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Critical appraisal for emergency medicine: 5 Evaluation of a diagnostic test
  1. S Goodacre
  1. Professor S Goodacre, Medical Care Research Unit, University of Sheffield, Regent Court, 30 Regent Street, Sheffield S1 4DA, UK; s.goodacre{at}

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  • Sensitivity and specificity are constant when the prevalence varies.

  • PPV increases with increasing prevalence.

  • NPV decreases with increasing prevalence.

Studies of diagnostic tests commonly inform practice in emergency medicine. They aim to determine whether the test under investigation accurately identifies patients with and without the disease, as defined by a reference standard test. Several key issues need to be considered in appraising whether a study is likely to yield a reliable estimate of the diagnostic parameters. It is also important to be able to interpret what the diagnostic parameters actually mean.


The reference standard is the criterion by which it is decided that the patient has, or does not have, the disease. Typical reference standards might be: a single diagnostic test that is known to be accurate, eg, contrast venography for deep vein thrombosis; a combination of diagnostic tests that will reliably rule in and rule out disease, eg, lung perfusion scanning for pulmonary embolus combined with pulmonary angiography in equivocal cases; diagnostic testing with follow-up for negative cases to identify cases of disease that may have initially been misclassified as no disease.

An ideal reference standard should correctly classify patients with and without disease. However, it should also be safe and simple to apply, because it would be unethical to ask patients to undergo dangerous or complex testing purely for research purposes. If an ideal reference standard does exist, then there is little need to evaluate new diagnostic tests! So we have something of a catch 22 situation. This is why judging whether a reference standard is acceptable involves weighing its potential accuracy against the feasibility of any alternative approaches.

The choice of reference standard will often involve a trade-off between validity and generalisability. A study that uses a highly accurate reference standard in all patients is likely to …

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  • Competing interests: None.