Blood, Electrolytes, and Intravenous Infusions

3-21. BLOOD TESTS

The results of blood examinations are required to definitely rule out practically every disease. A physician would hesitate to declare a patient free from a disease until certain blood tests have been performed and the results of these tests can be included with the patient's health examination by other means. Even if the specific suspected disease is not expected to produce changes in the patient's hematologic (or blood picture) profile, this fact is required to support diagnosis. Blood tests are normally ordered by the physician and completed in a clinical hematology laboratory. The medical NCO should be aware of the commonly ordered tests and how some of the test results may indicate or point toward disease diagnosis. Most blood changes do point toward disease. The more common blood examinations are frequently all that are required for a patient. Their chief purpose is to indicate whether more detailed hematologic procedures are required. Listed below are some of the commonly performed tests you might expect to encounter in a clinical setting. The tests to be discussed here are the complete blood count (CBC), hematocrit, hemoglobin, sedimentation rate, partial thromboplastin time, and prothrombin time. There are many others. The selection of the test(s) will depend on the suspected disease, physician's preference, and the laboratory facilities.

a. Complete Blood Count. The complete blood count (CBC) includes the red blood count and the white blood cell count. These may be done either by using manual or by using automated methods.

(1) The red blood cell count (RBC) results in million RBCs per cubic millimeter in the sample. The normal values are 4.2-5.4 million RBCs per cubic millimeter for adult males and 3.6-5.0 million RBCs per cubic millimeter for adult females. To perform the test, a sample of blood is diluted with a special isotonic solution. When the sample has been mixed enough, part of the sample is put into a ruled counting chamber. Five ruled sections are counted, and the RBC is calculated.

(2) When the total leukocytes (white blood cells) are counted, no distinction is made for the type of white cell (for example, lymphocyte, monocyte, and so forth). If distinction is required, further testing must be done. The normal range for adults is 4,500-11,500 per cubic millimeter. Leukocyte counting is usually done electronically, but can be performed manually. A blood sample is mixed with required solutions, and gentian violet is added for color. A measured sample is put into a ruled counting chamber. Four marked sections are counted, and the white blood cell count (WBC) is calculated.

b. Hematocrit (Packed Cell Volume). The hematocrit is the volume of erythrocytes expressed as a percentage of whole blood in a sample. An anticoagulant is added to a small blood sample, and the tube is tightly capped to avoid evaporation. The sample is placed on a centrifuge and turned for five to thirty minutes (depending on the method and equipment used). The red cell column is measured in height (millimeters) against the height (in millimeters) of the whole column. The normal hematocrit for males is between 40 and 54. For females, the normal range is between 38 and 47. A value below the patient's normal or below the normal range may indicate anemia. A higher reading may indicate polycythemia.

c. Hemoglobin. The hemoglobin concentration in the blood bears a direct relationship to its oxygen carrying capacity. Because of the relationship, this test is performed on practically every patient, especially for suspected diseases associated with anemia. There are several ways to measure hemoglobin. The most widely used and recommended method uses cyanide compounds to convert the hemoglobin. This process will eventually result in a compound called "cyanmethemoglobin". The hemoglobin content can then be determined. The use of cyanide compounds in this process increases the danger of accidental poisoning in the laboratory. Proper ventilation and protection for the technician must be available. The normal values are 14 to 17 grams hemoglobin (per deciliter) for adult males and 12 to 16 grams hemoglobin (per deciliter) for adult females.

d. Erythrocyte Sedimentation Rate. The erythrocyte sedimentation measures the rate at which the red blood cells settle out of the cellular-plasma suspension. The rate is usually increased in inflammatory infections, toxemia, cell or tissue destruction, severe anemia, active tuberculosis, syphilis, acute coronary thrombosis, rheumatoid arthritis, and malignant processes. The rate is generally decreased by sickle cell anemia, polycythemia, hypofibrinogenemia, and certain drugs. The procedure is to place a measured amount of anticoagulated blood in a tube and measure the distance the erythrocytes fall within a given time interval.

Normal values are 0 to 9 millimeters for adult males and 0 to 20 millimeters for adult females. This test is inconclusive. It indicates the need for further testing. In some cases, such as acute rheumatic fever or congestive heart failure, the sedimentation rate has remained within normal limits.

e. Partial Thromboplastin Time. The partial thromboplastin time is the most useful screening method for detecting blood coagulation disorders. This procedure tests all three stages of blood coagulation and can show abnormalities in almost all of the clotting factors. Freshly collected blood is combined with certain compounds and observed for clot formation. Using most commercially prepared and some laboratory prepared compound, the clot should form in less than 35 seconds to be considered normal.