Lab Results 101: Liver Function Tests
The first thing to know is that what is commonly called liver function testing (LFTs) has little if anything to do with the function of the liver. They are really tests that are clues to liver injury or disorders, but not really liver function. Still the name is likely here to stay, so liver function testing is what you will hear discussed. In the common test ordered at your doctor’s office, called the Comprehensive Metabolic Panel(CMP), several tests are included that give information about the liver.
The first two tests are the ALT (Alanine aminotransferase) and AST (Aspartate aminotransferase) were formally called SGPT and SGOT respectively. These are enzymes that normally function inside liver cells primarily, and are present in the blood stream in small quantities. When there is something causing injury to liver cells, these enzymes leak into the bloodstream in large quantities, and elevated serum levels of these enzymes what physicians call hepatocellular injury. At the top of the list of things that can lead to elevation of these enzymes is alcohol use. Excessive alcohol intake, especially chronic overuse, leads to hepatocellular damage, and eventually liver tissue scarring called cirrhosis. Elevation of the AST and ALT is often a clue to your physician that alcohol overuse may be a concern. Interestingly a high ratio of AST to ALT is especially suggestive of alcohol toxicity. Other toxic compounds, including acetaminophen (Tylenol) overuse, can cause similar liver cell injury and transaminase enzyme elevation.
Other causes of elevation of the AST and ALT include the various viral hepatitis infections. Currently hepatitis C is the cause we think of most commonly as a concern. It’s at epidemic proportions in the US now, and can be a silent cause of progressive liver damage, liver failure, and liver cancer. Often a physician will order testing for hepatitis C, B and sometimes A when evaluating for a cause of elevation of the liver transaminases. Less common causes of hepatitis include other medications, other ingested toxins, autoimmne hepatitis, and infiltrative diseases like cancer, hemochromatosis, amyloidosis, and fatty liver. Alcoholism, diabetes and obesity can be causes of fatty liver.
Another test result on the CMP is called alkaline phosphatase, or Alk Phos. This is an enzyme found mostly in the liver’s bile ducts. These are the tubes that bile flows through to exit the liver into the gallbladder and small intestine. Anything that causes obstruction to or inflammation of the bile ducts can lead to an elevated Alk Phos level. Examples of things your physician may look for include gall stones or other causes of bile duct obstruction, cancers or other masses that can block the bile ducts, and autoimmune disorders of the bile ducts, like primary sclerosing cholangitis.
Bilirubin is a breakdown product of the hemoglobin in our red blood cells. Red blood cells are constantly breaking down and being replenished. The end product of the metabolism of hemoglobin is bilirubin, and the liver excretes bilirubin in the bile as well as changes it in a process called conjugation into a form that can be excreted by the kidney in the urine. Elevation of bilirubin can be a clue to blockage of the bile ducts, to excessive red blood cell destruction or to failure of the liver to metabolize the bilirubin properly to be excreted. Significant elevation of bilirubin is a very concerning finding most of the time, and needs to be investigated.
Other tests in the CMP that reflect on liver function include the albumin level. The liver produces albumin, the primary intravascular protein, and low albumin levels can reflect malnutrition, chronic illness, or liver disease.
There are lots of additional tests available to investigate liver function, but these are the ones commonly seen on routine office testing. As you may have noted, most reflect problems with liver inflammation, blockage of the bile ducts, or liver cell damage rather than the true function of the liver, so they should be called liver disorder tests. The term is likely here to stay, so LFTs it is.
See a prior post Hepatitis C: A Sleeping Giant for more on this common cause of abnormal LFTs.