The blood count and film

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<ul><li> 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</li></ul><p> 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 automated instrument and will differ slightly between instruments; figures for manual differential counts will have broader limits. clinical staff it is necessary for all doctors to understand them. The terms used to describe quantitative abnormalities are shown in Table 2.3; unfor- tunately these terms are not necessarily logical or consistent so you just have to learn how they are used. The terms used to describe erythrocytes and their abnormalities are shown and illustrated in Table 2.4. Normal Ranges and Reference Ranges Once we have got a laboratory result how do we know if it is normal? Laboratories issue test results with a reference range or normal range for comparison. A reference range is derived from a carefully defined popula- tion, the reference population: the definition will include the age and gender of the subjects and sometimes the ethnic origin or other relevant details. If a requirement to be in good health is part of the definition then a reference range becomes a normal range. Conventionally, reference ranges and normal ranges include the central 95% of results from the reference population. Ideally, tests would give a clear separation of healthy subjects (the normal range) and an unhealthy population, as shown in Fig. 2.3a. More often, the situation will be as shown in Fig. 2.3c, with a small amount of overlap between sick and well people. Sometimes a test gives poor separation of sick and well (Fig. 2.3b). Either the laboratory or the clinician looking after the patient has to decide the threshold to accept. If the figure chosen includes all sick people then a lot of normal people will also be included; the test becomes sensitive but at the cost of a high rate of false positives. If the threshold is moved down so that no healthy person is HAEMATOLOGY - A Core Curriculum Imperial College Press 8. b1015_Chapter-02.qxd7/20/20103:19 PMPage 27b1015 Haematology: A Core CurriculumThe Blood Count and Film 27Table 2.3. Terminology used for describing quantitative abnormalitiesin blood counts.TermMeaningAnaemiaReduced HbPolycythaemia or Either term can be used to indicate an increaseerythrocytosis in the RBC, Hb and HctLeucocytosis Increased WBCLeucopenia Reduced WBCThrombocytosis Increased platelet countThrombocytopenia Reduced platelet countNeutrophilia Increased neutrophil countNeutropeniaReduced neutrophil countLymphocytosisIncreased lymphocyte countLymphopenia* orReduced lymphocyte countlymphocytopeniaMonocytosisIncreased monocyte countEosinophilia Increased eosinophil countBasophiliaIncreased basophil countReticulocytosisIncreased reticulocyte countReticulocytopeniaReduced reticulocyte count*The term lymphopenia is particularly illogical because it is used to mean a reducedlymphocyte count not a reduced amount of lymph.Basophilia has a quite different alternative meaning; it also means an increaseduptake of basic dyes by the cytoplasm of a cell so that when examined with a micro-scope it appears blue. HAEMATOLOGY - A Core Curriculum Imperial College Press 9. b1015_Chapter-02.qxd7/20/20103:19 PM Page 28b1015 Haematology: A Core Curriculum 28Chapter 2Table 2.4. Terminology used for describing erythrocytes and their abnormalities. TermSignificancePicture NormocyticOf normal size NormochromicWith normal staining characteristics AnisocytosisIncreased variation in size Poikilocyte An erythrocyte of abnormal shape PoikilocytosisIncreased variation in shape Microcyte Smaller than normal MicrocytosisThe presence of microcytes (Continued) HAEMATOLOGY - A Core Curriculum Imperial College Press 10. b1015_Chapter-02.qxd7/20/20103:19 PM Page 29b1015 Haematology: A Core CurriculumThe Blood Count and Film 29Table 2.4.(Continued ) TermSignificancePicture Macrocyte Larger than normal MacrocytosisThe presence of macrocytes Hypochromic Paler than normal (more than a third of the diameter of the cell is pale) Hypochromia The presence of hypochromic cells Polychromasia Having a blue tinge Elliptocyte An erythrocyte that is elliptical in shape (Continued) HAEMATOLOGY - A Core Curriculum Imperial College Press 11. b1015_Chapter-02.qxd7/20/20103:19 PM Page 30b1015 Haematology: A Core Curriculum 30Chapter 2Table 2.4.(Continued ) TermSignificancePicture Ovalocyte An erythrocyte that is oval in shape Macro-ovalocyte An oval macrocyte SpherocyteAn erythrocyte that is spherical in shape and is therefore lacking central pallor Irregularly An erythrocyte that lackscontracted cellcentral pallor and has an irregular outline Teardrop cell An erythrocyte that is shaped (dacrocyte) like a tear (Continued) HAEMATOLOGY - A Core Curriculum Imperial College Press 12. b1015_Chapter-02.qxd7/20/20103:19 PM Page 31b1015 Haematology: A Core CurriculumThe Blood Count and Film 31Table 2.4.(Continued ) TermSignificancePic...</p>


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