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Back pain - diagnosis by numbers

Dr Nick Summerton shows how likelihood ratios can help in establishing whether back pain might have a serious cause

Dr Nick Summerton shows how likelihood ratios can help in establishing whether back pain might have a serious cause

Many of us have received criticism for excessive use of imaging in patients with back pain. But knowing when to investigate remains a challenge, with GPs keen to pick out individuals with a potentially serious cause, such as the older patient with cancer or the younger person with ankylosing spondylitis.

Positive likelihood ratios have been calculated for both these problems (see boxes below).

Definitions

Likelihood ratio

This is the ratio of the probability of an event (such as a symptom) in affected persons to the probability of that same event in non-affected 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 a disease when the clinical information is present.

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


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Discussion

In the case of back pain and cancer, the magnitude of the likelihood ratios diminishes as we move down the table from features in the previous healthcare history, personal characteristics, to symptoms and, finally, to some aspects of the clinical examination.

Simple investigations are also helpful: an ESR greater than 20mm/hr provides a positive likelihood ratio of 2.4 and an ESR greater than 50 a positive likelihood ratio of 19.2.

With younger patients, back pain that occurs at night – especially if it makes the patient get out of bed – means the GP should consider ankylosing spondylitis.

It has been demonstrated that the application of such information to the assessment of patients with back pain can reduce the inappropriate use of imaging. In addition, it helps the doctor decide which patients with negative radiographic findings might require further investigation.

Dr Nick Summerton is a GP in Hull

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

AS cancer Key messages Key messages

• It can be difficult to work out which patients might have a serious cause for their back pain. GPs are often criticised for their ‘excessive' use of imaging in these circumstances.
• A previous history of cancer (LR+ 14.7), age over 50 (LR+ 2.7), unexplained weight loss (LR+ 2.7) and a duration of back pain for more than a month (LR+ 2.6) – especially where medical care has been sought with no improvement (LR+ 3.0) – all argue strongly in favour of cancer as a cause for the back pain.
• In a younger patient, pain at night (LR+ 1.5) – especially if it makes them get out of bed (LR+ 3.1) – should raise the possibility of ankylosing spondylitis.

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