ALT above 100 U/L, ALP above 120 U/L and a postprandial bile acid over 25 umol/L are three different alarms, and mixing them up is the most common reason a cat gets an ultrasound it did not need. Cats run lower ALP than dogs, hide liver disease behind a normal panel, and raise ALT from muscle as easily as from liver. The three numbers only mean something as a pattern.
Start with the reference set: ALT 10-100 U/L, ALP under 120 U/L, GGT under 5 U/L, total bilirubin under 0.3 mg/dL, fasting bile acids 0-5 umol/L and postprandial up to 15 umol/L.
Why the cat liver reads differently from the dog
ALP in cats sits far lower, so 130 U/L is a real signal where a dog would shrug. Cats also have less glycogen reserve and a smaller liver-to-body ratio than dogs, which is why 3-4 days without food can start hepatic lipidosis, and why anorexia is treated as a liver emergency in a fat cat.
Half-life matters too: ALT falls over 2-3 days once the insult stops, while ALP can stay up for 1-2 weeks. A single high ALT taken after a fight may only mean a bruised muscle.
ALT at 100 U/L: the ceiling and the traps
ALT leaks from hepatocytes when membranes are damaged, so it is sensitive and not specific. Values of 100-200 U/L are mild, 200-500 is moderate, and above 500 U/L usually means active damage: cholangitis, toxins, or hypoxia. Above 1000 U/L in a cat is uncommon and pushes toward acute injury.
The trap is muscle. Any cat with an abscess, a bite wound, or a fall can show ALT at 150-300 U/L with a CK over 1000 U/L; repeat in 7-10 days and the ALT normalizes without a liver problem. Hyperthyroidism raises ALT in about 30% of cases and it falls 4-8 weeks after treatment.
ALP under 120 U/L and the cholestatic pattern
ALP comes from the bile canaliculus, so a rising ALP with a rising GGT points at obstruction or cholangitis rather than liver cell death. In cats, cholangitis is the common inflammatory liver disease, and ALP may be the only enzyme that moves.
A cat with ALP 200 U/L, GGT 12 U/L and bilirubin 1.8 mg/dL needs imaging within 48 hours, not a diet change. A cat with ALP 130 U/L and everything else normal gets a recheck in 4 weeks and a look at its medication list.
Bile acids 5-25: the number that tests function
Bile acids measure what the liver does, not what leaked out of it. Fasting values of 0-5 umol/L are normal; 5-25 umol/L is the band that needs a second sample, because a 12-hour fast is hard to guarantee in a household with two cats. Postprandial values up to 15 umol/L are accepted.
Above 25 umol/L, especially above 40, a portosystemic shunt or cirrhosis is on the list, and the next step is a bile acid pair with a 2-hour postprandial sample plus ultrasound. Values of 5-25 in a kitten under 6 months can be normal maturation and are repeated at 6 months.
Three patterns, three different next steps
| Pattern | ALT U/L | ALP U/L | Bile acids | Usual cause |
|---|---|---|---|---|
| Hepatocellular | 200-1000 | under 150 | normal | Toxins, hypoxia, cholangitis |
| Cholestatic | under 200 | 200-600 | raised | Bile duct obstruction, cholangitis |
| Mixed or vacuolar | 100-250 | 120-250 | 5-25 | Lipidosis, endocrine disease, drugs |
Pattern three is the one that fools people. A fat cat that stopped eating for 4 days with ALT 180 U/L and ALP 160 U/L is developing lipidosis, and the treatment is calories, not a liver supplement. Wait a week to confirm the pattern and the cat can be in a worse place than when the sample was drawn, which is why anorexia over 72 hours outranks the enzyme numbers.
Sampling errors that rewrite the result
Hemolysis in the tube, a sample left 6 hours at room temperature, or a cat that struggled hard during the draw all change the picture. ALT is fairly stable, but bilirubin drops with light exposure, and a lipemic sample from a cat that ate 2 hours earlier distorts several values at once.
Practical rules: fast 8-12 hours, draw 0.5-1 mL into a plain tube, spin within 2 hours, and keep the plasma at 4 C if it will be run the same day or at minus 20 C if it waits. Ask for CK alongside ALT whenever the cat has had an injection, a seizure or a fall. Repeat the panel in 2-4 weeks rather than 2-4 days, because ALT needs that long to fall and a same-week retest mostly measures the same event twice.
What raises enzymes without liver disease
Muscle injury, seizures, and IM injections push ALT. Bone growth, healing fractures, and dental disease push ALP. Phenobarbital raises ALP over 2-6 weeks; corticosteroids can raise both. Hyperthyroidism and diabetes each shift the panel in their own direction.
This is why a cat with abnormal enzymes but a normal bile acid, normal bilirubin and a good appetite usually waits 2-4 weeks for a repeat rather than starting a drug.
Reading the whole panel in order
- Check CK and the muscle score before believing an ALT.
- Decide the pattern from ALT versus ALP ratio, not the absolute value.
- Add bilirubin and bile acids to see whether function is affected.
- Ask for a urine culture in any cat with fever or jaundice.
- Repeat in 2-4 weeks before imaging if the cat is well.
Same-day care for jaundice, a cat that has not eaten for 72 hours, or any sign of hepatic encephalopathy such as head pressing or circling. Within 48 hours for ALT above 500 U/L, bilirubin above 2 mg/dL, or vomiting more than 3 times in 24 hours. If the white cells are also abnormal, the white cell count guide explains what a left shift means in a liver case, and the breed quiz flags breeds with higher shunt risk.
Frequently asked
What ALT level is normal in cats?
Most laboratories use 10-100 U/L, so 100 U/L is the ceiling rather than the middle. Values of 100-200 are mild, 200-500 moderate and above 500 U/L suggests active injury such as cholangitis, a toxin or hypoxia. Muscle damage from a bite or a fall can push ALT to 150-300 U/L with a CK over 1000 U/L, so a repeat in 7-10 days settles it.
Are bile acids of 5-25 umol/L abnormal?
They sit in the band that needs a second sample, because a true 12-hour fast is hard to guarantee at home and the fasting reference is 0-5 umol/L with postprandial values up to 15 accepted. Above 25 umol/L, and especially above 40, raises portosystemic shunt or cirrhosis, so the next step is a fasting and 2-hour postprandial pair plus ultrasound.
Can a cat have liver disease with normal enzymes?
Yes. Cats hide chronic liver disease behind a normal panel because ALP runs low and enzyme rises can be intermittent. A normal bile acid with a normal bilirubin is reassuring, but a cat losing weight, eating poorly or jaundiced needs imaging regardless of the numbers, and any cat that has not eaten for 72 hours needs same-day care.