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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These posts contain high yield information collected from various educational resources including textbooks, journal articles, educational websites and more. They are intended for educational use only and should NOT be taken as medical advice. I strongly believe the spreading of knowledge and depth of learned information should be encouraged in today's society rather than coveted. Membership is required to view these posts  and should be used solely for educational purposes only.