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Introduction to Evidence Based Practice: Diagnosis

Testing.... Imagine I could screen 50 people present for previously undiagnosed diabetesYou all get a Random Blood Glucose (RBG) test (line up for a finger prick

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Introduction to Evidence Based Practice: Diagnosis

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    1. Introduction to Evidence Based Practice: Diagnosis Tony Roberts Clinical Effectiveness Specialist Advisor South Tees Hospitals NHS Trust, Tees PCTs and Honorary Research Fellow, Durham University John Blenkinsopp Clinical Effectiveness Advisor North Tees and Hartlepool NHS Trust Amanda McGough North Yorkshire and York PCT Emma Carter Clinical Audit Manager South Tees Hospitals NHS Trust www.evidencebasedpractice.org.uk

    2. Testing... Imagine I could screen 50 people present for previously undiagnosed diabetes You all get a Random Blood Glucose (RBG) test (line up for a finger prick) A cutoff of >8.0 mmol/L counts as positive. I also need to know the result of 2 fasting glucose levels and a standard Oral Glucose Tolerance Test...

    3. The results Diabetes Present Absent Positive 3 1 4 RBG a b c d Negative 2 44 46 5 45 50

    4. The results Diabetes Present Absent Positive 3 1 4 RBG a b c d Negative 2 44 46 5 45 50 Sensitivity = a / a + c = 3 / 5 = 0.6 or 60%

    5. The results Diabetes Present Absent Positive 3 1 4 RBG a b c d Negative 2 44 46 5 45 50 Specificity = d / b + d = 44 / 45 = 98%

    6. The results Diabetes Present Absent Positive 3 1 4 RBG a b c d Negative 2 44 46 5 45 50 LR+ = sen /100 spec = 60 / 2 = 30 The ratio of true positives to false positives

    7. The results Diabetes Present Absent Positive 3 1 4 RBG a b c d Negative 2 44 46 5 45 50 LR- = 100 -sen / spec = 40 / 98 = 0.41 The ratio of true negatives to false negatives

    8. The results Diabetes Present Absent Positive 3 1 4 RBG a b c d Negative 2 44 46 5 45 50 Prevalence = a + c / a +b + c + d = 5 / 50 = 10%

    9. What does it mean? The pre-test probability is effectively an estimate of the prevalence in the tested population. So if I know that 10% of my 50 volunteers have diabetes, the chance of any one of them having it is 10% before I test them

    10. What does it mean? If the result of the RBG test is positive, this changes the pre-test probability and the post-test probability can be estimated using Fagans Nomogram About a 75% chance that given a positive RBG, the person really has diabetes

    11. What does it mean? If the result of the RBG test is negative, this also changes the pre-test probability and the post-test probability can again be estimated using Fagans Nomogram About a 4% chance that given a negative RBG, the person really has diabetes A prevalence of 10% is very high. What if the prevalence were only 1%, 5% or the prevalence in the patients you see...

    12. Summary Likelihood ratios allow you to estimate how likely the target condition is in someone, if you know what their chance of having the condition is before you start This is often regarded as the most difficult bit of EBP (Clinical Epidemiology). We will be working more slowly through it, but you may need to use the JAMA Guides to work through it at your own pace

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