You have arrived here because you have a question that involves the performance of a diagnostic test as it relates to its ability to predict the presence or absence of a disease, condition, or injury. For example, usefulness of a DPL in predicting operable abdominal injuries, the use of occult blood test card on fluids other than stool, etc.
The first step is to map out your diagnosis question (the purpose of mapping a question is to facilitate a literature search). You must think about three things, Patient population, Intervention, and Outcome. Actually, what might help is for you to think about this as if you were actually designing a study.
Patient population - What population are you interested in, is it patients in an emergency department, patients referred to a specialist, etc. What demographics like age, sex, race, are important. Do you want patients with a certain condition (i.e. asthma, or diabetes). What stage of development should the disease or condition be at (see Spectrum definition).
Intervention - What diagnostic test are you interested in. What gold standard would you compare this test to.
Outcome - What threshold post-test probabilities are you interested in.