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Liver Function Tests: AST/ALT

As I mentioned in my previous post on " Liver Function Tests: An Overview", here I will discuss how the results of ALT and AST testing can be used to evaluate liver (dys)function. Some details may not be mentioned; more detailed posts will be coming soon.

Aspartate aminotransferase (AST)

& Alanine aminotransferase (ALT) Testing

A few basics

  • Simply speaking, AST and ALT are enzymes that function to transfer an amino group.

  • While not entirely specific for the liver, it is very commonly performed as "THE LFT"

  • ALT is located entirely in the cytoplasm of hepatocytes

  • 80% of AST is in the hepatocyte mitochondria; remainder is cytoplasmic

  • ALT can show diurnal variation (highest in afternoon); &greatest day to day variation (up to 30%)


  • AST: ALT ratio is called the DeRitis ratio.

  • Healthy individuals have an AST:ALT ratio = <1 (the ALT is higher than AST due to its cytoplasmic location)

  • If the AST or ALT are elevated >3x the upper limit of normal, it is nearly always a sign of hepatic origin (the exception being rhabdomyolysis).

  • ALT elevations = more specific to Liver (or kidney) injury

  • AST elevations = AlcoholicS; S(c)irroSiS (remember that alochol induces the release of mitochondrial AST into the blood)

Causes of Falsely Elevated AST/ALTs

AST and ALT elevations are NOT entirely liver specific! They can be elevated even up to 3x the upper limit of normal in NON-liver diseases. A few of the most common situations to be aware of that cause elevated AST or ALTs:

  • Strenuous exercise

  • Hemolysis

  • Heparin therapy

  • Adult males tend to have higher levels than adult females

  • African Americans usually are higher than non-AA

What to do with an abnormal result <3X ULN in an asymptomatic patient?

  • Repeat the test- If normal= good. But keep in mind that Hepatitis C can cause fluctuations! If still elevated, need further workup.

  • Review the clinical history & detailed physical exam- look for other clues!

  • Consider performing additional testing (below)

Useful Add-on Tests

  • Alkaline phosphatase & bilirubin levels can be useful (esp in jaundiced patients)

  • Serum creatinine kinase (CK) can rule out muscle etiology

  • Hepatitis serology testing (Anti-HCV, HBsAg, HBcAb, anti-HBs, anti-HAV total and IgM)

  • Serum protein electrophoresis

  • Autoimmune serologies (Anti-mitochondrial Ab, Anti-smooth muscle Ab, Anti-nuclear Ab)

  • Iron studies (Hemochromatosis, Transfusion history)

  • Ceruloplasmin (especially in young patients <40yo), Urine copper


Be sure to check out the separate posts on each test to see how it can be used to evaluate other diseases & explore any pitfalls to the diagnosis/testing process!

As always, be aware of pre-analytic, analytic, and post-analytic errors when interpreting any test result.

Disclaimer: this is NOT medical advice... consult your physician, medical literature etc before making any assumptions from this article

Reference: Daniel D. Mais, MD. "Practical Clinical Pathology". pg 1. Amer Soc for Clin Path. 2014.


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