the blood count and film

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  • 1. b1015_Chapter-02.qxd7/20/20103:19 PM Page 20b1015 Haematology: A Core Curriculum2The Blood Count and FilmWhat Do You Have to Know?The constituent parts of a blood countThe meaning and clinical significance of a reticulocyte countThe meaning of the words that are frequently used to describeabnormalities in the blood count and blood film and their possibleclinical significanceThe meaning of normal range and reference rangeThe approximate normal ranges for the white cell count, haemo-globin concentration, mean cell volume and platelet count inhealthy adultsHow to interpret a blood count and develop a differential diagnosisThe meaning and clinical significance of an erythrocyte sedi-mentation rate The Full Blood Count The term full blood count (FBC) refers to a group of tests performed simultaneously on a blood sample to assess whether there is any haema- tological abnormality. The tests that are almost always included in an FBC are shown in Table 2.1. In modern haematological practice these tests are performed on large automated analysers capable of processing many hun- dreds of samples in a day. Further tests are sometimes also included. The20 HAEMATOLOGY - A Core Curriculum Imperial College Press

2. b1015_Chapter-02.qxd7/20/20103:19 PM Page 21b1015 Haematology: A Core CurriculumThe Blood Count and Film21Table 2.1. The full blood count. Normal rangeNormal range TestAbbreviation Unitsin men*in women* White blood cell countWBC 109/l Red blood cell countRBC 1012/l 4.325.663.884.99 HaemoglobinHbg/dl 13.316.711.814.8 concentration Haematocrit Hct l/l 0.390.500.360.44 Mean cell volumeMCV fl8298 Mean cell MCH pg27.332.6 haemoglobin Mean cellMCHC g/dl31.634.9 haemoglobin concentration Platelet count 109/l168411188445 * From Bain, B.J. (2006). Blood Cells. Fourth Edition. Blackwell Publishing, Oxford. Reported ranges from different instruments vary. blood count is performed on a blood specimen that has been anticoagu- lated by being mixed with ethylene diaminetetra-acetic acid (EDTA), a chelating agent that prevents clotting by binding calcium. If you are obtaining a blood specimen from a patient for an FBC be sure to use a tube containing EDTA, add the correct volume of blood and mix the blood sample with the anticoagulant. The white blood cell count The white blood cell count (WBC, also known as the white cell count) was initially determined by counting cells using a microscope and a glass counting chamber. Nowadays it is counted by an automated instrument that detects individual cells flowing in a stream through a sensor. Recognition is either because the cell interrupts a beam of light or because it alters the electrical current flowing between two electrodes. A WBC is performed on a sample in which the mature red cells have been lysed by solutions to which they are exposed within the instrument so that the only cells that will be counted are the white cells and any nucleated red blood cells that might be present. HAEMATOLOGY - A Core Curriculum Imperial College Press 3. b1015_Chapter-02.qxd7/20/20103:19 PMPage 22b1015 Haematology: A Core Curriculum 22Chapter 2 The red blood cell count The red blood cell count (RBC, also known as the red cell count) was ini- tially determined with a counting chamber. As for the WBC, it is now determined by an instrument that counts red cells flowing through a sen- sor. However, this time the red cells are not lysed. White cells will actually also be included in the count but because they are infrequent in relation to the red cells this does not usually introduce much error. The haemoglobin concentration The haemoglobin concentration (Hb) is determined by lysing red cells and measuring light transmitted through a diluted sample of the blood at a specific wavelength after conversion of the haemoglobin to a stable form. The haematocrit It is possible to measure the proportion of red cells in an anticoagulated blood sample by centrifuging a tube of the blood and comparing the height of the column of red cells with the total height of the blood sample. This test is called a packed cell volume (PCV). It is no longer performed because it is not suitable for dealing with large numbers of blood speci- mens. The modern equivalent is called a haematocrit (Hct). It has the same significance as a PCV but instruments calculate it rather than meas- uring it directly. This is done by multiplying the average size of a red cell, the mean cell volume (MCV), by the number of red cells in a litre of blood (the RBC). The mean cell volume The mean cell volume (MCV) was once estimated by dividing the PCV (obtained by centrifugation) by the RBC (from a counting chamber). This was a very laborious technique so it was not done very often. Modern instruments estimate the MCV from the height of the electrical impulse generated