Laboratory Blood Tests..

Doctors depend on many different laboratory tests of blood samples to diagnose and monitor diseases. Because the liquid portion of the blood (plasma) carries so many substances essential to the body's functioning, blood tests can be used to find out what is happening in many parts of the body.

Testing blood is easier than obtaining a tissue sample from a specific organ. For example, thyroid function can be evaluated more easily by measuring the level of thyroid hormones in the blood than by directly sampling the thyroid. Likewise, measuring liver enzymes and proteins in the blood is easier than sampling the liver. However, certain blood tests are used to measure the components and function of the blood itself. These are the tests that are mostly used to diagnose blood disorders.

Complete Blood Count:

The most commonly performed blood test is the complete blood count (CBC), which is an evaluation of all the cellular components (red blood cells, white blood cells, and platelets). Automated machines perform this test in less than 1 minute on a small drop of blood. The CBC is supplemented in some instances by examination of blood cells under a microscope.

The CBC determines the number of red blood cells and the amount of hemoglobin (the oxygen-carrying protein in red blood cells) in the blood. In addition, the average size, degree of variability of size, and hemoglobin content of red blood cells is assessed by a CBC and can alert laboratory workers to the presence of abnormal red blood cells (which may then be further characterized by microscopic examination). Abnormal red blood cells may be fragmented or shaped like teardrops, crescents, needles, or a variety of other forms. Knowing the specific shape and size of red blood cells can help a doctor diagnose a particular cause of anemia. For example, sickle-shaped cells are characteristic of sickle cell disease, small cells containing insufficient amounts of hemoglobin are likely due to iron deficiency anemia, and large oval cells suggest anemia due to a deficiency of folic acid or vitamin B₁₂.

The CBC also determines the number of white blood cells. The specific types of white blood cells can be counted (differential white blood cell count) when a doctor needs more information. If the total number of white blood cells or the number of one of the specific types of white blood cells is above or below normal, the doctor can examine these cells under a microscope. The microscopic examination can identify features that are characteristic of certain diseases. For example, large numbers of white blood cells that have a very immature appearance (blasts) may indicate leukemia (cancer of the white blood cells).

Platelets are usually also counted as part of a CBC. The number of platelets is an important measure of the blood's protective mechanisms for stopping bleeding (clotting). A high number of platelets (thrombocytosis or thrombocythemia) can lead to blood clots in small blood vessels, especially those in the heart or brain. In some disorders, a high number of platelets may paradoxically result in excess bleeding.