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Interpreting test results –Sensitivity, Specificity, positive and negative predicted values

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Sensitivity The chance of having a positive test result given that you have the disease (not to be confused with the positive predicted value which is the other way around.) REMEMBER SnNOut – If the sensitivity is high and the test negative, the disease (infection) can be ruled out. A very high sensitivity is associated with a very high negative predicted value. Specificity The chance of having a negative test result given that you do not have a disease (not to be confused with the negative predicted value which is the other way around.) REMEMBER SpPIn – If the specificity is high and the test positive, the disease (infection) can be ruled in. A very high specificity is associated with a very high positive predicted value The Take home message first – then how we got there

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Disease present ( ICP) Disease absent Test positive (no pulsation) 100 AB 200 Test negative (pulsation) 0 CD 1000 Sensitivity Retinal vein pulsation and raised ICP On fundoscopy, look for vein pulsation. Normally the vein pulsates. The test looks for absence of pulsation. If there is NO pulsation, the test is positive Sensitivity = A / (A+C) Sensitivity = 100/100+0 =100%

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Disease present ( ICP) Disease absent Test positive (no pulsation) 100 AB 200 Test negative (pulsation) 0 CD 1000 Sensitivity Positive predictive value = A / (A+B) Positive predictive value = 100/100+200 = 33.3% On fundoscopy you cannot see venous pulsation. How do you interpret the test? Sensitivity = 100% Negative predictive value = D / (D+C) Negative predictive value = 1000/1000+0 = 100%

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Disease present ( ICP) Disease absent Test positive (no pulsation) 100 AB 200 Test negative (pulsation) 0 CD 1000 Sensitivity Sensitivity = 100% On fundoscopy you cannot see venous pulsation. +ve predictive value = 33.3% Raised ICP in 33.3% of cases – but with a sensitivity of 100% it will pick up all cases of those patients with raised ICP, but with such a poor +ve pred value, it is not a very good diagnostic test, i.e. you cannot diagnose raised ICP by absent venous pulsation -ve predictive value = 100%

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Disease present ( ICP) Disease absent Test positive (no pulsation) 100 AB 200 Test negative (pulsation) 0 CD 1000 Sensitivity Sensitivity = 100% +ve predictive value = 33.3% -ve predictive value = 100% However, if the test is negative i.e. you see venous pulsation, then you can be sure the patient DOES NOT have raised ICP, i.e., raised ICP is excluded and you can go ahead and do an LP without a CAT scan. If the test is negative, the disease can be ruled out SeNsitivity Negative test means disease ruled OUT ( or SnNOut)

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Disease present (alcoholism) Disease absent Test positive (yes to CAGE ) 100 AB 1 Test negative (no to GAGE) 50 CD 99 Specificity CAGE question for alcoholism. Cutting down, Annoyance, Guilt, Eye opener are the questions asked. Specificity = D / (D+B) Specificity = 99/99+1 =99%

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Disease present (alcoholism) Disease absent Test positive (yes to CAGE ) 100 AB 1 Test negative (no to CAGE ) 50 CD 99 Specificity Positive predictive value = A / (A+B) Positive predictive value = 100/100+1 = 99% The patient scores 0 with the CAGE question. How do you interpret the test? Specificity = 99% Negative predictive value = D / (D+C) Negative predictive value = 99/99+50 = 66%

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Disease present (alcoholism) Disease absent Test positive (yes to CAGE ) 100 AB 1 Test negative (no to CAGE ) 50 CD 99 Specificity Specificity = 99% The patient scores 0 with the CAGE question. +ve predictive value = 99% The patient does not have an alcohol problem in 66% of cases – but with a specificity of 99%, it will be a negative test in virtually all patients without alcoholism. but with such a poor -ve predictive value, it is not a very good screening test, i.e. you cannot exclude alcoholism on the basis of a negative CAGE question. -ve predictive value = 66%

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Disease present (alcoholism) Disease absent Test positive (yes to CAGE ) 100 AB 1 Test negative (no to CAGE ) 50 CD 99 Specificity Specificity = 99% +ve predictive value = 99% -ve predictive value = 66% However, if the test is positive i.e. the patient replies in agreement to the CAGE questionnaire, then you can be sure the patient DOES have an alcohol problem. If the test is positive, the disease/illness can be ruled in SPecificity Positive test means disease ruled IN ( or SpPIN)

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