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Liver Function Tests Definition
Liver Function Tests – a panel of blood tests that measures the levels of albumin, bilirubin, and certain liver enzymes in the blood. More specifically targeted tests further identify the reasons for abnormal results, as the findings may also indicate dysfunctions of other organs.
Reasons for Doing This Test
Liver function tests provide a general assessment of how effectively the liver is performing its metabolic tasks. They also allow the doctor to monitor the progression of liver disease and the effectiveness of treatment.
The liver synthesizes (produces) the key amino acids that make up albumin, the primary protein in blood plasma. Albumin transports numerous substances—including other proteins, nutrients, and hormones—through the blood. When albumin levels are low the blood cannot carry these substances, which has a variety of consequences throughout the body. Many liver conditions cause ascites (abdominal edema), in which a deficiency of albumin in the plasma allows plasma to seep across cell membranes to accumulate in the abdominal cavity. Decreased blood albumin levels suggest liver dysfunction. The normal range of serum albumin is 3.4 to 5.4 grams per deciliter (g/dL).
One key function of the liver is to complete the break down of old erythrocytes (red blood cells) to recycle their ingredients for other uses in the body. The spleen initiates this process, splitting the hemoglobin erythrocytes contain into its core proteins, heme and globin, and extracting the pigment bilirubin from the heme. Bilirubin is a waste product that the liver chemically alters to use in synthesizing bile. The chemically processed bilirubin is conjugated or direct bilirubin; bilirubin that circulates in the blood is unconjugated or indirect bilirubin.
A healthy liver excretes most of the conjugated bilirubin it produces in the bile. A damaged liver cannot keep up the pace of bile production, allowing conjugated bilirubin to escape into the blood. Elevated circulation of conjugated, or direct, bilirubin suggests liver or gallbladder dysfunction. The normal range of serum direct bilirubin is 0.0 to 0.4 milligrams per deciliter (mg/dL).
Enzymes are catalytic substances that expedite chemical processes within the liver. In a healthy liver enzymes remain within the lobules, the working communities of hepatocytes. Hepatocytic damage results in enzymes leaking from the cells and spilling out into the blood. The commonly measured liver enzymes are
- the aminotransferases (also called transaminases), which catalyze amino acid METABOLISM for example, aspartate aminotransferase (AST), also called serum glutamic oxaloacetic transaminase (SGOT), and alanine aminotransferase (ALT), also called serum glutamic pyruvic transaminase (SGPT)
- alkaline phosphatase (ALP), which also goes up in biliary obstruction (blockage of the flow of BILE)
- gamma-glutamyltranspeptidase (GGT)
Collectively elevated levels of these enzymes in the blood indicate damage to the liver that has caused the death of hepatocytes (hepatocellular necrosis). Individual elevations may indicate damage to other tissues in the body such as might occur in the heart with heart attack. The AST to ALT ratio is also significant; a ratio greater than 2:1 is common with liver disease of alcoholism.
The prothrombin time (PT) measures the amount of time it takes for the clotting process to take place in the plasma. The normal range (for a person who is not taking anticoagulation therapy) is 11 to 13.5 seconds. Clotting time longer than normal suggests a general dysfunction of the body’s clotting mechanisms. Liver function becomes suspect with an elevated clotting time because the liver synthesizes many of the proteins (clotting factors) necessary for coagulation.
Preparation, Procedure, and Recovery
Liver function tests require a blood sample, typically drawn from a vein in the arm. No preparation is necessary and there is no recovery period.
Risks and Complications
Some people experience minor bruising at the site of the venipuncture, which usually heals in a few days (though liver disease that affects clotting mechanisms may extend healing).
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