ASAT and ALAT are two transaminases systematically measured in a liver function test, but their interpretation is not limited to checking if the value exceeds the laboratory’s upper limit. The tissue localization of each enzyme, the patient’s metabolic context, and the thresholds used radically change the reading of the results.
Laboratory thresholds and clinical thresholds: an underestimated gap for ALAT
Most analysis laboratories set the upper limit of ALAT between 35 and 55 U/L depending on the technique used. These statistical thresholds, calculated on heterogeneous reference populations, do not necessarily reflect a healthy liver.
Since 2023, the Nara Declaration from the Japan Society of Hepatology recommends considering that an ALAT greater than 30 U/L already justifies further investigation, especially in patients with a metabolic background (diabetes, obesity, metabolic syndrome). This threshold, well below the norms displayed by most French laboratories, aims to detect early signs of developing metabolic liver steatosis.
In practice, general practitioners reassure patients whose ALAT is at 40 or 45 U/L because the result appears within the “normal” range of the report. This is precisely the trap: a result within the laboratory’s norms does not exclude liver damage if the clinical context demands it.
Exploring the differences between ASAT and ALAT in a blood test helps to better understand why these two enzymes do not react the same way depending on the affected organ.
Tissue specificity of ALAT and ASAT: why the ASAT/ALAT ratio guides diagnosis
ALAT is predominantly cytosolic and concentrated in hepatocytes. Its isolated elevation almost systematically points to hepatocellular damage: steatosis, acute viral hepatitis, direct drug toxicity.
ASAT, on the other hand, is distributed in the liver but also in the myocardium, skeletal muscles, kidneys, and red blood cells. Part of the hepatic ASAT is mitochondrial, which has a direct implication: when cell necrosis is profound (mitochondrial damage), ASAT rises more than ALAT.
The ASAT/ALAT ratio as a sorting tool
The ASAT/ALAT ratio (also known as the De Ritis ratio) provides a quick etiological orientation:
- Ratio less than 1: ALAT predominates, suggesting metabolic steatosis, chronic viral hepatitis, or early hepatic toxicity. This is the most common profile in outpatient medicine.
- Ratio greater than 1: ASAT predominates, indicating alcoholic hepatitis, established cirrhosis, or muscle damage (rhabdomyolysis, intense exercise, myopathy).
- Ratio greater than 2 with elevated gamma-GT: a very suggestive combination of chronic alcohol consumption with advanced liver damage.
This ratio loses its value in cases of massive elevation of both enzymes (greater than ten times normal), where acute severe hepatitis, hepatic ischemia, or acetaminophen toxicity is suspected.
Normal transaminases and metabolic steatosis: the clinical false negative
One of the most documented traps in recent years concerns metabolic liver steatosis (formerly NAFLD, now MASLD). Transaminases within the laboratory’s norms do not exclude significant liver fibrosis.
Specialized recommendations emphasize that a diabetic or obese patient with apparently normal ALAT and ASAT should still undergo a fibrosis evaluation. Two calculated scores are particularly used as first-line assessments:
- The FIB-4 score, which combines age, ASAT, ALAT, and platelet count to estimate the risk of advanced fibrosis.
- The NAFLD Fibrosis Score, which additionally incorporates body mass index, blood glucose, and albumin.
- In cases of intermediate or high scores, hepatic elastography (FibroScan) is recommended to confirm or rule out fibrosis.
We recommend never using “normal” transaminases as the sole argument to reassure a patient at metabolic risk. Isolated transaminase measurement was not designed to screen for fibrosis, and its exclusion power in this context is low.
Elevated ASAT without liver damage: extra-hepatic causes not to be missed
An isolated elevation of ASAT with normal or slightly increased ALAT should prompt consideration of non-hepatic causes before any extensive liver evaluation.
The most common cause in everyday practice is intense physical exercise. A weightlifting session or prolonged endurance effort can elevate ASAT for 24 to 72 hours. Measuring CPK (creatine phosphokinase) helps clarify: if CPK levels are proportionally high, the muscle origin is almost certain.
Hemolysis is another classic trap. Since ASAT is present in red blood cells, any hemolysis (in vivo or even in vitro, related to a difficult blood draw) skews the result upwards. Checking haptoglobin and free bilirubin helps rule out this hypothesis.
Myocardial damage (heart attack, myocarditis) also elevates ASAT, although troponin has largely supplanted ASAT as a cardiac marker. In cases of unexplained elevated ASAT in a patient without hepatic risk factors, measuring troponin remains relevant.
Medications and isolated ASAT
Some statins, certain antiepileptics, and macrolides can cause a moderate elevation of ASAT due to subclinical muscle toxicity, without liver damage. Again, measuring CPK is discriminating.
Reading a blood test including ASAT and ALAT is not limited to comparing two numbers against reference limits. The tissue distribution of each enzyme, the De Ritis ratio, the patient’s metabolic context, and fibrosis scores transform two simple measurements into a true diagnostic sorting tool, provided they are interpreted together and not in isolation.
