Clinicians use tests that are usually referred to as “diagnostic”—including signs and symptoms, imaging, biochemistry, pathology, and psychological testing—for various purposes.1
These purposes include identifying physiological derangements, establishing prognosis, monitoring illness and response to treatment, and diagnosis. This article focuses on diagnosis: the use of tests to establish the presence or absence of a disease (such as tuberculosis), target condition (such as iron deficiency), or syndrome (such as Cushing’s syndrome).
Whereas some tests naturally report positive and negative results (for example, pregnancy), other tests report their results as a categorical (for example, imaging) or continuous variable (for example, metabolic measures), with the likelihood of disease increasing as the test results become more extreme. For simplicity, in this discussion we assume a diagnostic approach that ultimately categorises test results as positive or negative.
Guideline panels considering a diagnostic test should begin by clarifying its purpose. The purpose of a test under consideration may be for triage (to minimise use of an invasive or expensive test), replacement (of tests with greater burden, invasiveness, or cost), or add-on (to enhance diagnosis beyond existing tests).2
The panel should identify the limitations for which alternative tests offer a putative remedy; for example, eliminating a high proportion of false positive or false negative results, enhancing availability, decreasing invasiveness, or decreasing cost. This process will lead to identification of sensible clinical questions that, as with other management problems, have four components: patients, diagnostic intervention (strategy), comparison, and outcomes of interest.3 4
The box shows an example of a question for a replacement test.
Example question for replacement test
In patients in whom coronary artery disease is suspected, does multislice spiral computed tomography of coronary arteries as a replacement for conventional invasive coronary angiography reduce complications with acceptable rates of false negatives associated with coronary events and false positives leading to unnecessary treatment and complications?5 6
Clinicians often use diagnostic tests as a package or strategy. For example, in managing patients with apparently operable lung cancer on computed tomography, clinicians may proceed directly to thoracotomy or may apply a strategy of imaging the brain, bone, liver, and adrenal glands, with subsequent management depending on the results. Furthermore, a testing sequence may use an initial sensitive but non-specific test, which, if positive, is followed by a more specific test (for example, faecal occult blood followed by colonoscopy). Thus, one can often think of evaluating or recommending not a single test but a diagnostic strategy.