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Diagnosis by numbers - ankle injuries

Dr Nick Summerton discusses how likelihood ratios can reduce unnecessary X-rays in ankle injuries

Dr Nick Summerton discusses how likelihood ratios can reduce unnecessary X-rays in ankle injuries

It is often difficult to determine whether an ankle injury involves bone. Although only between 8% and 14% of patients presenting to A&E with ankle injuries turn out to have a fracture, many are still subjected to unnecessary X-rays and waiting. A number present first to the GP, who has to decide whether to send the patient to hospital.

Definitions

41207132Likelihood ratio This is the ratio of the probability of an event (such as a symptom) in diseased persons to the probability of that same event in non-diseased persons.

Likelihood ratios indicate how many times more (or less) likely a result is in a patient with the disease compared with a patient free of disease.

The adjectives ‘positive' or ‘negative' indicate whether the likelihood ratio refers to the presence of the clinical information (positive) or the absence of the clinical information (negative).

Positive likelihood ratios with the highest value argue most for disease when the clinical information is present.

Negative likelihood ratios with the value closest to zero argue the most against disease when that clinical information is absent.


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Discussion

Tenderness over the posterior medial malleolus has the highest positive likelihood ratio and is the most suggestive of a fracture. The smallest individual negative likelihood ratio is provided by the ‘inability to bear weight for four steps in A&E' (LR -0.3) – so the absence of this sign argues the most against a fracture.

In seeking to reduce inappropriate X-rays, we are particularly interested in negative likelihood ratios. Combining clinical findings with the Ottawa ankle rules can reduce the negative likelihood ratio to -0.1. The Ottawa ankle rules state that an ankle radiographic series is only necessary if the patient has pain near the malleoli and one or more of these findings:

• age over 18 (these rules are not applicable to those under 18)

• inability to bear weight immediately after the injury and for four steps in A&E

• bone tenderness at the posterior edge or tip of either malleolus or in the bone within 6cm above the malleolus.

Dr Nick Summerton is a GP in Yorkshire

This article is based on a chapter of Patient-Centred Diagnosis by Dr Nick Summerton, published by Radcliffe Publishing, ISBN 9781846190551. Pulse readers can claim a 15% discount when they buy direct from Radcliffe by calling 01235 528820 or online by putting PULSEPCD in the discount code box at the checkout

Swollen ankle Swollen ankle key ratios

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