• Understanding diagnostic tests 2: likelihood ratios, pre- and post-test probabilities and their use in clinical practice

    by Alan Batt. Last modified: 30/01/15

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    Akobeng AK1. Understanding diagnostic tests 2: likelihood ratios, pre- and post-test probabilities and their use in clinical practice. Acta Paediatr. 2007 Apr;96(4):487-91. PMID: 17306009.

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    Free full text from: http://onlinelibrary.wiley.com/doi/10.1111/j.1651-2227.2006.00179.x/pdf

    Abstract

    The sensitivity and specificity of a test cannot be used to estimate probability of disease in individual patients. They can, however, be combined into a single measure called the likelihood ratio which is, clinically, more useful than sensitivity or specificity. Likelihood ratios provide a summary of how many times more (or less) likely patients with a disease are to have a particular result than patients without the disease. Using the principles of the Bayes theorem, likelihood ratios can be used in conjunction with pre-test probability of disease to estimate an individual’s post-test probability of disease, that is his or her chance of having disease once the result of a test is known. The Fagan’s nomogram is a graphical tool which, in routine clinical practice, allows one to combine the likelihood ratio of a test with a patient’s pre-test probability of disease to estimate post-test probability.

    Likelihood ratios summarize information about a diagnostic test by combining sensitivity and specificity. The Fagan’s nomogram is a useful and convenient graphical tool that allows likelihood ratios to be used in conjunction with a patient’s pre-test probability of disease to estimate the post-test probability of disease.

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    Alan Batt

    Alan Batt

    Paramedic, educator, researcher
    Alan is a critical care paramedic, paramedic educator and prehospital researcher, currently working around the world as an educator and researcher. He has previously worked and studied across Europe, North America and the Middle East. He holds a Graduate Certificate in Intensive Care Paramedic Studies, and an MSc in Critical Care. His main interests are in care of the elderly, end-of-life care, patient safety, professionalism (including role and identity), and paramedic education.

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