Chapter 9 – “Liver Function Tests”
Welcome to the ninth installment of “Inconceivable: medical terms that don’t mean what you think they mean.”
You are evaluating a patient presenting to the ED with right upper quadrant pain. Her laboratory testing reveals an elevated AST and ALT and her focused ultrasound shows shadowing gallstones and a slightly dilated common bile duct. You call the admitting team and endorse that the patient has “elevated liver function tests.” Unfortunately, you never even checked how well the liver is functioning. Don’t fret, this mistake is made so frequently in medicine that even most EMRs have it wrong!
There are many laboratory values that help a clinician assess the hepatobiliary system. Many hospitals and labs use this imprecise term “Liver Function Tests” in their ordering system to obtain a panel that typically includes AST, ALT, bilirubin, and alkaline phosphatase, among other things. Despite the colloquialism of “LFTs,” none of these tests are actually measuring how well the liver is functioning. Let’s take a look at what each of these four values actually are:
- Aspartate aminotransferase (AST) and Alanine aminotransferase (ALT) – These are enzymes contained within the hepatocytes and their levels in the serum are a measure of the integrity of the hepatocytes. Elevated AST and ALT indicate a hepatocellular injury.
- Bilirubin – Bilirubin is the chemical end product of heme degradation. Its level in the serum is a measure of the biliary excretory function – not the liver function.
- Alkaline phosphatase and Gamma-glutamyl transpeptidase (GGT) – These are both enzymes in the plasma membrane and are elevated in the presence of damage to the bile canaliculus.
Knowing what these tests actually mean is important, not only so you sound smart when talking to a consultant, but also so you can correctly diagnose and treat your patient. Our patient has an elevation of her AST and ALT, which could accurately be described as an elevation in her “liver enzymes” or a “transaminitis.”She may have an obstructing common bile duct stone, and a resulting backup of bile that has damaged the hepatocytes. She probably also has a “hyperbilirubinemia” because the stone has impaired bile excretion, and she probably has an elevation in her alkaline phosphatase from said backup causing an injury to the bile duct and canaliculi.
Now, if you do want to check how well our patient’s liver is functioning, instead take a look at her PT/INR, PTT, and serum albumin. These tests measure the synthetic function of the liver. Additionally, if you want to see how well the metabolic function of the liver is performing, check the ammonia level.
AST and ALT are enzyme levels that measure damage to the hepatocytes and are NOT a measure of liver function. They are more accurately referred to as “liver enzymes” or “transaminases.” True liver function can be measured with PT/INR, PTT, albumin, and/or ammonia levels.
Kumar, V., Abbas, A. K., Fausto, N., Robbins, S. L., & Cotran, R. S. (2005). Robbins and Cotran pathologic basis of disease. Philadelphia: Elsevier Saunders.
Dr. Krueger is currently an emergency medicine attending physician at Advocate Lutheran General Hospital and a former UIC EM Chief Resident.