An ALT of 45 U/L usually gets one of two responses. Either the lab does not flag it, because many upper limits sit at 40 to 55, or it gets flagged and waved off as mildly elevated, probably nothing, we will recheck sometime.
Both responses are common and both miss the same thing. In most people who are otherwise well, an ALT in the forties means fat in the liver, that fat is driven by insulin resistance, and it is the most common liver disease in the world sitting quietly on an otherwise unremarkable panel.
The upper limit is too generous
The reference range for ALT was derived from populations that included a great many people with undiagnosed fatty liver, which pulled the ceiling upward.
Work on healthier reference populations has suggested substantially lower thresholds, in the region of 30 for men and lower still for women. Under that standard, a 45 is not borderline. It is elevated, and in a woman it is meaningfully elevated.
This is the same structural problem as the TSH range and the ferritin floor. The range describes the population that got tested. It does not describe health.
What it usually is
Metabolic dysfunction-associated steatotic liver disease, formerly called non-alcoholic fatty liver disease, is the common answer. Fat accumulates in hepatocytes, the cells become irritated, and ALT leaks into the blood.
The most common contributor is insulin resistance. Which means an ALT of 45 rarely arrives alone. Look at the rest of the panel and you will often find triglycerides trending up, HDL trending down, a fasting insulin nobody ordered, an ApoB above target, uric acid high-normal, and blood pressure creeping. That is one process with several readouts, not six unrelated findings.
It matters because a meaningful fraction of people with fat in the liver go on to inflammation and then to fibrosis, and fibrosis is the part that determines what happens to you. The good news is that the early stages are reversible in a way most chronic disease is not.
The free calculation nobody runs
Here is the most actionable thing on this page.
FIB-4 is a fibrosis risk score calculated from four things you almost certainly already have: your age, your AST, your ALT, and your platelet count. It costs nothing, requires no new test, and stratifies you into low, indeterminate, or high risk of advanced fibrosis.
A low FIB-4 in someone with a mildly elevated ALT is genuinely reassuring and means the work is lifestyle and re-testing. An indeterminate or high result is the trigger for actual imaging, usually elastography, which measures liver stiffness directly.
I bring this up because it is the clearest example I know of information sitting unused. The numbers are already on the report. Nobody calculates it. And it is the calculation that decides whether an ALT of 45 is something to work on over a year or something to investigate this month.
What has to be ruled out first
Fatty liver is the most likely explanation. It is not the only one, and the others are specifically worth excluding because several are treatable and one is inherited.
Hepatitis B and C serology. Iron studies with ferritin and transferrin saturation, because hemochromatosis is genetic, is more common than people expect, and presents exactly this quietly. An honest alcohol history, which is a conversation rather than a test. A full review of medications and supplements, and supplements are not a throwaway here, because several popular ones are hepatotoxic. Celiac serology. Thyroid function. Autoimmune markers if the picture warrants.
| The question a 45 raises | What answers it | Can a chatbot do it? |
|---|---|---|
| Is this even abnormal? | Comparing to a healthy-population limit, not the lab's | Yes, and it usually will |
| What is my fibrosis risk? | FIB-4 from age, AST, ALT, platelets | Yes, if you give it all four |
| Is it fat, and how much? | Ultrasound or elastography | No |
| Is it hepatitis? | Hepatitis B and C serology | No |
| Is it iron overload? | Ferritin and transferrin saturation | No |
| Is it alcohol? | An honest history | No, not honestly |
| Is a supplement doing it? | Review of everything you take | Partly |
| Is it improving? | Re-testing at three to six months | No |
Two rows say yes, and they are the two that matter most for triage. A model will calculate your FIB-4 correctly and will tell you the healthy-population limits are lower than the lab's. That is real value and I am not going to pretend it is not.
What it cannot do is order the elastography, take an alcohol history that is worth anything, examine you, or notice in eight months that the number has not moved.
What actually works
This is the encouraging part, and it is worth stating plainly because people arrive expecting to be told there is no treatment.
Weight loss is the intervention with the best evidence, and the thresholds are specific rather than vague: modest loss improves the fat, more improves the inflammation, and more still can improve fibrosis. Resistance training helps independently of weight. Fructose and alcohol are the two inputs that punish the liver most directly. And treating the insulin resistance upstream treats the liver downstream, which is the whole argument for finding the driver rather than chasing the readout.
Then re-test at three to six months, because an ALT that is falling tells you the theory was right, and an ALT that is not tells you it was not.
The point
An ALT of 45 is the most ignorable abnormal result in common practice. It is barely outside the range, it produces no symptoms, and it is the earliest visible sign of a process that is reversible now and much less so later.
The whole response fits in a paragraph. Calculate the FIB-4 from numbers you already have. Rule out hepatitis and iron overload. Be honest about alcohol. Find the insulin resistance underneath it. Re-test in six months to see whether you were right.
None of that is exotic. It just requires somebody to look at a 45 and not shrug.
If you have liver enzymes in the forties and nobody has calculated your FIB-4, that is what the Precision Health Assessment is for. The metabolic driver is covered at Cardiometabolic and in the fasting insulin piece, and if you want to talk it through first, apply to work with me.
