Free glycerol causes a positive error in the triacylglycerol value in the total glycerol measurement method for canine and feline blood.

Two methods of measuring triacylglycerol (TG) in the blood are used in clinical laboratories. The glycerol-blanked TG analysis method is used primarily in Japan; the total glyceride measurement method is used in most countries, including the United States. The latter method includes free glycerol in the blood, which is known to be slightly higher in humans. However, the extent to which the 2 methods differ in companion animals is unknown. We used cryopreserved blood from dogs and cats that visited a secondary veterinary hospital to investigate the relationship between the data obtained using the 2 TG analysis methods. The median ratio (5th and 95th percentiles) of total glyceride measurement:glycerol-blanked TG measurement ratios was 1.24 (1.08 and 1.77) for dogs and 2.00 (1.44 and 3.66) for cats, with the total glycerol method clearly having higher values. The Passing-Bablok regression equation comparing the total glyceride method (y) and glycerol-blanked method (x) was y = 1.049x + 0.119 for dogs and y = 1.476x + 0.177 for cats. The estimated free glycerol value calculated from the difference between the 2 TG measurements was strongly correlated with the measured free glycerol value (dog,  = 0.7905; cat,  = 0.8708), indicating that free glycerol in the blood was the cause of the TG measurement discrepancy. Therefore, the TG concentrations obtained from total glyceride assays in dogs and cats may contain non-negligible levels of free glycerol.