absent in the mature cells that circulate in the bloodstream. nucleated red blood cells are normal in newborn babies, but when