Nurse practitioners and other clinicians use laboratory studies and ancillary tests to confirm or rule out suspected conditions. Deciding which tests to use and interpreting results are essential to the practice of high-quality, cost-effective primary care. A working knowledge of test characteristics and performance facilitates these decisions. This article illustrates how a conceptual understanding of sensitivity, specificity and predictive value applies to daily practice. In addition, the article explains how to evaluate the quality and relevance of literature that reports test performance. While discussion focuses on laboratory tests, the same principles may be applied to the history and physical examination, and to other types of diagnostic data.