Protein components tests measure the amounts and types of protein in the blood. Proteins are constituents of muscle, enzymes, hormones, transport proteins, hemoglobin, and other functional and structural elements of the body. Albumin and globulin make up most of the protein within the body and are measured in the total protein of the blood and other body fluids. Thus, the serum (blood) protein components test measures the total protein, as well as its albumin and globulin components in the blood.
The protein components test is used to diagnose diseases that either affect proteins as a whole, or that involve a single type of protein. The test is also used to monitor the course of disease in certain cancers, intestinal and kidney protein-wasting states, immune disorders, liver dysfunction, and impaired nutrition.
Drugs that may cause increased protein levels include the anabolic steroids, androgens (male hormones), growth hormone, insulin, and progesterone. Drugs that may decrease protein levels include estrogen, drugs poisonous to the liver, and oral contraceptives.
Proteins are large molecules (complex organic compounds) that consist of amino acids, sugars and lipids. There are two main types of proteins: those that are made of fiber and form the structural basis of body tissues, such as hair, skin, muscle, tendons, and cartilage; and globular proteins (generally water soluble), which interact with many hormones, various other proteins in the blood, including hemoglobin and antibodies, and all the enzymes (substances that promote biochemical reactions in the body).
Proteins are needed in the diet to supply the body with amino acids. Ingested proteins are broken down in the digestive system to amino acids, which are then absorbed and rebuilt into new body proteins. One of the most important functions of proteins in the body is to contribute to the osmotic pressure (the movement of water between the bloodstream and tissues). An example of this is seen in diseases that result in damage to the filtering units of the kidneys (nephrotic syndrome). A severe loss of protein from the bloodstream into the urine (proteinuria) results, lowering the protein content of the blood and resulting in fluid retention, or edema.
Albumin and globulin are two key components of protein. Albumin is made in the liver and constitutes approximately 60% of the total protein. The main function of albumin is to maintain osmotic pressure and to help transport certain blood constituents around the body via the bloodstream. Because albumin is made in the liver, it is one element that is used to monitor liver function.
Globulin is the basis for antibodies, glycoproteins (protein-carbohydrate compounds), lipoproteins (proteins involved in fat transport), and clotting factors. Globulins are divided into three main groups, the alpha-, beta-, and gammaglobulins. Alpha-globulins include enzymes produced by the lungs and liver, and haptoglobin, which binds hemoglobin together. The beta-globulins consist mostly of low-density lipoproteins (LDLs), substances involved in fat transport. All of the gamma-globulins are antibodies, proteins produced by the immune system in response to infection, during allergic reaction, and after organ transplants.
Both serum albumin and globulin are measures of nutrition. Malnourished patients, especially after surgery, demonstrate greatly decreased protein levels, while burn patients and those who have protein-losing syndromes show low levels despite normal synthesis. Pregnancy in the third trimester is also associated with reduced protein levels.
The relationship of albumin to globulin is determined by ratio, so when certain diseases cause the albumin levels to drop, the globulin level will be increased by the body in an effort to maintain a normal total protein level. For example, when the liver is unable to synthesize sufficient albumin in chronic liver disease, the albumin level will be low, but the globulin levels will be normal or higher than normal. In such cases, the protein components test is an especially valuable diagnostic aid because it determines the ratio of albumin to globulin, as well as the total protein level. It should be noted, however, that when globulin is provided as a calculation (total protein - albumin = globulin), the result is much less definitive than other methods of determining globulin.
Consequently, when the albumin/globulin ratio (A/G ratio) is less than 1.0, more precise tests should be ordered. These tests include protein electrophoresis, a method of separating the different blood proteins into groups. If the protein electrophoresis indicates a rise, or "spike" at the globulin level, an even more specific test for globulins, called immunoelectrophoresis, should be ordered to separate out the various globulins according to type. Some diseases characterized by dysproteinemia (derangement of the protein content of the blood), have typical electrophoretic globulin peaks.
Unless this is requested by the physician, there is no need that the patient restrict food or fluids before the test.
Risks posed by this test are minimal, but may include slight bleeding from the blood-drawing site, fainting or lightheadedness after venipuncture, or hematoma (blood accumulating under the puncture site).
Reference values vary from laboratory to laboratory, but can generally be found within the following ranges: Total protein: 6.4-8.3 g/dL; albumin: 3.5-5.0 g/dL; globulin: 2.3-3.4 g/dL.
Increased total protein levels are seen in dehydration, in some cases of chronic liver disease (like autoimmune hepatitis and cirrhosis), and in certain tropical diseases (for example, leprosy). Very low total protein levels (less than 4.0 g/dl) and low albumin cause the edema (water retention) usually seen in nephrotic syndromes. Decreased protein levels may be seen in pregnancy, chronic alcoholism, prolonged immobilization, heart failure, starvation, and malabsorption or malnutrition.
Increased albumin levels are found in dehydration. Decreased albumin levels are indicative of liver disease, protein-losing syndromes, malnutrition, inflammatory disease, and familial idiopathic (of unknown cause) dysproteinemia, a genetic disease in which the albumin is significantly reduced and globulins increased.
Increased globulin levels are found in multiple myeloma and Waldenström's macroglobulinemia, two cancers characterized by overproduction of gamma- globulin from proliferating plasma cells. Increased globulin levels are also found in chronic inflammatory diseases such as rheumatoid arthritis, acute and chronic infection, and cirrhosis. Decreased globulin levels are seen in genetic immune disorders and secondary immune deficiency.
- Nephrotic syndromes
- A collection of symptoms that result from damage to the filtering units of the kidney (glomeruli) causing severe loss of protein from the blood into the urine.
For Your Information
- Cahill, Mathew. Handbook of Diagnostic Tests. Springhouse, PA: Springhouse Corporation, 1995.
- Jacobs, David S. Laboratory Test Handbook. 4th ed. Hudson, OH: Lexi-Comp Inc., 1996.
- Pagana, Kathleen Deska. Mosby's Manual of Diagnostic and Laboratory Tests. St. Louis, MO: Mosby, Inc., 1998.
Gale Encyclopedia of Medicine. Gale Research, 1999.