Diagnostic tests

A test can be defined as ‘any measurement used to identify individuals who could benefit from therapeutic intervention’

These could be:

  • The presence or absence of a symptom
  • The presence or absence of a sign
  • Laboratory or radiological measurements

A test is often used synonomously for a diagnostic test. However, tests may have functions besides that of diagnosis

  • To monitor the effect of treatment
  • To determine if treatment should be continued, stopped or changed
  • To provide information about prognosis
  • To indicate presence/absence of a degree of risk

Diagnostic tests are used for a range of purposes; at their simplest they are either positive or negative. Most people who have the disease will test positive (true positives) with most who do not have the disease testing negative (true negatives). However, as few tests are perfect some of those without the disease will test positive (false positives) and some with the disease will test negative (false negatives).

The balance between these is expressed by two criteria which are used to judge all diagnostic tests.

  • Sensitivity: Probability that a subject with the disease will test positive
  • Specificity: Probability that a subject who is disease free will test negative

Two are parameters are useful in assessing diagnostic tests:

  • Positive Predictive Value: probability of those testing positive actually having the disease
  • Negative Predictive Value: probability of those testing negative not actually having the disease

Resources

Guidance

  • The STARD statement - for reporting studies of diagnostic accuracy: explanation and elaboration
  • QUADAS - quality assessment tool for use in systematic reviews of diagnostic accuracy studies

Journal Articles

Evidence base of clinical diagnosis - BMJ Series 2002

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Other Resources