Bile acids are the most sensitive single blood test for a portosystemic shunt in a cat, and they only work if the sample timing is right. Preprandial values sit under 5 µmol/L in a healthy cat; the postprandial sample is drawn 2 hours after a meal, when the curve peaks. Get the timing wrong by 60 minutes and a normal cat can read abnormal, or the reverse. The test is cheap, the arithmetic is simple, and the 3 patterns below cover most of what a first-round result can say.
What the cycle does after every meal
The liver builds bile acids from cholesterol and conjugates them, mostly with taurine in the cat. They leave in bile, enter the small intestine, and emulsify fat. About 95% are reabsorbed in the ileum and return to the liver through the portal vein; the other 5% leave in feces and are replaced by new synthesis. The whole pool cycles 4 to 6 times a day, and the portal vein carries the entire reabsorbed load to the liver on the first pass.
That first pass is the point of the test. A healthy liver clears 80% to 90% of the bile acids arriving in portal blood, so peripheral blood stays low. A liver with a shunt, or with fewer working hepatocytes, lets them through, and the peripheral number rises.
Why the sample is drawn at 2 hours
After a meal, peripheral bile acids climb from the fasting baseline and peak at about 2 hours, then fall back toward baseline by 4 to 6 hours. Draw at 30 minutes and the curve has not risen; draw at 5 hours and it is on the way down. The standard protocol is a 12-hour fast, a preprandial sample, a meal of 1/2 to 1/4 of the daily ration, then a second sample at 120 minutes. Some labs accept 90 to 150 minutes, which is why the time is written on the tube.
| Preprandial (µmol/L) | Postprandial (µmol/L) | Pattern | Usual meaning |
|---|---|---|---|
| 0 to 5 | 0 to 15 | Both normal | Normal hepatic clearance |
| above 5 | above 20 | Both high | Shunt or hepatocellular dysfunction |
| 0 to 5 | 15 to 25 | Post only high | Early or mild insufficiency, microvascular dysplasia |
| 5 to 15 | 0 to 15 | Pre high, post normal | Delayed gastric emptying, gallbladder stasis, fasting artifact |
| above 40 | above 40 | Both very high | Cholestasis or severe dysfunction, imaging needed |
The 3 liver patterns
Pattern 1 is both values high, and it is the classic shunt signature: a young cat with 8 to 20 µmol/L fasting and 40 to 90 µmol/L after eating, dilute urine, and a small liver on film. Pattern 2 is a normal fasting value with a raised postprandial one, which usually means the clearance reserve is gone but the resting liver still copes; early fibrosis and microvascular dysplasia both sit here. Pattern 3 is a raised fasting value with a normal postprandial one, which is more often a gut-motility or gallbladder-filling problem than a liver problem.
Caution 1: the 12-hour fast has a limit in cats
Twelve hours is the protocol. Sixteen to 24 hours is not, and in a cat it is a risk: a cat that fasts beyond 12 to 16 hours starts mobilizing fat to the liver, and an overweight cat can begin the cascade toward hepatic lipidosis inside 48 to 72 hours. Feed a thin cat or a kitten at 8 hours and note the shorter fast on the form. A fasting sample from a cat that last ate 20 hours ago can read high for reasons that have nothing to do with a shunt.
Caution 2: 3 things that fake the number
Hemolysis and lipemia both interfere with the assay, and a markedly lipemic sample after a fatty meal can shift the result either way. Ursodiol, given for cholestasis, is itself a bile acid and reads as a rise. Metronidazole and some anticonvulsants alter hepatic enzymes and can change clearance. Tell the lab every drug the cat takes, and repeat a borderline result rather than treating it.
Units and lab differences
Results are reported in µmol/L, and some laboratories print mg/L. Reference intervals are not interchangeable: 1 lab calls the postprandial cut-off 15, another prints 20, a third 25. Hemolysis, delayed separation, and the assay method all move the number by several µmol/L. Compare a cat only with its own earlier values from the same laboratory, and read the interval printed on that report rather than a figure from a textbook.
What bile acids do not tell you
They do not measure liver function in real time, they do not grade fibrosis, and they cannot separate a shunt from severe parenchymal disease on their own. Ammonia, albumin, glucose, BUN, and a bile acid pair together narrow the list; ultrasound and, in some cats, scintigraphy or a portal vein study confirm it. A cat with a normal bile acid pair and a high ammonia still has a problem worth imaging.
5 steps before the test
- Confirm the fast: 12 hours, no longer, and no treats after midnight.
- Feed 1/2 to 1/4 of the daily ration, and write down the exact time.
- Draw the second sample at 120 minutes, not at 90 or 180.
- List ursodiol, metronidazole, and any anticonvulsant on the submission form.
- Ask for the laboratory's own reference interval in writing.
For how the rest of a feline blood panel reads, see white cell counts in cats. If the cat is also showing visual changes, colour vision explains what an eye exam can and cannot pick up, and the breed quiz covers how body size shifts normal values.
Terms mentioned in this article
Frequently asked
Why does the post-meal bile acid sample have to be at 2 hours?
Because that is where the curve peaks. Peripheral bile acids rise after a meal, top out near 120 minutes, and drift back toward baseline by 4 to 6 hours. A sample at 30 minutes catches the rise too early and a sample at 5 hours catches the fall, so either can be called normal when the true peak was high, or the reverse.
Is a fasting bile acid of 6 µmol/L a problem?
It sits just above the usual 0 to 5 µmol/L ceiling and means little on its own. Read it with the postprandial value: 6 fasting with 10 after eating is usually benign, while 6 fasting with 40 after eating needs imaging. Lipemia, hemolysis, and ursodiol can each push the number up, so repeat a borderline pair before treating.
Can I fast my cat 20 hours to get a cleaner result?
No. Twelve hours is the protocol and longer is a risk, because a cat fasting past 12 to 16 hours starts moving fat to the liver, and an overweight cat can slide toward hepatic lipidosis within 48 to 72 hours. Feed a thin cat or kitten at 8 hours and record the shorter fast, since a 20-hour fast can raise the value on its own.