Analyzing liver function: how to use lab values
In this video, we'll look at liver function tests in more detail, what each of these tests can tell us about our patient's health, and when they may signal something sinister.
The liver function panel involves testing for levels of bilirubin, liver enzymes, albumin, and also prothrombin time. In this video, we'll look at each of these tests in more detail. We'll explore what each test can tell us about our patient's health and when they may signal something sinister.
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The liver is the largest internal organ of the body. It is involved in processing dietary amino acids, carbohydrates, lipids and vitamins. It also metabolizes cholesterol and toxins, produces clotting factors and stores glycogen. The liver is the principal site for synthesis of all circulating proteins, except gamma globulins.
The liver function panel involves testing for the levels of bilirubin, liver enzymes, and the serum protein albumin. Prothrombin time, a measure of blood clotting is also routinely measured. Let's look at each of these tests in a bit more detail. Bilirubin is a waste product produced from the breakdown of red blood cells.
Heme, derived from the breakdown of hemoglobin ultimately becomes unconjugated bilirubin, which is a non-water-soluble molecule that is bound to serum albumin in the blood. Unconjugated bilirubin is taken up by the liver, where it is converted to conjugated bilirubin, which is water soluble, and can be excreted in the bile.
The liver enzymes that are commonly measured in a liver functional panel, include the serum aminotransferases, such as alanine aminotransferase, and aspartate aminotransferase. They also include alkaline phosphatase, gamma glutamyl transpeptidase, 5'-nucleotidase, and lactate dehydrogenase. Prothrombin time is a test that measures blood clotting.
All blood clotting factors, except for factor viii are produced in the liver. Therefore when liver function is significantly impaired, in either the acute or chronic setting, clotting factor levels will be low. Prothrombin time is one way of measuring how long it takes the blood to form a clot. Prothrombin time is a common measure included in a liver function panel.
Albumin is the most common protein found in the blood. It is used by the body for growth and tissue repair. When liver function is impaired over a prolonged period, albumin synthesis is impaired, which will result in low levels of albumin in the blood. Thus, hypoalbuminemia is a common finding in chronic liver disease, but not an acute liver failure.
What do these liver function tests tell us? Alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase and bilirubin are biochemical markers of liver injury. So a change in their levels suggests a pathologic process, causing liver damage or injury. In contrast, albumin, bilirubin and prothrombin time are markers of hepatocellular function, meaning that changes in these markers suggest altered or reduced function of the liver cells themselves.