Is it important to correct apparent drug tissue concentrations for blood contamination in the dog?

The goal of this study was to quantify in the dog the error that is made in assessing drug tissue concentrations when no correction for blood contamination is performed and hence to determine in which organs such a correction should be made. The organs investigated were the heart, the brain, the liver and the skeletal muscle, and the test drug used was the H1-antihistamine, cetirizine (0.1 or 0.6 mg/kg/day for 3 days, orally, n = 6 dogs). Radiolabelled serum albumin was used to quantitate blood trapped in the tissues. Blood and tissue samplings were performed 2 h after the last drug administration. Mean (+/-SEM) volumes of blood trapped in the liver, heart, muscle and brain were 263 +/- 12, 91 +/- 3, 27 +/- 1 and 20 +/- 2 microL/g, respectively. Apparent tissue/blood concentration ratios of cetirizine were 2.39 +/- 0.33, 1.11 +/- 0.09, 0.77 +/- 0.07 and 0.37 +/- 0.05 in the four organs. When correction for residual blood is not performed, cetirizine concentrations are underestimated (-13.6 +/- 3.2%) in the liver, slightly overestimated (+4.7 +/- 1.5 to +6.3 +/- 2.8%) in the brain, and neither over nor underestimated in the heart and muscle. Simulation data over a wide range of theoretical drug tissue/blood concentration ratios indicate that in the dog: (a) for the liver, correction of apparent tissue concentration for residual blood should be performed when the drug tissue/blood concentration ratio achieved is <0.8 or >4, (b) for the heart, correction should be made when this ratio is < or =0.4 and (c) for the brain and muscle, no correction is necessary unless the ratio is < or =0.1.