Evidence-based Medicine Toolkit

38 Evidence-based Medicine ToolkitTarget disorder(prostate cancer)PresentAbsentTotalsDiagnostic testresult (prostateserum)Positive 26Negative46acbd6995249 295Totals 72 318Sensitivity a/(a + c) 26/72 = 36%Specificity d/(b + d) 249/318 = 78%Positive predictive value a/(a + b) 29/95 = 27%Negative predictive value d/(c + d) 249/295 = 84%Pre-test probability(prevalence)LR for a positive resultLR for a negative resultPre-test odds(a + c)(a + b + c + d)sens(1 – spec)(1 – sens)specPrevalence(1 – prevalence)72/390 = 18%0.36/0.22 = 1.660.64/0.78 = 0.820.18/0.82 = 0.22For a positive test result:Post-test odds pre-test odds × LR 0.22x1.66 = 0.37Post-test probability Post-test odds(post-test odds + 1)0.37/1.37 = 27%Using sensitivity and specificity: SpPin and SnNoutSometimes it can be helpful just knowing the sensitivity and specificityof a test, if they are very high.If a test has high specificity, i.e. if a high proportion of patientswithout the disorder actually test negative, it is unlikely to producefalse positive results. Therefore, if the test is positive it makes thediagnosis very likely.