The clinical value of using a new diagnostic test depends on whether it improves patient outcomes beyond the outcomes achieved using an old diagnostic test. When can studies of diagnostic test accuracy provide sufficient information to infer clinical value, and when do clinicians need to wait for results from randomized trials? The authors argue that accuracy studies suffice if a new diagnostic test is safer or more specific than, but of similar sensitivity to, an old test. However, if a new test is more sensitive than an old test, it leads to the detection of extra cases of disease. Results from treatment trials that enrolled only patients detected by the old test may not apply to these extra cases. Clinicians need to wait for results from randomized trials assessing treatment efficacy in cases detected by the new diagnostic test, unless they can be satisfied that the new test detects the same spectrum and subtype of disease as the old test or that treatment response is similar across the spectrum of disease.