An ALT of 100 U/L in a 12-year-old cat is about twice the top of most feline reference intervals (10–50 U/L) and is a reason to recheck in 14 days, not a reason to book an ultrasound the same week. Mild single-enzyme rises are common in old cats and often transient; a rise that doubles, or one with a raised GGT, bilirubin or bile acid, earns imaging and sometimes a biopsy.
What ALT 100 U/L actually tells you
ALT leaks from hepatocytes when they are damaged or turning over faster, so it reports liver cell injury, not liver function. A result of 100 on a 10–50 U/L interval is 2 times the upper limit; 150 is 3 times; 400 is 8 times. The size of the number correlates loosely with the number of dying cells and hardly at all with how sick the cat is.
Three things move ALT with no liver disease: a seizure or 20 minutes of struggling during the draw, a 12-hour fast that mobilises fat, and muscle injury, since cat muscle carries some ALT. Cats over 10 also run higher than 3-year-olds: 60 U/L in a 14-year-old is often that cat's new normal.
What changes management is the shape of the curve. A single 100 that falls to 60 in 14 days needs nothing further. A 100 that becomes 220 at 14 days, or 100 with a bilirubin of 1.5 mg/dL, is a different cat and a different workup. Typical intervals for the other markers are on the reference range page.
The 2-week recheck, done properly
Recheck at 14 days, not 3 and not 60. Three days is too soon for a transient cause to settle; 8 weeks hides the trend. Use the same laboratory: two labs can differ by 15–25% on the same sample, which is the size of the change you are trying to detect.
Hold the conditions steady. Fast 8–12 hours, water free; take the sample before any dental work or vaccination; record any drug given in the previous 30 days, because corticosteroids, phenobarbital and some antibiotics raise ALT 2–5 times. Ask for ALT, ALP, GGT, bilirubin, albumin, glucose, bile acids and a full CBC on the same tube draw, so one visit answers the whole question.
If the second ALT is under 100 and the cat is eating, stop testing and recheck in 6 months. If it is over 100 at 14 days, add bile acids and abdominal ultrasound before repeating bloods again. Full kinetics, including the 24–48 hour half-life of the enzyme, are in the kinetics explainer.
ALP: the enzyme that is not liver-specific
Alkaline phosphatase in cats has a short half-life of about 6 hours and normally sits at 10–40 U/L, lower than in dogs. A raised ALP in an old cat is as likely to come from bone, thyroid or gut as from the biliary tree, and 3 common confounders explain most of it.
Hyperthyroidism raises ALP in roughly a third of cases, along with ALT, because thyroid hormone speeds bone turnover; a T4 of 4.5 µg/dL with ALP at 90 is thyroid, not cholestasis. Osteoarthritis raises it through bone remodelling: 2 swollen stifles and ALP 70 need a pain plan, not a liver workup. Steroids, including depot injections given for itching 6 weeks earlier, push ALP up 2–4 times and can hold it there for 8 weeks.
The pattern that does point at the liver: ALP above 150 with GGT above 5, or ALP rising over 2 consecutive samples 4 weeks apart. Isolated ALP of 60–90 with normal GGT, bilirubin and bile acids is usually rechecked in 4–6 weeks, not imaged.
GGT: the bile duct marker
GGT is the enzyme that localises disease to the biliary tree. Normal in cats is 0–5 U/L, and a value above 5 with a raised ALP means cholestasis or cholangitis until proven otherwise. A cat with ALP 200, GGT 12 and bilirubin 0.8 mg/dL has a biliary problem; a cat with ALP 200, GGT 2 and a T4 of 6.0 has a thyroid problem.
Ratios help when the numbers are borderline. An ALT to ALP ratio above 5 points to hepatocellular injury; below 2 with a raised GGT points to the ducts. Neither replaces ultrasound above a bilirubin of 1.0 mg/dL, but both tell the sonographer where to look: ducts or parenchyma.
Enzyme, threshold, action
| Test | Typical range | Action threshold | Recheck |
|---|---|---|---|
| ALT | 10–50 U/L | over 100 U/L | 14 days |
| ALP | 10–40 U/L | over 150 U/L | 28 days |
| GGT | 0–5 U/L | over 5 U/L | 14–28 days |
| Bilirubin | under 0.3 mg/dL | over 1.0 mg/dL | 48–72 hours |
| Bile acids, pre | under 5 µmol/L | over 20 µmol/L | with next panel |
| Albumin | 2.2–4.0 g/dL | under 2.0 g/dL | 7 days |
Read the table as a ranking, not a diagnosis. Bilirubin over 1.0 and albumin under 2.0 are the 2 that change the week's plan, because they speak to function and to how much liver is working, not to how many cells died. A cat with ALT 400 and normal bilirubin and albumin can wait 14 days; a cat with ALT 90, bilirubin 1.8 and albumin 1.9 cannot.
When a biopsy is actually indicated
Biopsy answers a question blood cannot: is this inflammation, lipidosis, lymphoma or cholangitis. The indications that most internists accept are ALT above 300 U/L on 3 samples over 8–12 weeks, ALP above 300 with a GGT above 10, bile acids above 30 µmol/L, a liver that is nodular or enlarged on ultrasound, or any of the above with weight loss over 10% of body weight.
Before any sampling, check clotting: platelets above 80,000/µL and a normal PT, plus a vitamin K trial of 3 doses if the cat is jaundiced. A fine-needle aspirate under ultrasound guidance costs 150–400 and answers inflammation versus neoplasia about 70–80% of the time; a core biopsy under sedation costs 400–900 and gives architecture. A small, irregular, hypoechoic liver with low albumin is a poor surgical candidate; bleeding risk is 5–10 times higher.
One more number to hold onto: 3 days. A cat that stops eating for 3 days is on the road to hepatic lipidosis, and the enzyme panel is the least of it. If a cat with a liver value has eaten under 50% of its calories for 72 hours, start assisted feeding before the next blood test; recovery timelines for that condition are in the lipidosis recovery guide.
Five steps after an abnormal panel
- Write down the lab's own upper limit for ALT; 100 means different things at 45 and at 75.
- Book the recheck at 14 days, same lab, 8–12 hours fasted, and list every drug from the past 30 days.
- Ask for ALP, GGT, bilirubin, albumin and bile acids on the same draw, not one marker at a time.
- Check T4 in any cat over 10 with a raised ALP, because thyroid disease explains a third of them.
- Weigh the cat weekly; a 5% loss in 2 weeks moves biopsy up the list faster than any single enzyme value.
Terms mentioned in this article
Frequently asked
Is ALT 100 in an old cat an emergency?
No. It is about 2 times the upper limit and calls for a recheck at 14 days on the same laboratory, fasted 8-12 hours. The exception is a cat that is not eating, jaundiced, or has bilirubin over 1.0 mg/dL or albumin under 2.0, which needs imaging within days.
Why does a hyperthyroid cat have high liver enzymes?
Thyroid hormone speeds bone turnover and liver cell turnover, so ALP and ALT rise together. About a third of hyperthyroid cats show a raised ALP, and values often fall within 4-8 weeks of treatment. Check T4 in any cat over 10 with a raised ALP before ordering liver imaging.
When does a liver value justify a biopsy?
ALT above 300 U/L on 3 samples over 8-12 weeks, ALP above 300 with GGT above 10, bile acids above 30 umol/L, a nodular liver on ultrasound, or any of those plus more than 10% weight loss. Clotting is checked first, since bleeding risk rises 5-10 times in a cat with poor liver function.