Background <p>Randomized test-treatment studies are performed to evaluate the clinical effectiveness of diagnostic tests by assessing patient-relevant outcomes. The assumptions for a sample size calculation for such studies are often uncertain.</p> Methods <p>An adaptive design with a blinded sample size recalculation based on the overall success rate in a randomized test-treatment trial with restricting randomization to discordant pairs is proposed and evaluated by a simulation study. The results of the adaptive design are compared to those of the fixed design.</p> Results <p>The empirical type I error rate is sufficiently controlled in the adaptive design as well as in the fixed design and the estimates are unbiased. The adaptive design achieves the desired theoretical power, whereas the fixed design tends to be over- or under-powered.</p> Conclusions <p>It may be advisable to consider blinded recalculation of sample size in a randomized test-treatment study with restriction of randomization to discordant pairs in order to improve the conduct of the study. However, there are a number of study-related limitations that affect the implementation of the method which need to be considered.</p>