Feline urinary F2-isoprostanes measured by enzyme-linked immunoassay and gas chromatography–mass spectroscopy are poorly correlated

15-F2T-isoprostanes are byproducts of lipid peroxidation and were determined to be the best marker of oxidative injury in a rodent model of oxidative stress. A previous study compared methods for measurement of urinary F2-isoprostanes (gas chromatography and negative ion chemical ionization–mass spectrometry, GC-NICI-MS; and ELISA) and found poor agreement in dogs, horses, and cows. Surprisingly, fair agreement between these methods was identified in a small population of cats. We evaluated the agreement between GC-NICI-MS and ELISA of urinary F2-isoprostanes in the urine of 50 mature cats ranging from healthy to systemically ill. All urine samples had detectable levels of F2-isoprostanes by both methods. Significant proportional bias and poor agreement were identified between the 2 methods (ρ = 0.364, p = 0.009) for all cats, and in subgroup analysis based on health status. The concentration of urinary F2-isoprostanes was significantly lower in systemically ill cats compared to healthy cats when measured by ELISA ( p = 0.002) but not by GC-NICI-MS ( p = 0.068). Our results indicate that GC-NICI-MS and ELISA have poor agreement when measuring urinary F2-isoprostanes in cats.