Diagnostic Accuracy: Sensitivity, Specificity and Likelihood Ratios in Physiotherapy

Start with pre-test probability

A test does not diagnose in isolation. History, prevalence, mechanism and examination create a pre-test probability; the test result should update that probability rather than replace reasoning.

Sensitivity and specificity

• Sensitivity asks how often a test is positive among people with the target condition.
• Specificity asks how often a test is negative among people without the target condition.
• Neither is identical to the probability that this patient has the condition after a result.

Likelihood ratios

LR+ describes how much more likely a positive result is in someone with the condition than without it. LR- describes how much a negative result shifts probability downward. Values farther from 1 generally change probability more.

Clinical example

A 2025 systematic review of acromioclavicular-joint tests reported limited utility for single tests but larger probability shifts for selected combinations. Test clusters still require validation in an appropriate population.

Common mistakes and pearls

• Do not treat a highly sensitive test as automatically confirmatory when positive.
• Predictive values change with prevalence.
• Do not transport accuracy estimates across very different settings without caution.
• Check confidence intervals, not just point estimates.

Summary

Use diagnostic tests when the result can realistically change what you believe or what you do, integrating pre-test probability, evidence quality and consequences of error.