Albumin
The two serum proteins measured to assess liver function are albumin and globulin. Values for total serum proteins range from 6 to 8 g/dl. Of this total, between 52 and 68% (3.5-5g/dl) is albumin; the remainder are globulins.
Albumin, produced only in the liver, is the major plasma protein that circulates in the bloodstream. Albumin is essential for maintaining the oncotic pressure in the vascular system. A decrease in oncotic pressure due to a low albumin level allows fluid to leak out from the interstitial spaces into the peritoneal cavity, producing ascites. Albumin is also very important in the transportation of many substances such as drugs, lipids, hormones, and toxins that are bound to albumin in the bloodstream. Once the drug or other substance reaches the liver, it is detached from the albumin and made less toxic by conversion to a water-soluble form that can be excreted.
A low serum albumin indicates poor liver function. Decreased serum albumin levels are not seen in acute liver failure because it takes several weeks of impaired albumin production before the serum albumin level drops. The most common reason for a low albumin is chronic liver failure caused by cirrhosis. The serum albumin concentration is usually normal in chronic liver disease until cirrhosis and significant liver damage has occurred. In advanced liver disease, the serum albumin level may be less than 3.5 g/dl. Albumin levels can be low in conditions other than liver disease, such as severe malnutrition and some kidney diseases that cause extensive protein wasting. A loss of albumin in the urine caused by renal dysfunction (nephrotic syndrome) can cause a decrease in the serum albumin. Albuminuria or protein in the urine is a key sign of both renal pathology and pre-eclampsia. Severe burns that damage capillaries and blood vessels cause a huge loss of serum proteins. The increased capillary permeability caused by the burn damage allows a continual leakage of serum proteins out of the vascular system.
When there is inadequate protein intake, the body begins to breakdown muscle to obtain enough amino acids for the synthesis of serum albumin. The United States Department of Agriculture (USDA) recommends that women over age 25 consume 50 gms. of protein daily and men over 25 consume approximately 65 gms. of protein a day. When a patient has liver disease, dietary protein will be decreased to lessen stress on the liver, based on the results of liver function tests. Albumin levels do not drop in fasting states or in malnutrition until the condition is severe. A combination of severe illness with prolonged protein deprivation is eventually reflected in a reduced serum albumin level. There are no pathological conditions that cause the liver to produce extra albumin; thus, an increased rate is a reflection of dehydration.
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Albumin is important for maintaining oncotic pressure, and for transporting many substances in the blood.
When the plasma albumin level falls due to liver disease the colloid osmotic pressure of the blood is also reduced. Albumin produces the largest fraction of colloid osmotic pressure. Insufficient albumin allows fluid to move across vessel walls and into the interstitial spaces, causing generalized edema. The collection of fluid in the abdominal cavity (ascites) may make it extremely difficult for the patient to breathe in a reclining position. A paracentesis or abdominal "tap" may be done to remove fluid and relieve pressure on the diaphragm. A disadvantage of a parencentesis is that proteins are lost when the peritoneal fluid is drained. Diuretics and fluid and sodium restriction may be used to help treat edema.
The exact amounts of albumin and each type of globulin are measured through a process called serum protein electrophoresis. This test separates the major proteins in the serum in an electric field to determine the relative concentration of each. Serum protein electrophoresis is a useful test for patients with liver disease because it provides several diagnostic clues. The Albumin/Globulin or A/G ratio describes the relationship between albumin and globulins. The normal ratio is 1.0 or greater. For example, a patient's albumin is 3.5 and the globulin is 2.5, the ratio is 1.4 Although the A/G ratio may still be used, serum protein electrophoresis is now used to compare the amount of albumin with globulin.