Alanine aminotransferase, shortened to ALT on almost every lab report you'll see, is an enzyme released when liver cells are damaged. It's one of the numbers on a standard canine chemistry panel, and it's the one most families end up Googling at 10:47 PM because the printout came home with a little arrow next to it and the clinic closed at six. On Plumb's Veterinary Drug Handbook, the reference most US vets keep on their desk, a mildly elevated ALT isn't a diagnosis. It's a flag.
The short version is this. ALT is a liver-cell enzyme. When the cells are stressed or damaged, some of that enzyme leaks into the bloodstream and the lab picks it up. A high ALT means the liver is reacting to something. It doesn't tell you what that something is. It also doesn't tell you whether the reaction is the start of a real problem or a one-time bump the dog will clear on his own. The rest of the panel, the clinical exam, the drug history, and usually a recheck in two to four weeks are what sort those apart.
What ALT actually measures
Liver cells run a large share of the body's metabolic chemistry, and to do that work they hold high concentrations of certain enzymes inside them. ALT is the most liver-specific of those in dogs. When a liver cell is injured enough for its membrane to leak, or for the cell itself to die, ALT spills into the bloodstream and the lab can measure it. The half-life in circulation is short, which is why a single panel catches the signal but doesn't tell you how long it's been going on. A recheck panel at two to four weeks is how the clinic separates a transient bump from a trend.
The Merck Veterinary Manual, the reference textbook clinicians use, is explicit that ALT elevation alone doesn't point to one cause. Causes run from the genuinely uninteresting (recent exercise, a fatty meal, mild trauma from a long car ride) through common and manageable (a medication side effect, a resolving GI bug) to genuinely serious (chronic hepatitis, toxin exposure, hepatic neoplasia). The number on the page is the same in all of those scenarios. The pattern over time and the rest of the panel are what distinguish them.
How to read the number on your dog's panel
Most labs print ALT alongside its reference interval, usually somewhere in the range of 10 to 125 U/L. The exact band depends on the lab and the analyzer, which is one reason comparing a new ALT to a remembered number from a different clinic is a quick way to worry yourself. The useful comparison is against the reference range printed on the same sheet and, if you have one, against a prior panel from the same lab.
A value one to two times the upper reference limit, on an otherwise healthy dog with an unremarkable exam, is common and usually not acted on beyond a recheck. Three to four times the upper limit starts getting the clinic's attention, especially if ALP (alkaline phosphatase, a second liver enzyme read alongside ALT) is also up. Much higher than that, or combined with clinical signs like jaundice, vomiting, or appetite loss, is usually when a bile acid test or imaging gets scheduled without waiting for a recheck. Those thresholds are clinical rules of thumb, not strict cutoffs. Your DVM is reading the whole picture, not a single bold number.
Why the liver enzyme went up
The honest answer at the first elevated ALT is usually “we don't know yet, and we often don't need to.” The more useful question is what's changed recently for this dog. A new medication is the first thing most clinics check, because a handful of commonly prescribed drugs bump ALT as a known effect. Phenobarbital for seizures does it. Steroids including prednisone do it. Certain NSAIDs can. A recent heartworm preventive dose can produce a transient uptick. The dose your vet set is the dose for your pet, and the bump is usually tolerable, but the recheck is how the clinic confirms it's tolerable rather than worsening.
Non-drug causes run a similarly broad list. A fatty-meal episode, a minor GI upset, acute pancreatitis that hasn't fully declared itself, a tick-borne infection in endemic regions, and genuine hepatic disease are all on the table. One common and genuinely serious cause worth naming: xylitol ingestion. Xylitol is a sugar substitute in many gums, candies, baked goods, and some peanut butters, and dogs get into it from purses and counters with distressing frequency. ASPCA Animal Poison Control, the ASPCA's Animal Poison Control Center, keeps a hotline for suspected exposures. Xylitol-induced liver injury is an emergency, not a recheck conversation.
What the clinic typically does next
For a mild, isolated ALT elevation on a healthy-looking dog, the standard next step is a recheck panel in two to four weeks. If the value has normalized, that's usually the end of the story. If it's stable or rising, most clinics will add a bile acid test (which measures actual liver function rather than just cell leakage), often a urine specific gravity, and sometimes abdominal ultrasound. Cornell's College of Veterinary Medicine has a useful owner-facing overview of the workup sequence that mirrors how most general practices handle it.
