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.
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