Liver Enzymes in Cats: ALT, ALP and GGT Half-Lives

A cat with an ALT of 180 U/L is not automatically sicker than one at 90 U/L. What separates them is time. ALT leaves the blood in 3 to 5 days and ALP in 1 to 3 days, so the same rise means different things at different clocks. One rule covers most feline cases: an ALT above 100 U/L earns a recheck at 2 weeks, not an immediate biopsy.

Every cutoff below belongs to the laboratory that printed it. A common feline panel reads ALT 10 to 50 U/L, AST 10 to 40 U/L, ALP 10 to 40 U/L and GGT 0 to 5 U/L, but methods shift those limits by 20 to 50 percent between laboratories. Compare a cat only against its own lab range.

Half-life decides what the number means

An enzyme has no clock of its own: clearance sets it. With a 3 to 5 day half-life, ALT drops 50 percent every 3 to 5 days once the injury stops. One half-life leaves 50 percent, 2 leave 25, 3 leave about 12, and 4 leave 6. A real hepatocellular hit therefore needs 15 to 25 days to vanish from the page.

ALP turns over faster in cats, at 1 to 3 days, so its signal fades in 5 to 15 days. In dogs ALP is slower, near 3 days, which is one reason a dog can carry an abnormal ALP for a month after a biliary episode. A sample drawn 2 weeks after the first falls at 3 to 4 ALT half-lives, where 87 to 94 percent of the original rise should be gone.

That arithmetic turns one test into a curve. Falling values mean a closed wound; flat values mean the liver is still being hit, and the next step is bile acids, an ultrasound and a drug history.

ALT leaks; it does not measure function

ALT sits in the hepatocyte cytosol, so any membrane damage leaks it: hypoxia, toxins, trauma, inflammation, seizures. A cat that spent 12 hours hypotensive under anesthesia can post an ALT of 200 to 400 U/L with no liver disease at all. That is why AST and CK belong on the same panel: ALT up with CK up usually means muscle, not liver.

Two thresholds matter in practice. Above 2 times the upper limit, meaning roughly 100 U/L on a 50 U/L ceiling, most clinicians recheck in 14 days before imaging. Above 500 U/L, or any value with jaundice or vomiting, the workup starts within 24 to 48 hours. Between 50 and 100 U/L, a recheck at 2 to 4 weeks is reasonable if the cat is eating and active.

ALT also lies at the other end. A cat with advanced fibrosis or a portosystemic shunt can hold a normal ALT while its liver function is already poor, because there is little tissue left to leak from. Function tests, not leakage tests, catch that cat: bile acids above 20 umol/L preprandial or 25 umol/L postprandial point to shunting. How the liver handles those acids and the drugs that induce ALP is covered in liver metabolism in cats.

ALP is induced, not just spilled

ALP rises when bile flow stalls, and also when drugs induce the enzyme. Cholestasis raises ALP within 48 to 72 hours because bile acids dissolve the membrane-bound enzyme into circulation. Phenobarbital, often dosed at 2 to 5 mg/kg twice daily for epilepsy, lifts ALP within 2 to 4 weeks. Hyperthyroid cats often run ALP at 1.5 to 3 times the top of the range, and it normalizes over 3 to 6 months once the thyroid is controlled.

Growth and bone add their own isoenzyme. A kitten under 6 months can show ALP 3 to 5 times adult limits while growth plates are open, and a healing fracture does the same for 4 to 8 weeks.

GGT is the biliary marker

GGT lives on the biliary epithelium, so it is the cleanest signal of a bile duct problem. Cats normally sit at 0 to 5 U/L, and GGT climbs within 24 to 72 hours of obstruction. Unlike ALP it is not induced by steroids, which makes it the tiebreaker: ALP up with GGT up points to cholestasis, while ALP up with a normal GGT points to induction, bone or a corticosteroid.

Feline neutrophilic cholangitis, common in middle-aged cats, pushes GGT 5 to 10 times above the ceiling while ALT rises only 2 to 4 times. It may also run a fever of 39.5 to 40.5 C and a bilirubin of 3 to 8 mg/dL, which is a same-day problem.

Four numbers, one picture

Enzyme	Half-life in cats	Main source	Typical cat range	Days to clear	Recheck
ALT	3-5 days	Hepatocyte cytosol	10-50 U/L	15-25	14 days
AST	1-2 days	Liver and muscle	10-40 U/L	5-10	7-14 days
ALP	1-3 days	Bile canaliculi, bone	10-40 U/L	5-15	14-21 days
GGT	1-3 days	Biliary epithelium	0-5 U/L	7-14	14 days
Bile acids	Hours	Liver function	0-5 umol/L fasted	1-2	Same day

Read the table in rows, not columns. A cat with ALT 300 U/L, ALP 60 U/L and GGT 4 U/L has a hepatocellular pattern. A cat with ALT 80 U/L, ALP 300 U/L and GGT 40 U/L has a biliary pattern and needs an ultrasound within 48 hours.

What makes a value urgent

Four findings move a cat from the recheck column to the same-day column: visible jaundice, which appears once bilirubin passes about 2 to 3 mg/dL; anorexia lasting 24 hours in an obese cat, because hepatic lipidosis can start after 2 to 7 days without food; weight loss above 10 percent in 3 months; and any neurological sign such as head pressing, circling or seizures, which suggests ammonia above 100 umol/L. Add bleeding from the gums or a temperature under 37.5 C and the case is an emergency.

Hepatic lipidosis deserves its own line. An overweight cat that stops eating for 48 hours is at risk, and the risk climbs steeply after 72 hours. For a cat that has not eaten since Tuesday, feed it or call, but do not wait for the recheck.

Five steps that change the outcome

1. Keep the printed reference range with the result, and compare only same-lab numbers.
2. Repeat an isolated ALT of 100 to 300 U/L at 14 days before ordering imaging.
3. Add GGT and bile acids when ALP is the dominant rise, since GGT separates bile from bone and steroid.
4. Ask about phenobarbital, steroids, thyroid status and recent anesthesia at every recheck.
5. Photograph the cat weekly and weigh it monthly; a 300 g drop in a 4 kg cat is 7.5 percent and counts. If the cat is new, the breed quiz gives the adult weight to compare against.

A single abnormal liver value in a well cat is usually a scheduling problem, not a diagnosis. The 2 week recheck and the GGT tiebreaker resolve most of them for the price of 2 blood draws.

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