Blood

Blood consists of several different types of cells suspended in plasma. The function of blood is to provide oxygen to the tissues, to prevent invasion by microorganisms and to promote haemostasis. The formed elements of the blood or blood cells consist of the red cells (erythrocytes), the white cells and the platelets. The cells are produced primarily by bone marrow. Plasma fluid derives from the intestines and organs and provides vehicle for cell measurement.

Haemoglobin (Hb)

Haemoglobin is the oxygen-carrying component contained in red blood cells (RBCs). It is composed of a single protein called globin and a compound called haem, which contains iron atoms and the red pigment porphyrin. The oxygen-combining capacity of the blood is directly proportional to the haemoglobin concentration, rather than to the number of RBC. Haemoglobin determination is part of a complete blood count  that screens for disease associated with anaemia, determines the severity of anaemia, follows the response to treatment for anaemia and evaluates polycythemia. Anaemia is defined as Hb below normal for age and gender.

Use of Test: Suspected anaemia or erythrocytosis.

Red Blood Cell (RBC) Count

Red blood cells contain haemoglobin and a variety of other proteins, salts, and vitamins. The cells are shaped like biconcave discs to enable the maximum amount of haemoglobin to be used. The primary functions of RBCs are to transport oxygen from the lungs to the body tissue and to carry carbon dioxide from the tissues to the lungs. RBC count determines the total number of red blood cells found in a cubic millimeter (mm3) of blood. It is an important measurement in the determination of anaemia or polycythemia.

Use of Test: Investigation of anaemia or erythrocytosis.

Haematocrit (Hct), (PCV)

Haematocrit determines red blood cell mass. The results are expressed  as the percentage of packed red cells in a volume of whole blood. It is part of the complete blood count.

Use of Test: Assessment of anaemia and erythrocytosis. Monitor haemodilution, haemoconcentration.

Mean Cell Volume (MCV)

The mean cell volume gives an indication of the size of RBCs. This index expresses the volume occupied by a single RBC and is a measure in cubic microns (um3) of the mean volume. It is useful for classifying anaemias.

Use of Test: Guide to investigation of anaemia; blood film should also be requested.

Mean cell Haemoglobin (MCH)

MCH gives an indication of the amount of haemoglobin per RBC. It is expressed in picograms. An increase of MCH is associated with macrocytic anaemia and a decrease of MCH is associated with microcytic anaemia.

Use of Test: Guide to investigations of anaemia; blood film should also be requested.

Mean Cell Haemoglobin Concentration (MCHC)

This test measures the average concentration of haemoglobin in the average RBC. It gives the ratio of the weight of haemoglobin to the volume of the red blood cell.

Use of Test: Guide to investigation of anaemia; blood film should also be requested.

White Blood Cell Count (WBC)

White blood cells or leucocytes fight infection and defend the body by phagocytosis, a process in which the leucocytes encapsulate foreign organisms. They also produce, transport and distribute antibodies as part of the immune response. White blood cells can be differentiated into neutrophils (polymorphonuclear leucocytes), lymphocytes, monocytes, eosinophils and basophils. WBC count determines the total number of white blood cells circulating in the blood. Variation in count can occur in the same individual at different times, such as during the menstrual cycle, normal human development, activity and others.

Use of Test: Possible infection, inflammatory disease, bone marrow failure, haematological or other malignancy. Monitoring drugs with potential or predictable bone marrow toxicity.

White Blood Cell (WBC) Differential Count

The white blood cells are categorized into 5 main types, each of which performs a specific function. The differential count is expressed as a percentage of the total number of white cells. The percentage is the relative number of each type of WBC in the blood.

Lymphocytes: T lymphocytes are principally concerned with cell-mediated immune processes.

B lymphocytes are concerned with humoral immunity and are the precursors of  antibody-producing plasma cells.

Use of Test: To establish the numbers of individual leukocytes such as neutrophils, lymphocytes, monocytes, eosinophils and basophils.

白细胞分类计数

白细胞分为五大类，各有特定的功能。分类计数是各类细胞占白细胞总数 的百分比。也就是血液中每种白细胞的相对数量。

中性白细胞： 吞噬和消灭细菌。

具有吞噬功能但比中性白细胞的效果低。受到过敏 和寄生 嗜酸性细胞： 虫侵袭时数量增加。

嗜碱性细胞： 是白细胞中数量最少的细胞，在骨髓及外骨髓增殖 疾病及 其他非恶性疾病时有增加的可能性 .

检查的作用：确定各种白细胞的数量。

Blood Film

An examination of the stained blood film is the most important investigation in haematology. Each of the cell types will be studied separately.

Use of Test: Evaluation of changes in numbers or morphology of red blood cells, white blood cells and platelets. It clarifies abnormalities detected by automated haematology instruments and guides further investigation.

Platelet Count

Platelets are the smallest of the formed elements in the blood. They are developed primarily in the bone marrow. They are necessary for blood clotting, vascular integrity, vasoconstriction and formation of plugs to occlude breaks in small vessels.

Use of Test: History of excessive and/or inappropriate bleeding, bruising; purpura. Monitoring drugs with potential or predictable bone marrow toxicity; monitoring heparin therapy.

Erythrocyte Sedimentation Rate (ESR)

ESR is the rate at which erythrocytes clump or aggregate together and settle out of anticoagulated blood in 1 hour. A normal ESR does not exclude active disease. The ESR increases with age and is raised in pregnancy, anaemia, acute and chronic inflammatory diseases, neoplastic diseases, collagen diseases, renal insufficiency and any disorder associated with a significant change in the plasma proteins.

Use of Test: A non-specific indicator of inflammatory and neoplastic disease. Should not be used to screen asymptomatic patients for the presence of disease.