The clinical reasoning process for decision-making in medicine is complex and involves multiple factors, including diagnostic probabilities, study characteristics, costs, and patient preferences. This article highlights the role of pre-test probability in calculating the positive predictive value (PPV) and negative predictive value (NPV) of diagnostic tests and also likelihood ratios. It also discusses concepts such as sensitivity, specificity, and predictive values, and their relationship with pre-test and post-test probabilities in various clinical contexts. The notion of treatment and diagnostic thresholds is introduced, emphasizing their impact on medical decision-making. Finally, it underscores that diagnostic tests should only be requested if their results can change clinical management, promoting a rational and cost-effective approach to medical practice.