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Using likelihood ratios to diagnose chest problems

Dr Nick Summerton on how likelihood ratios can help to diagnose pneumonia and left ventricular dysfunction

Dr Nick Summerton on how likelihood ratios can help to diagnose pneumonia and left ventricular dysfunction

We might assess a patient's respiratory status for a number of reasons but, most commonly, we are interested in detecting evidence of infection or heart failure. In determining whether an adult has a community-acquired pneumonia a number of clinical features have been suggested – see box below.

Definitions

Likelihood ratio
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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 (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 information is absent.

Kappa value
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Kappa values measure reliability – the extent to which repeated measurements of a stable phenomenon produce similar results. This level of agreement is expressed as a chance-corrected proportional value.

Kappa values greater than 0.80 may be taken to represent almost perfect agreement, values between 0.61 and 0.80 substantial agreement, values between 0.41 and 0.60 moderate agreement, values between 0.21 and 0.40 slight agreement and values of 0.20 or below represent poor agreement beyond chance.

Discussion

Although some individual findings – such as a raised respiratory rate, an elevated temperature, dullness to percussion and bronchial breath sounds – provide substantial positive likelihood ratios, clusters of findings are more powerful.

A combination of a temperature higher than 37.8°C, a heart rate more than 100 beats per minute, crackles and diminished breath sounds in a patient without asthma provides a positive likelihood ratio of 8.2, while the absence of this combination produces a negative likelihood ratio of 0.3.

With regard to predicting left ventricular dysfunction in patients aged 70-84 living in the community, basal crackles in the chest confer a positive likelihood ratio of 2.4.

But this needs to be considered in relation to a history of breathlessness (LR+5.4), MI (LR+4.3) or angina (LR+3.3). In many elderly patients, the presence of COPD confuses the issue.

In such cases, the combination of a history of ischaemic heart disease, a BMI over 30, a laterally displaced apex beat, a heart rate greater than 90 beats per minute and a raised BNP level confers a positive likelihood ratio of left ventricular dysfunction of 5.6 (negative LR 0.7).

Unfortunately, one of the particular difficulties of chest examination is the unreliability of some of the physical findings (see box of kappa values left).

Dr Nick Summerton is a GP in Yorkshire

LRs for Pneumonia Kappa values/chest signs Key messages

• A combination of a temperature higher than 37.8°C, a heart rate more than 100 beats per minute, crackles and decreased breath sounds in a patient without asthma provides a positive likelihood ratio of 8.2 for the diagnosis of pneumonia.
• The absence of this combination produces a negative likelihood ratio of 0.3.
• For left ventricular dysfunction, basal crackles in the chest confer a positive likelihood ratio of 2.4 (with a kappa value of 0.41), whereas a history of breathlessness provides a positive likelihood ratio of 5.4. So in this context, the history is more valuable than the examination.
• The kappa values show that some chest findings on examination are unreliable.

COPd will making diagnosing LVD more difficult

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