APPLICABILITY OF THE CRITICAL DIFFERENCE

The present study is concerned with the critical difference, which may help to judge whether or not the difference between two consecutive m easurements with a certain probability (i.e. 95 percent) may be ascrib ed to natural variation. Knowledge of the applicability of the critica l difference in veterinary medicine is sparse and therefore, to justif y future use of the critical difference, it is important to know wheth er or not the critical difference performs as expected. The hypothesis to be tested in this study was that at least 95 percent of the differ ences between consecutive measurements, that have been obtained in ani mals where the component measured is known to be unchanged, should be within the critical difference. From previous studies it was known tha t a low-sodium diet had no influence on the plasma potassium concentra tion. The critical difference of the plasma potassium concentration in dogs was calculated as 0.5 mmol/l. using weekly measurements of this plasma component in a group of twenty healthy dogs. To test the hypoth esis, this value was compared to the differences between consecutive w eekly measurements of this plasma component in another group of eight dogs fed a low-sodium diet for five weeks. In agreement with previous studies, the plasma potassium concentration in the eight dogs did not change significantly during the feeding experiment. Of the fourty diff erences between consecutive weekly measurements, thirty-six were withi n the critical difference. This number was not different from the numb er expected from the hypothesis and thus, the critical difference perf ormed as expected. Furthermore, the present study also indicated that a critical difference calculated from one group of dogs can be used to evaluate differences between consecutive measurements in another grou p of dogs under other dietary and management conditions.