by interruption of a light beam or an electrical current. The same electrical impulse is therefore used both to count the cells and to size them. HAEMATOLOGY - A Core Curriculum Imperial College Press 4. b1015_Chapter-02.qxd7/20/20103:19 PM Page 23b1015 Haematology: A Core CurriculumThe Blood Count and Film23 The mean cell haemoglobin The mean cell haemoglobin (MCH) is the average amount of haemoglo- bin in an individual red cell. Instruments calculate it by dividing the haemoglobin in a given volume of blood by the number of red cells in the same volume. The mean cell haemoglobin concentration The mean cell haemoglobin concentration (MCHC) is the average con- centration of haemoglobin, rather than the absolute amount, in an individual red cell. Fig. 2.1 is a visual representation of the difference between the MCH and the MCHC. Instruments calculate it by dividing the haemoglobin in a given volume of blood by the proportion of the whole blood sample that is occupied by red cells. The red cell distribution width The red cell distribution width (RDW) is a measurement of the amount of variation in the size of the red cells. Fig. 2.1. A diagram to illustrate the difference between the mean cell haemoglobin (MCH) and the mean cell haemoglobin concentration (MCHC). HAEMATOLOGY - A Core Curriculum Imperial College Press 5. b1015_Chapter-02.qxd7/20/20103:19 PMPage 24b1015 Haematology: A Core Curriculum 24Chapter 2 The platelet count Platelets are counted by the same principles as red cells and white cells. They can be distinguished from red and white cells by their smaller size. The red cell indices The term red cells indices refers to the RBC, MCV, MCH and MCHC. The formulae (which you do not need to know) for calculating the latter three values are: PCV (l/l) 1000MCV (fl) =RBC (cells/l) 1012Hb (g/dl) 10MCH (pg) =RBC (cells/l) 1012Hb (g/dl) MCHC (g/dl) =PCV (l/l) The red cell indices are very useful for indicating the likely cause of anaemia. The reticulocyte count A reticulocyte count is a supplement to an FBC, rather than a normal part of it. It can be performed using a microscope, counting the percentage of erythrocytes that have developed a reticulum or network of precipitated dye after incubation of the blood sample with a dye such as methylene blue (Fig. 2.2). This is a method for identifying ribonucleic acid (RNA) within red cells, thus demonstrating that these are cells newly released from the bone marrow (13 days old). Reticulocyte counts can also be performed by automated instruments, using either the same principle as the reticulocyte count with a microscope or alternatively, using a fluores- cent dye that binds to RNA. The Differential White Cell Count The different types of white cell can be distinguished from each other by examining a stained blood film. If a hundred cells are counted and the HAEMATOLOGY - A Core Curriculum Imperial College Press 6. b1015_Chapter-02.qxd7/20/20103:19 PM Page 25b1015 Haematology: A Core CurriculumThe Blood Count and Film 25 Fig. 2.2. A reticulocyte preparation, showing a reticulum of precipitated dye following incubation of blood with methylene blue. percentages are multiplied by the WBC, the absolute number of cells of each type can be calculated. Calculating the percentage or absolute num- ber of each cell type is referred to as a differential count. It is much more useful to calculate the absolute count than to try to make deductions from the percentages. For example, if a patient had 5% neutrophils and 95% lymphocytes this could indicate either a severe reduction of neutrophils or a marked increase in lymphocytes.Many automated instruments can recognise the five normal types of white cell and can detect the presence of nucleated red blood cells (NRBC) and other cells that are not normally present in the circulating blood. If only normal cells are present they are capable of performing a differential count. If abnormal cells are present it is usually still necessary to perform a differential count on a blood film. However, some instruments are also capable of counting NRBC. Normal ranges for the differential white cell count are shown in Table 2.2. Haematological Terminology Haematologists use precise terms to refer to abnormalities in the blood count and film. Since they use these terms to report abnormalities to HAEMATOLOGY - A Core Curriculum Imperial College Press 7. b1015_Chapter-02.qxd7/20/20103:19 PMPage 26b1015 Haematology: A Core Curriculum 26 Chapter 2Table 2.2. The differential white cell count.Normal rangeNormal range Test Units in men*in women* Neutrophil count 109/l1.76.1 1.77.5 Lymphocyte count 109/l 1.03.2 Monocyte count 109/l 0.20.6 Eosinophil count 109/l0.030.46 Basophil count 109/l0.090.29 *From Bain, B.J. (2006). Blood Cells. Fourth Edition. Blackwell Publishing, Oxford. Figures are for an autom


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