Liver function tests
Background
A liver function test is a group of blood tests that, when evaluated as a whole, allow the clinician to determine if there is any damage to the liver, determine if that damage is acute or chronic, and determine if there is hepatobiliary stasis for any reason. The essential components of an LFT includes liver transaminases (AST, ALT), which are surrogates markers of liver health, coagulation studies (PT, PTT, INR), albumin, total protein, lactate dehydrogenase, and liver enzymes GGT and Alkaline Phosphatase which, in conjunction with direct and total bilirubin, are surrogate markers of biliary patency and function.
Aminotransferases
Alanine aminotransferase (ALT)
- the function of ALT is to catalyze the transfer of an amino group from alanine to alpha-ketoglutarate, which reversibly produces pyruvate and L-glutamate. As pyruvate is a glycolysis intermediate, and glutamate is a Krebs cycle intermediate, ALT is an essential enzyme in controlling catabolism/anabolism through the production or break down of amino and keto-acids into and/or from essential intermediates of cellular respiration. As will be discussed below, it's elevation, in conjunction with an elevation in AST, may suggest different types of hepatocellular injury.
Aspartate aminotransferase (AST)
- the function of AST is to catalyze the transfer of an amino group from aspartate to alpha-ketogluterate, which reversibly produces oxaloacetate and glutamate.
- Oxaloacetate is yet another Kreb's cycle intermediate, thus AST is also an important enzyme in catabolism/anabolism similar to ALT.
Alkaline Phosphatase
This enzyme is responsible for removing phosphate groups off of proteins and nucleotydes throughout the body. It is predominantly found in the liver, bile ducts, kidney, and bones. Because of its abundant production in biliary duct tissue, it's elevation reflects a problem with stasis or obstruction to bile, however false positivity may occur in patients with increased bone turnover (puberty, osteoporosis, hyperparathyroidism, Paget's disease, etc.). Therefore, alkaline phosphatase should always be interpreted in conjunction with other LFTs.
Gamma-glutamyl Transferase
This enzyme reversibly transfers gamma-glutamyl groups from the antioxidant gluathione to create glutamate. Like alkaline phosphatase, it is also found in abundance in the liver and bile ducts (among other organs, including the spleen, heart, and brain). Therefore it's elevation, in conjunction with an elevation in alkaline phosphatase suggests biliary stasis/obstruction.
Lactate Dehydrogenase
This enzyme reversibly catalyzes the conversion of lactate to pyruvic acid. Although it is found in cells throughout the body, it is commonly obtained as part of LFTs. An elevation in LDH can occur as a result of cellular rupture, the reasons for which are described below:
- Cardiac - Demand ischemia, myocarditis
- Pulmonary - pulmonary embolism
- GI - hepatitis, small or large bowel obstruction
- Hematologic - hemolytic process, ineffective erethropoiesis
- Other - pregnancy, cancer, infection, systemic autoimmune disorders
Normal Values
- ALT: 7 - 55 U/L
- AST: 8 - 48 U/L
- Alk Phos: 45 - 115 U/L
- Albumin: 3.5 to 5.0 g/dL
- Total protein: 6.3 - 7.9 g/dL
- Bilirubin: 0.1 - 1.2 mg/dL
- GGT: 9 - 48 U/L
- LD: 122 - 222 U/L
- PT: 11 - 14 seconds
- PTT: 25 - 35 seconds
- INR: 0.8 - 1.2