The liver is the largest internal organ and unlike the kidneys or the lungs, there is only one in the body. It is a metabolically active organ that has several general functions and is often called both the body’s manufacturing center and its filtering plant.
The liver produces many important compounds. It makes bile, which is responsible for the digestion and absorption of fats, cholesterol and the fat-soluble vitamins, A, D, E and K. It manufactures proteins such as albumin and most of the clotting factors which allow the body to clot when it is injured. It also produces angiotensinogen, a hormone involved in blood pressure regulation.
The liver also acts as a storage warehouse. It stores ferritin, which is involved in the production of red blood cells. It stores vitamins such as A, D, E and K and B12 as well as the mineral, copper. Complex sugars are stored in the liver so that the liver can break them down into glucose and release them into the bloodstream when needed to maintain normal glucose levels in the blood.
All blood flows through the liver. The liver acts as a filter to detoxify the blood. It removes many different compounds from the blood such as hormones and alcohol. Perhaps one of the most important functions of the liver is to metabolize drugs into either active or inactive metabolites, depending on the medication.
Blood tests used to evaluate the liver can be divided into those representing liver cell damage, cholestasis or liver function. The serum aminotransaminases, alanine aminotransferase (ALT or SGPT) and aspartate aminotransferase (AST or SGOT) are part of most automated blood chemistry panels. Elevation of these enzymes is caused by damage to the hepatocyte or liver cell. The degree of elevation may be important in acute disease but is unimportant in chronic disease. The most common causes of elevated aminotransaminases are fatty liver, viral hepatitis, medication induced hepatitis, autoimmune hepatitis and alcoholic liver disease. The tests are a reflection of cell damage and death but are not liver function tests. Although many patients and physicians refer to these tests as “liver function tests”, this term is incorrect as they do not reflect the liver’s ability to either synthesize or metabolize various chemicals. Therefore, an abnormality in these tests does not mean that the liver is not functioning. In fact, the vast majority of patients with elevated aminotransaminases, regardless of degree, have normal liver function.
Cholestatic liver diseases are any conditions leading to the obstruction of bile ducts in either the liver or biliary tree. Elevations of the alkaline phosphatase and gamma-glutamyl transpeptidase are indicative of this type of disease. Conditions that commonly lead to the elevation of these enzymes include primary biliary cholangitis, primary sclerosing cholangitis and gallstone disease.
Albumin and blood clotting factors are proteins made in the liver. Blood tests such as the serum albumin and prothrombin time are measures of these proteins. As these tests evaluate the functional integrity of the liver, they can be correctly called “liver function tests.” Abnormalities of these tests are of concern and are indicative of extensive liver damage.
The adequate interpretation of laboratory test results is very important in the evaluation of liver disease. Unfortunately, in many cases, blood tests are unable to predict current disease stage or possible disease progression. Therefore, despite all these advanced tests, the performance of a liver biopsy cannot be emphasized enough as this is the best test to accurately stage the disease and predict disease progression.