Diagnosis

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 (see Section 10, Chapter 134) 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 a basic evaluation of the cellular components of blood (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 most instances by examination of blood cells under a microscope.

The CBC determines the number of red blood cells and the amount of hemoglobin (the protein that enables the cells to carry oxygen) in the blood. In addition, the size of red blood cells is usually assessed and can alert laboratory workers to the presence of abnormally shaped 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 or 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 may signal iron deficiency anemia, and large oval cells suggest anemia due to folic acid or vitamin B₁₂ deficiency (pernicious anemia).

After putting together information about the number, size, and shape of the red blood cells, a doctor might order additional tests to evaluate the cause of an anemia. These include tests for increased red blood cell fragility, abnormal types of hemoglobin, and the quantities of certain other substances contained within red blood cells.

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 detailed information on a person's condition. 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 suggest 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.

See the table Complete Blood Count (CBC).

Reticulocyte Count: The reticulocyte count measures the number of newly formed (young) red blood cells (reticulocytes) in a specified volume of blood. Reticulocytes normally make up about 1% of the total number of red blood cells. When the body needs more red blood cells, as in anemia, the bone marrow normally responds by producing more reticulocytes. Thus, the reticulocyte count is a measure of bone marrow function.

Blood Typing: Blood type, which is determined by the presence of proteins on the surface of red blood cells, can be identified by measuring the reaction of a small sample of a person's blood to certain antibodies. Blood typing must be done before blood can be transfused (see Section 17, Chapter 171).

Bleeding Time and Other Clotting Tests: The body's ability to stop bleeding may be determined by the bleeding time (in addition to a count of the number of platelets). In this test, a small cut is made on the person's forearm, and the examiner measures the amount of time that elapses before bleeding stops. This test is largely a measure of platelet function; most tests of platelet function are done by automated methods. Other tests can be performed to measure the overall function of the many proteins needed for normal blood clotting (clotting factors). The most common of these tests are the prothrombin time (PT) and the partial thromboplastin time (PTT). The levels of individual clotting factors can also be determined.

Other Blood Tests: Specialized blood tests can be used to determine other blood disorders. For example, on rare occasions, doctors must measure the total volume of blood or the total number of certain blood cells in the body. These measurements can be done using radioactive isotopes that mix in the blood or attach to blood cells.

Bone Marrow Examination

Sometimes a sample of bone marrow must be examined to determine why blood cells are abnormal. A doctor can take two different types of bone marrow samples: a bone marrow aspirate and a bone marrow core biopsy. Both types are usually taken from the hipbone (iliac crest), although aspirates are sometimes taken from the breastbone (sternum). In very young children, bone marrow samples are occasionally taken from a leg bone (tibia).

See the figure Taking a Bone Marrow Sample.

When both types of samples are needed, they are taken at the same time. After the skin and tissue over the bone are numbed with a local anesthetic, the sharp needle of a syringe is inserted into the bone. For a bone marrow aspirate, the doctor pulls back on the plunger of the syringe and draws out a small amount of the soft bone marrow, which can be spread on a slide and examined under a microscope. Special tests, such as cultures for bacteria, fungi, or viruses, chromosomal analysis, and analysis of cell surface proteins (flow cytometry), can be performed on the sample. Although the aspirate often provides enough information for a diagnosis to be made, the process of drawing the marrow into the syringe breaks up the fragile bone marrow. As a result, determining the original arrangement of the cells is difficult.

When the exact anatomic relationships of cells must be determined and the structure of the tissues evaluated, the doctor also performs a core biopsy. A small core of intact bone marrow is removed with an internal coring device. This core is preserved and sliced into thin sections that are examined under a microscope.

A bone marrow sampling generally involves a slight jolt of pain, followed by minimal discomfort. The procedure takes a few minutes.