ALT healthy upper limit

The ACG's 2017 guideline draws this line lower than most laboratories do, and says why. Its first summary statement: "A true healthy normal ALT level in prospectively-studied populations without identifiable risk factors for liver disease ranges from 29-33 IU/L for males and 19-25 IU/L for females, and levels above this should be assessed by physicians." The studies behind those numbers excluded people with viral hepatitis, alcohol risk and the metabolic risk factors for fatty liver, and found 30 and 19 (blood donors), 33 and 25 (liver donors with normal biopsies), and 29 and 22 (US survey data). Ordinary lab ranges are built from populations that were never screened that way; the ACG cites one state in which 67 laboratories set the upper limit anywhere from 31 to 72 U/L. If your report prints 40 or 45 as the top of normal, that is the lab's population, not the healthy one.

What a healthy ALT does not rule out. ACG summary statement 4: "A normal ALT level may not exclude significant liver disease." The guideline gives the evidence: 9 percent of hepatitis C patients with normal or near-normal ALT had bridging fibrosis and 11 percent had cirrhosis; 40 percent of a hemochromatosis cohort had normal enzymes; and in one imaging study 79 percent of people with excess liver fat had an ALT under 40 in men and 31 in women. The AASLD's 2023 guidance says the same for fatty liver: aminotransferases "are frequently normal in patients with advanced liver disease due to NASH and should not be used in isolation to exclude the presence of NASH with clinically significant fibrosis." That is why both the AASLD and the AGA send people with type 2 diabetes, or two or more metabolic risk factors, through a FIB-4 fibrosis score regardless of their ALT.

What to do with a healthy number. Nothing liver-specific, unless you carry metabolic risk. The AGA's pathway asks that all patients with type 2 diabetes be screened for fibrosis, and that people with two or more of central obesity, triglycerides of 150 or higher, low HDL, a systolic pressure of 130 or higher or a diastolic of 85 or higher, or a fasting glucose of 100 to 125 be screened too. FIB-4 uses age, AST, ALT and platelets; under 1.3 (under 2.0 past age 65) it rules out advanced fibrosis with a negative predictive value of 90 percent or better, and the AGA suggests repeating it every 2 to 3 years at low risk, the AASLD every 1 to 2 years with prediabetes, diabetes or two or more risk factors. The AASLD adds two limits: FIB-4 "has low accuracy in those under age 35," and it should not be used during acute illness.

One number, two conventions. The AASLD prints the ACG's sex-specific pair and also a single rule of thumb, "As a general rule, ALT > 30 U/L should be considered abnormal." For a man the two agree within three points; for a woman the sex-specific limit is five points lower. This page keeps the ACG pair because it is the one built from sex-specific data. And one cohort finding worth knowing at this level: among US veterans with fatty liver on imaging, those whose ALT stayed normal had a cirrhosis rate of 1.22 per 1,000 person-years, no different from people without fatty liver, against 3.85 in those with an elevated ALT. That study defined normal as up to 40 in men and 31 in women, above the ACG limit, and its population was over 90 percent men, so it describes the reassurance of a persistently normal ALT, not a license to skip the risk score.