For a higher ALT with clinical signs, the sequence compresses. Imaging, a bile acid test, and sometimes a referral to an internal medicine specialist happen in the same week. A referral visit in most of the country runs $300 to $600 and is usually a one-time workup rather than the start of ongoing care. If your DVM recommends referral, it's because the case has hit the edge of what general practice is set up to do, not because the general practitioner is failing you.
What to log at home before the recheck
The two weeks between a flagged ALT and a recheck are the window where a home log earns its keep. The four things DVMs actually read in a home log, in order: appetite, water intake, stool, and any vomiting or new behavioral changes. A missed breakfast is information. Three missed meals in five days is a phone call. A list of every medication and supplement the dog has been on, with exact dose and start date, belongs on the same page. If the clinic ends up pulling a bile acid test or scheduling imaging, those notes shape the appointment more than any Google search will.
If your household is the kind that keeps vet records scattered across a shared email, an envelope in a drawer, and a photo of a prescription bottle on somebody's phone, that's exactly the starting point most families bring to the pet health records conversation. The senior-years piece on senior pet math walks through how a timeline of panels, not a single snapshot, is what actually makes a number like ALT interpretable.
Questions that come up at the recheck
What is a normal ALT range for dogs?
Most labs report a reference interval somewhere between 10 and 125 U/L, and the exact band depends on the lab and the machine it was run on. The Merck Veterinary Manual's chapter on liver disease treats mildly elevated ALT as a signal to recheck rather than a diagnosis. The number on your dog's panel will be printed next to the reference range that specific lab uses, and that's the band to read against, not a remembered number from the last visit at a different clinic.
Is an elevated ALT always bad?
No. ALT rises for many reasons, and some of them are transient and clinically uninteresting. Medications including phenobarbital, prednisone, and certain heartworm preventives commonly bump ALT. Vigorous exercise the day before the draw can do it. Mild trauma, a recent GI upset, and sample handling can all produce a value above the reference range that drops back on recheck. A single mildly elevated ALT on an otherwise healthy dog is rarely the start of a bad story.
What does it mean when ALT and ALP are both elevated?
ALT rises when liver cells are damaged. ALP (alkaline phosphatase) rises for a broader list of reasons including cholestasis, steroid exposure, and bone turnover in young dogs. When both are up, most DVMs read the pattern rather than either number alone. The ratio, the trend across prior panels, and the rest of the chemistry tend to matter more than the isolated values. A bile acid test is often the next step the clinic will propose.
How high is dangerously high for ALT?
There isn't a clean threshold. A dog with ALT of 500 U/L and no other abnormalities is a different clinical picture than a dog with ALT of 250 U/L, jaundice, and a dropping albumin. As a rough clinical reference, Plumb's and most internal-medicine textbooks treat values more than four or five times the upper reference limit as worth investigating promptly, especially when paired with clinical signs. Your DVM is reading the whole panel, not a number in isolation.
Can diet or supplements raise ALT?
Some can. Xylitol exposure causes acute hepatic injury and dramatic ALT elevation, and it's in many sugar-free products that dogs find in purses and counters. Certain herbal supplements marketed for joints or behavior have been linked to idiosyncratic liver reactions in case reports collected by FDA CVM (the FDA's Center for Veterinary Medicine). Chronic low-grade elevation on a dog eating a normal commercial diet is usually about something other than food, but the diet and supplement list is always fair game to review at the recheck.
Should I ask for a bile acid test?
The bile acid test measures how well the liver is actually doing its job, not just whether its cells are leaking enzymes into the blood. It's usually the follow-up after an unexplained or persistently elevated ALT. Not every mildly high ALT needs one. Your DVM will weigh the rest of the panel, the clinical picture, and the trend over time. If ALT has been elevated on two panels several weeks apart, asking about the bile acid workup is a reasonable question.
A number is not a verdict
An ALT above the reference range is a flag. A flag is information. On its own it's neither a diagnosis nor a reason for the kind of 10:47 PM spiral that starts with an arrow on a printout and ends two hours later with a forum thread from 2011. The panel gets read as a whole. The trend across panels gets read as a whole. The recheck is how the clinic turns a flag into either “nothing to worry about” or a next step. Call the clinic and ask what they want to see at the recheck, and bring the home log.
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Rachel Howland, CVT (ret.), spent a decade in clinic: seven years in a mixed practice in upstate New York, then three on the internal-medicine floor at Angell Animal Medical Center in Boston. She left practice in 2017 and has written about small-animal health since. She does not diagnose or prescribe; she explains what your vet's records are telling you and what questions are fair to ask.