Many kinds of studies can be done on blood, and some of these have become a standard part of a routine physical examination. Machines that are able to perform several tests at the same time have largely replaced manual procedures, particularly in large institutions.
The hematocrit, the volume percentage of red cells in whole blood, is determined by spinning a blood sample in a high-speed centrifuge for 3 to 5 minutes to separate the cellular elements from the plasma (Fig. 9-11). The hematocrit is expressed as the volume of packed red cells per unit volume of whole blood. For example, “hematocrit, 38%” in a laboratory report means that the patient has 38 mL red cells per 100 mL (dL) of blood; red cells comprise 38% of the total blood volume. For adult men, the normal range is 42% to 54%, whereas for adult women the range is slightly lower, 36% to 46%. These normal ranges, like all normal ranges for humans, may vary depending on the method used and the interpretation of the results by an individual laboratory. Hematocrit values much below or much above these figures point to an abnormality requiring further study.
Figure 9-11 Hematocrit. The tube on the left shows a normal hematocrit. The middle tube shows that the percentage of red blood cells is low, indicating anemia. The tube on the right shows an excessively high percentage of red cells, as seen in polycythemia.
A sufficient amount of hemoglobin in red cells is required for adequate oxygen delivery to the tissues. To measure its level, the hemoglobin is released from the red cells, and the color of the blood is compared with a known color scale. Hemoglobin is expressed in grams per 100 mL whole blood. Normal hemoglobin concentrations for adult males range from 14 to 17 g per 100 mL blood. Values for adult women are in a somewhat lower range, at 12 to 15 g per 100 mL blood. A decrease in hemoglobin to below normal levels signifies anemia. Normal and abnormal types of hemoglobin can be separated and measured by the process of electrophoresis. In this procedure, an electric current is passed through the liquid that contains the hemoglobin to separate different components based on their electrical charge. This test is useful in the diagnosis of sickle cell anemia and other disorders caused by abnormal types of hemoglobin.
Blood Cell Counts
Most laboratories use automated methods for obtaining the data for blood counts. Visual counts are sometimes done using a hemocytometer, a ruled slide used to count the cells in a given volume of blood under the microscope.
Red Cell Counts The normal red cell count varies from 4.5 to 5.5 million cells per nL (mm3) of blood. An increase in the red cell count is called polycythemia. People who live at high altitudes develop polycythemia, as do patients with the disease polycythemia vera, a disorder of the bone marrow.
White Cell Counts The leukocyte count varies from 5000 to 10,000 cells per nL of blood. In leukopenia, the white count is below 5000 cells per mL. This condition indicates depressed bone marrow or a bone marrow neoplasm. In leukocytosis, the white cell count exceeds 10,000 cells per mL. This condition is characteristic of most bacterial infections. It may also occur after hemorrhage, in cases of gout (a type of arthritis), and in uremia, the presence of nitrogenous waste in the blood as a result of kidney disease.
Platelet Counts It is difficult to count platelets visually because they are so small. More accurate counts can be obtained with automated methods. These counts are necessary for the evaluation of platelet loss (thrombocytopenia) such as occurs after radiation therapy or cancer chemotherapy. The normal platelet count ranges from 150,000 to 450,000 per nL of blood, but counts may fall to 100,000 or less without causing serious bleeding problems. If a count is very low, a platelet transfusion may be given.
The Blood Slide (Smear)
In addition to the above tests, the complete blood count (CBC) includes the examination of a stained blood slide (see Fig. 9-2). In this procedure, a drop of blood is spread thinly and evenly over a glass slide, and a special stain (Wright) is applied to differentiate the otherwise colorless white cells.
The slide is then studied under the microscope. The red cells are examined for abnormalities in size, color, or shape and for variations in the percentage of immature forms, known as reticulocytes (See Box 9-3 to learn about reticulocytes and how their counts are used to diagnose disease). The number of platelets is estimated. Parasites, such as the malarial organism and others, may be found. In addition, a differential white count is done. This is an estimation of the percentage of each white cell type in the smear. Because each type has a specific function, changes in their proportions can be a valuable diagnostic aid.
Reticulocytes. Some ribosomes and rough ER appear as a network in a late stage of erythrocyte development.
Figure 9-2 Blood cells as viewed under the microscope. All three types of formed elements are visible.
Blood Chemistry Tests
Batteries of tests on blood serum are often done by machine. One machine, the Sequential Multiple Analyzer (SMA), can run some 20 tests per minute. Tests for electrolytes, such as sodium, potassium, chloride, and bicarbonate, may be performed at the same time along with tests for blood glucose, and nitrogenous waste products, such as blood urea nitrogen (BUN), and creatinine. Other tests check for enzymes. Increased levels of CPK (creatine phosphokinase), LDH (lactic dehydrogenase), and other enzymes indicate tissue damage, such as damage that may occur in heart disease. An excess of alkaline phosphatase could indicate a liver disorder or metastatic cancer involving bone. Blood can be tested for amounts of lipids, such as cholesterol, triglycerides (fats), and lipoproteins, or for amounts of plasma proteins. Many of these tests help in evaluating disorders that may involve various vital organs. For example, the presence of more than the normal amount of glucose (sugar) dissolved in the blood, a condition called hyperglycemia, is found most frequently in patients with unregulated diabetes. Sometimes, several sugar evaluations are done after the administration of a known amount of glucose. This procedure is called the glucose tolerance test and is usually given along with another test that determines the amount of sugar in the urine. This combination of tests can indicate faulty cell metabolism. The list of blood chemistry tests is extensive and is constantly increasing. We may now obtain values for various hormones, vitamins, antibodies, and toxic or therapeutic drug levels.
Before surgery and during treatment of certain diseases, hemophilia for example, it is important to know that coagulation will take place within normal time limits. Because clotting is a complex process involving many reactants, a delay may result from a number of different causes, including lack of certain hormonelike substances, calcium salts, or vitamin K. The amounts of the various clotting factors are evaluated by percentage to aid in the diagnosis and treatment of bleeding disorders. Additional tests for coagulation include tests for bleeding time, clotting time, capillary strength, and platelet function.
Bone Marrow Biopsy
A special needle is used to obtain a small sample of red marrow from the sternum, sacrum, or iliac crest in a procedure called a bone marrow biopsy. If marrow is taken from the sternum, the procedure may be referred to as a sternal puncture. Examination of the cells gives valuable information that can aid in the diagnosis of bone marrow disorders, including leukemia and certain kinds of anemia.
Figure 9-6 Phagocytosis. (A) A phagocytic leukocyte (white blood cell) squeezes through a capillary wall in the region of an infection and engulfs a bacterium. (B) The bacterium is enclosed in a vesicle and digested by a lysosome.