Direct spectrometric determination of serum bile acids in the dog and cat
Authors: Center S. A. , Leveille C. R. , Baldwin B. H. , Tennant B. C. 
Published: February 2025

SUMMARY
Serum bile acid concentrations have been shown to be a predictive indicator of hepatobiliary disease in persons. However, there has been only limited use of bile acid values in the clinical diagnosis of hepatobiliary disease in the dog and cat because of technical difficulties associated with many bile acid assays. A rapid enzymatic method previously developed for the quantitation of 3-hydroxy bile acids in persons has been adapted for use in the dog and cat. Nonsulfated 3-hydroxy bile acids are converted to 3-oxo bile acids by 3 α-hydroxysteroid dehydrogenase and reduction of NAD + to NADH. In a coupled diaphorase catalyzed reaction, H+ is transferred to nitrotetrazolium blue to produce a diformazan dye, which is measured spectrophotometrically at 540 nm. Nonspecific interfering dehydrogenase activities present in the dog and the cat serums were inhibited by heating the serum to 60 C for 30 minutes or by the addition of sodium pyruvate. Standard curves prepared from various serum sodium taurocholate concentrations in dogs and cats are linear to 250 μmol/L.
The assay is sensitive for the detection of bile acid concentrations as low as 2.5 μmol/L in sera from dogs and cats. In validation studies quantitative recovery of known concentrations of 7 primary and secondary, conjugated and unconjugated, 3-hydroxy bile acids from pooled canine serum was 95.3 ± 7.9% (mean ± sem) and that from pooled feline serum was 101.4 ± 8.2%. Intra-assay precision on 10 replicate samples of pooled serums resulted in a mean value of 3.2 ± 0.2 μmol/L, with a 6.6% coefficient of variation (cv) for the dog, and a mean value of 2.7 ± 0.3 μmol/L with a 12.5% cv for the cat. Interassay precision on 11 separate assays of a known bile acid standard containing 29.0 μmol/L gave a mean value of 29.0 ± 3.5 μmol/L, with a 12.2% cv.
Serum bile acid concentrations were determined in 20 healthy mature male and female dogs after they were fasted and after they were fed. The dogs were allotted to 2 groups of 10 dogs each and those in the 2 groups were fed diets having different protein and fat content. Group 1 dogs were fed a diet containing 11.4% protein and 6.9% fat, and group 2 dogs were fed a diet containing 8.9% protein and 1.1% fat. Serum bile acid concentrations were determined after 12 hours' fasting and at 2, 4, 6, and 8 hours after the dogs were fed. The mean value ± se of the mean (X ± sem) of serum bile acids in fasting animals for both groups was 2.3 ± 0.4 μmol/L. A significant (P < 0.05) increase in mean serum bile acid concentrations occurred in each group at the 2- and 4-hour postprandial times, but not at the 6- and 8-hour times. The maximal increases in mean serum bile acid value occurred in group 1 at 2 hours after feeding with a mean of 8.3 ± 2.2 μmol/L, and in group 2 at 4 hours after feeding with a mean of 4.8 ± 1.0 μmol/L.
Serum bile acid concentrations were determined in 20 healthy mature male and female cats fasted for 12 hours and then fed a diet containing 13.6% protein and 8.4% fat. The mean serum bile acid concentration after fasting was 1.7 ± 0.3 μmol/L. The mean serum bile acid concentration at 2 hours after the cats were fed was significantly (P < 0.05) elevated to 8.3 ± 0.8 μmol/L. There were no significant differences between the dog and cat in the serum bile acid values after a 12-hour fast or at 2 hours after feeding.