The Hepatic Function Panel (HFP), often referred to as a Liver Function Test (LFT), is a common group of blood tests designed to evaluate the health and performance of the liver. This panel measures the levels of various enzymes, proteins, and waste products in the bloodstream, providing information about liver cell integrity, bile flow, and the organ’s ability to produce essential substances. Analyzing these components helps medical professionals diagnose specific liver conditions, monitor the progression of known liver diseases, and screen for potential liver damage caused by medications or other factors.
Enzymes Signaling Acute Liver Injury (ALT and AST)
One primary function of the hepatic panel is to identify damage to liver cells, or hepatocytes, by measuring the levels of Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST). These are intracellular enzymes that primarily function within the liver cells to assist with amino acid metabolism. When hepatocytes are injured or die due to inflammation, infection, or toxins, the cell membranes rupture, releasing these enzymes into the general circulation, signaling acute liver cell damage.
ALT is considered a more specific indicator of liver injury because it is predominantly concentrated in the liver. While AST is also abundant in the liver, it is found in other tissues, including the heart, skeletal muscles, and kidneys. An elevation in AST without a corresponding rise in ALT may suggest that the damage originated from a non-liver source, such as a muscle injury. The ratio between AST and ALT levels is a significant diagnostic clue, as a ratio where AST is significantly higher than ALT, particularly greater than 2:1, can be highly suggestive of alcohol-related liver disease or cirrhosis.
Markers of Bile Duct Function (ALP and GGT)
The hepatic panel also includes enzymes that help assess the function of the bile ducts, the small channels that transport bile from the liver to the gallbladder and small intestine. Alkaline Phosphatase (ALP) and Gamma-Glutamyl Transferase (GGT) are enzymes situated in the lining of these bile ducts. When bile flow is obstructed, a condition known as cholestasis, the pressure causes the cells lining the ducts to release higher amounts of these enzymes into the blood.
An elevated ALP level suggests a problem with the bile ducts but can also be caused by conditions affecting the bones, kidneys, or placenta during pregnancy. To confirm that the elevated ALP is specifically related to the liver or bile ducts, GGT is measured concurrently. An elevation in both ALP and GGT strongly points toward a cholestatic or bile duct disorder, such as gallstones or inflammation. If ALP is high but GGT is normal, the source of the elevation is more likely to be non-hepatic, such as a bone disorder.
Measuring the Liver’s Synthetic Capacity (Albumin and Total Protein)
The liver’s ability to manufacture necessary substances is measured by assessing the levels of Albumin and Total Protein. Albumin is the most abundant protein in plasma, and the liver is its exclusive production site, synthesizing about 10 to 18 grams daily in a healthy adult. This protein regulates the osmotic pressure of the blood, preventing fluid from leaking out of blood vessels, and serves as a transporter for hormones and drugs.
Because albumin has a relatively long half-life of about 20 days, its levels do not drop immediately following acute liver injury. Low albumin levels, or hypoalbuminemia, reflect a failure of the liver’s production machinery over a prolonged period, often indicating chronic liver disease or malnutrition. Total Protein measures the combined amount of all proteins in the blood, including albumin and globulins, which are largely related to the immune system.
Assessing Waste Processing (Bilirubin)
The final component of the hepatic function panel is Bilirubin, a yellowish pigment that is a byproduct of the normal breakdown of old red blood cells. The liver processes this waste product so it can be excreted from the body. High levels of bilirubin in the blood, a condition called hyperbilirubinemia, lead to jaundice, which is the yellowing of the skin and eyes.
The panel differentiates between two forms of bilirubin: unconjugated (indirect) and conjugated (direct). Unconjugated bilirubin is the initial form released into the blood; it is not water-soluble and must be bound to albumin for transport to the liver. Once in the liver, it is chemically modified, or conjugated, to become water-soluble (direct bilirubin) so it can be excreted into the bile and eliminated in the stool. High unconjugated bilirubin suggests a problem before the liver, such as excessive red blood cell breakdown, while high conjugated bilirubin indicates an issue with the liver’s ability to excrete it or a blockage in the bile ducts.