Objective-To evaluate markers of in vivo platelet function (urinary 11-dehydro-thrombox-ane B2 [11-dehydroTXB 2 ] and 2,3-dinorTXB 2 ) and assess their response to administration of 2 commonly used dosages of aspirin in healthy dogs.Animals-20 healthy dogs.Procedures-Urine was collected prior to aspirin administration and on the morning following the last evening administration. Twenty dogs received aspirin (1 mg/kg, PO, q 24 h) for 7 consecutive doses. After a washout period of 5 months, 10 dogs received a single dose of aspirin (10 mg/kg, PO). Concentrations of urinary thromboxane metabolites 11-dehydroTXB 2 and 2,3-dinorTXB 2 were measured via ELISA, and values were normalized to urine creatinine concentration. Results-Median baseline 11-dehydroTXB 2 concentrations were 0.38 ng/mg of creatinine (range, 0.15 to 1.13 ng/mg). Mean SD baseline 2 at a 3-dinorTXB 2 concentrations were 6.75 ± 2.77 ng/mg of creatinine. Administration of aspirin at a dosage of 1 mg/kg, PO, every 24 hours for 7 days did not significantly decrease urinary 11-dehydroTXB 2 concentration, but administration of the single aspirin dose of 10 mg/kg did significantly decrease 11-de-hydroTXB 2 concentration by a median of 45.5% (range, 28.2% to 67.1%). Administration of the 1 mg/kg aspirin dosage significantly decreased urinary 2,3-dinorTXB 2 concentration by a mean SD of 33.0 ± 23.7%. Administration of the single aspirin dose of 10 mg/kg also significantly decreased 2,3-dinorTXB 2 concentration by a mean SD of 46.7 ± 12.6%. Conclusions and Clinical Relevance-Aspirin administration (1 mg/kg/d) may be insufficient for reliable platelet inhibition in healthy dogs.
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