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DR.PRIYAMADHABA BEHERADR.GIRIDARA GOPAL
OUTLINE OF PRESENTATION INTRODUCTION USES OF CLINICAL EPIDEMIOLOGY SENSITIVITY AND SPECIFICITY PPV & NPV – PREVALENCE ...
Introductionanswer clinical questions andguide clinical decision makingClinical medicine EpidemiologyInvolved in methodsus...
Clinical epidemiology was introduced by –John Paulin1938According to him “a new basic science for preventivemedicine” T...
What is clinical epidemiology? The science of making predictions about individualpatients by counting clinical events in ...
Why learn about clinicalepidemiology?Clinical training has been oriented toward the mechanismsof disease through study of ...
Types of questions addressedbyclinical epidemiology
Health Outcomes of clinicalepidemiologic studiesClinical epidemiology can be studied directly only in intact humansnot in ...
Methods of clinicalepidemiology1. Formulate question (hypothesis)2. Choose study design3. Choose study population and samp...
Interpretation of diagnostic dataComprise clinical data and paraclinical dataClinical data comes from history and physic...
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If we set cut off at 320 IU100% of non infarct patient can be excluded but only37% of infarct patient includedWill the ...
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Factors deciding rule in and rule out cut off :1. If over diagnosis produced no harm we can relax rulein cut off to exclu...
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Sensitivity ( positivity in disease) : proportion ofpatients with the target disorder who have a positivetest resultSpec...
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Positive predictive value: proportion of patients withpositive test results who have the target disorderNegative predict...
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Receiver operatorcharacteristic curve(ROC curve)Express the relationship between sensitivity andspecificityFor a single ...
A test that discriminates well the curve should lie inupper left curve. little change in specificity unless testreaches h...
Receiver operator characteristic curve(ROC curve)
LIKELIHOOD RATIO Probability used to express the sensitivity, specificityand predictive value Likelihood ratios expresse...
Likelihood RatioBayes’ theorem no doubt helps to calculate PVsBut it is difficult to evaluate the posttest probswithout a ...
LIKELIHOOD RATIOA likelihood ratio is defined as the probability of a given level of atest result in those with disease d...
CONTINUEDDiseasePositive(D+)DiseaseNegative(D-)Test positive(T+) 70(a) 20(b) 90(a+b)Test negative(T-) 30(c) 80(d) 110(c+d)...
References Clinical Epidemiology – THE ESSENTIALS – 3rdEdition by Robert.H. Fletcher , Suzanne.W.Fletcher Clinical Epide...
Clinical epidemiology
Clinical epidemiology
Clinical epidemiology
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Clinical epidemiology

