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Clinical Effectiveness: Interpreting test results Nick Price 17 th October 2006

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Aims to reflect on the implications of a study of health professional's interpretation of a test result to reflect on the implications of a study of health professional's interpretation of a test result to develop skills in interpreting test results to develop skills in interpreting test results

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Objectives By the end of the session you should be able to: Define sensitivity in ordinary language Define sensitivity in ordinary language Define specificity in ordinary language Define specificity in ordinary language Understand how the prevalence of a condition in your test population influences the significance of a positive test result in a particular patient. Understand how the prevalence of a condition in your test population influences the significance of a positive test result in a particular patient. Understand how 'testing more patients, just in case' will influence the likelihood of a patient with a positive result having the condition. Understand how 'testing more patients, just in case' will influence the likelihood of a patient with a positive result having the condition. Understand to term 'positive predictive value'. Understand to term 'positive predictive value'. Have an opportunity to try explaining the result of a test to your peers. Have an opportunity to try explaining the result of a test to your peers.

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Sensitivity How many true positives in comparison to the gold standard. How many true positives in comparison to the gold standard. Or (most accurately) The chance of having a positive test, assuming that you do have the condition. The chance of having a positive test, assuming that you do have the condition.Or So with a very Sensitive Test a Negative will rule Out the condition – SnNOut So with a very Sensitive Test a Negative will rule Out the condition – SnNOutOr So a sensitive test is likely to pick up the condition. So a sensitive test is likely to pick up the condition.

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Sensitivity 2 Can you think of some tests with very high sensitivity in comparison to a gold standard? e.g. D-dimer (99%), Leucocytes on Multistix (87%), random blood sugar

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Specificity (most accurately) The chance of having a negative test given that you do not have the disease. The chance of having a negative test given that you do not have the disease.Or How many false negatives. How many false negatives.Or With a very Specific test a Positive result rules the condition IN -SpPin So with a specific test a positive test is likely to mean you have the condition. So with a specific test a positive test is likely to mean you have the condition.

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Specificity 2 Can you think of some very specific tests? 3+ of glucose and ketones on multistix? A hard craggy breast lump? A yes score of 3+ on CAGE (99.8%) Some not very specific ones: Moderately raised random blood sugar in general population

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The Truth Table TRUTH POSITIVENEGATIVE TESTTEST POSITIVE ab NEGATIVE cd Sensitivity is the probability [a / (a + c) in the table] that a true positive has been correctly classified as positive by the test. Specificity is the probability [d / (b + d)] that a true negative is correctly classified negative by the test

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Example With leukocyte esterase dipstix (LED) for chlamydia vs gold standard In a GUM clinic 500 patients were tested, 100 tested positive with gold standard, 90 tested positive with LED. Of these 90, 5 were in fact negative with the gold standard. What is the sensitivity and specificity of LED

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Example 2 Sensitivity = 85/100 = 85% Specificity = 395/400 = 98% Truth Truth Test+-Total +85590 -15395410 Total100400500

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So what is the chance that a positive LED test means you have chalmydia? Aka what is the positive predictive value (PPV). Aka what is the positive predictive value (PPV). This is the true positives / true positives and the false positives This is the true positives / true positives and the false positives PPV = a/a+c = 85/90 = 94%. PPV = a/a+c = 85/90 = 94%. Excellent, so this is a good test to use in GP e.g. routinely when taking smears!

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PPV 1 So the incidence of chlamydia in the general population of all women having smears in GP is say 5%. We do 500 smears a year We have a test that has sensitivity of 85% and a marvellous specificity of 98%. What chance the patient with a positive test actually has chlamydia in this context?

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Example 3 Sensitivity = 85% Specificity = 98% PPV = 21/31 = 67% NPV = 465/469 = 99% Truth Truth Test+-Total +211031 -4465469 Total 500x5% = 25 475500

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So the incidence of the disease greatly effects the PPV or how many patients you will see with false positive test result

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So what about the case in the experimental study? 1% of babies have Downs 1% of babies have Downs If the baby has Downs 90% will have +ve test. If the baby has Downs 90% will have +ve test. If the baby does not have Downs 1% chance the result will be positive If the baby does not have Downs 1% chance the result will be positive With a +ve result what is the chance baby has Downs? With a +ve result what is the chance baby has Downs?

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So what about the case in the experimental study? 2 1% of babies have Downs (incidence) 1% of babies have Downs (incidence) If the baby has Downs 90% will have +ve test. (90% sensitivity) If the baby has Downs 90% will have +ve test. (90% sensitivity) If the baby does not have Downs 1% chance the result will be positive (99% specificity) If the baby does not have Downs 1% chance the result will be positive (99% specificity) With a +ve result what is the chance baby has Downs? (PPV) With a +ve result what is the chance baby has Downs? (PPV)

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Example 4 – Maths solution Sensitivity = 90% Specificity = 99% PPV = 90/190 = 47% NPV = 9800/9810 = 99.9% Truth Truth Test+-Total +90100190 -1098009810 Total100990010000

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Example 4 – narrative solution Read the paper! Read the paper! Now practice explaining one of these example in trios, then rotate.

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Objectives By the end of the session you should be able to: Define sensitivity in ordinary language Define sensitivity in ordinary language Define specificity in ordinary language Define specificity in ordinary language Understand how the prevalence of a condition in your test population influences the significance of a positive test result in a particular patient. Understand how the prevalence of a condition in your test population influences the significance of a positive test result in a particular patient. Understand how 'testing more patients, just in case' will influence the likelihood of a patient with a positive result having the condition. Understand how 'testing more patients, just in case' will influence the likelihood of a patient with a positive result having the condition. Understand to term 'positive predictive value'. Understand to term 'positive predictive value'. Have an opportunity to try explaining the result of a test to your peers. Have an opportunity to try explaining the result of a test to your peers.

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