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Clinical epidemiology

  1. 1. DR.PRIYAMADHABA BEHERADR.GIRIDARA GOPAL
  2. 2. OUTLINE OF PRESENTATION INTRODUCTION USES OF CLINICAL EPIDEMIOLOGY SENSITIVITY AND SPECIFICITY PPV & NPV – PREVALENCE BASED ROC CURVE ANALYSIS LIKELIHOOD RATIOS
  3. 3. Introductionanswer clinical questions andguide clinical decision makingClinical medicine EpidemiologyInvolved in methodsused to answer thequestions
  4. 4. Clinical epidemiology was introduced by –John Paulin1938According to him “a new basic science for preventivemedicine” The shift in the focus of clinical epidemiology fromcommunity ecology to individual patients and groupsof patients took place in the 1960s
  5. 5. What is clinical epidemiology? The science of making predictions about individualpatients by counting clinical events in similar patients,using strong scientific methods for studies of groups ofpatients to ensure that the predictions are accurate Used as an aid to clinical decision makingLead to valid conclusions by avoiding being misled bysystematic error (bias) and chance
  6. 6. Why learn about clinicalepidemiology?Clinical training has been oriented toward the mechanismsof disease through study of biochemistry, anatomy,physiology and other traditional basic scienceKnowledge of the biology of disease should ordinarily beconsidered hypothesis, to be tested by clinical researchb/o-mechanism are only partly understood and many otherfactors in the genetic, physical and social environmentsaffect outcomeeg:some antiarrhythmic drugs actually cause arrhythmiasClinical epidemiology – foundation on which modern
  7. 7. Types of questions addressedbyclinical epidemiology
  8. 8. Health Outcomes of clinicalepidemiologic studiesClinical epidemiology can be studied directly only in intact humansnot in animals or part of humans, tissue cultures, cell membranesand genetic sequences
  9. 9. Methods of clinicalepidemiology1. Formulate question (hypothesis)2. Choose study design3. Choose study population and samplefrom that4. Collect and analyze data5. Interpret results
  10. 10. Interpretation of diagnostic dataComprise clinical data and paraclinical dataClinical data comes from history and physicalexaminationParaclinical data is lab tests10
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  16. 16. If we set cut off at 320 IU100% of non infarct patient can be excluded but only37% of infarct patient includedWill the same result be obtained in an another set of360 patients ?? 80-160 range mild infarct480 above is severe infarct40 IU below is without infarctRange of CK become larger with more patientsexamined because of outliers16
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  18. 18. Factors deciding rule in and rule out cut off :1. If over diagnosis produced no harm we can relax rulein cut off to exclude only 95% of patients.2.If early diagnosis and therapy were essential forsatisfactory clinical outcome as in neonatalscreening , low rule out cut off is set. So virtually allthe babies are screened.18
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  22. 22. Sensitivity ( positivity in disease) : proportion ofpatients with the target disorder who have a positivetest resultSpecificity ( Negativity in non diseased) : proportionof non diseased who have negative test result22
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  24. 24. Positive predictive value: proportion of patients withpositive test results who have the target disorderNegative predictive value: proportion of patients withnegative test results who don’t have the target diseasedisorderPredictive values are not constant. They change withthe prevalence of disease24
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  30. 30. Receiver operatorcharacteristic curve(ROC curve)Express the relationship between sensitivity andspecificityFor a single test it helps to decide cut-off value fordiseases where many cut-off are possibleUseful when comparing two or more methodAccuracy of a test can be described area under thecurve, larger the area better is the test
  31. 31. A test that discriminates well the curve should lie inupper left curve. little change in specificity unless testreaches high sensitivityGenerally the cut-off point is near the shoulder pointuntil the clinician is interested to minimize false positiveor false negative
  32. 32. Receiver operator characteristic curve(ROC curve)
  33. 33. LIKELIHOOD RATIO Probability used to express the sensitivity, specificityand predictive value Likelihood ratios expresses odds .
  34. 34. Likelihood RatioBayes’ theorem no doubt helps to calculate PVsBut it is difficult to evaluate the posttest probswithout a pen and paper or a calculatorAnother simple method of obtaining posttestprobabilities is by the use of ‘odds’ instead ofprobabilitiesThe discriminating ability of a test between diseasedindividuals and normal individuals can be measuredby an index called the Likelihood Ratio (LR)Like the predictive values, LR is calculatedseparately for positive test and negative tests
  35. 35. LIKELIHOOD RATIOA likelihood ratio is defined as the probability of a given level of atest result in those with disease divided by the probability of thatsame result in those without the disease. A likelihood ratio indicates how many times more (or less) likelya test result of a given level is obtained in disease than in nodisease.Likelihood ratios are a method of characterizing diagnostic tests.They offer several important advantages compared to traditionalmeasures such as sensitivity and specificity37
  36. 36. CONTINUEDDiseasePositive(D+)DiseaseNegative(D-)Test positive(T+) 70(a) 20(b) 90(a+b)Test negative(T-) 30(c) 80(d) 110(c+d)100(a+c) 100(b+d)38LR+=Pr(T+/D+)/Pr(T+/D-)=sensitive/1-specificity=(70/100)/(20/100)=7/2=a/bLR-=Pr(T-/D+)/Pr(T-/D-)=1-sensitive/specificity=(30/100)/(80/100)=3/8=c/dLikelihood Ratio=LR+/LR- =(a/b)/(c/d)=ad/bc=ORLikelihood ratio is numerically equal to odd ratio
  37. 37. References Clinical Epidemiology – THE ESSENTIALS – 3rdEdition by Robert.H. Fletcher , Suzanne.W.Fletcher Clinical Epidemiology – A Basic science for ClinicalMedicine – 2ndEdition by David.L.Sackett Modern Epidemiology – 2ndEdition by Kenneth.J.Rothman

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