ClinicalReasoning1:Introductiontodiagnostictests

DrNicolaCooperMBChBFAcadMEdFRCPEFRACPConsultantPhysician&HonoraryClinicalAssociateProfessor

Therearetwoclinicalreasoningworkshopsinthefirstyear.

Bytheendofthis sessionyoushould:

• Understandwhatismeantby‘clinicalreasoning’

• Knowthedifferentcomponentsofclinicalreasoning

• Understandtheprinciplesofusingandinterpretingdiagnostictests

Learningobjectives

Whatismeantby‘clinicalreasoning’?

‘Aclinician’sabilitytomakedecisions,oftenwithothers,basedontheavailableclinicalinformation,whichincludeshistory(sometimesfrommultiplesources),physicalexaminationfindingsandtestresults– againstabackdropofuncertainty.[It]alsoincludeschoosingappropriatetreatments(ornotreatmentatall)anddecision-makingwithpatientsand/ortheircarers.’

CooperN&FrainJ[Eds].ABCofClinicalReasoning.Wiley,2016.

Whatisclinicalreasoning?

Inalldefinitionsintheliterature,several‘components’(i.e.elementsofalargerwhole)oftheclinicalreasoningprocessaredescribed:

• History• Physicalexamination• Useandinterpretationofdiagnostictests• Reasoning/rationality• Shareddecisionmaking(withpatients,carers,teams,guidelinesetc.)

• Formalandexperientialknowledgeofmedicine

Whatisclinicalreasoning?

Clinicalreasoningdescribesthethinkinganddecision-makingprocessesassociatedwithclinicalpractice.

Clinicalreasoningisnotthesameas‘criticalthinking’… criticalthinkingisacomponentofrationality– one’sabilitytoreasonbasedonfacts–andrationalityisonlyonecomponentofclinicalreasoning.

Whatisclinicalreasoning?

Whydoesclinicalreasoningmatter?

Thescaleofdiagnosticerror

• 1in10diagnosesareincorrect

• Diagnosticerrorcausessignificantharm

• Diagnosticerroraccountsfor40,000– 80,000deathsannuallyintheUS,somewherebetweenbreastcanceranddiabetes

• Chancesare,wewillallexperienceadiagnosticerrorinourlifetime

USInstituteofMedicine.(2013).25-yearsummaryofUSmalpracticeclaimsfordiagnosticerrors1986-2010:ananalysisfromtheNationalPractitionerDataBank.BMJQual Saf;22(8):672-680

Results-‘System-relatedfactorscontributedtodiagnosticerrorin65%ofthecasesandcognitivefactorsin74%… themostcommoncognitivefactorsinvolvedfaultysynthesis.’

SherbinoJ&NormanGR.(2014).AcademicEmergencyMedicine;21(8):931-933.

‘Theprevailingopinionthatdiagnosticerrorisacognitiveprocessingerror… isincorrect.Thisperspectivepresupposesthatalloftheavailableknowledgeispresent… Incontrast,adiagnosticerrormayreflectnotaprocessingerror,butanincompleteknowledgebaseorinadequateexperience.’

Thecomponentsofclinicalreasoning

Basic science

& clinical medicine

Shared decision-making*

Evidence-based physical

examination

Evidence-basedhistory

Reasoning/rationality

Use and interpretation of diagnostic

tests

Clinical Reasoning

Whatdowemeanby‘knowledge’?

Knowledge dimensionA.Factualknowledge Thebasicelementsthatstudentsmustknowtobe

acquaintedwithadisciplineorsolveproblemsinit• Terminology• Specificdetailsandelements

B.Conceptualknowledge Theinter-relationshipsamongthebasic elementswithinalargerstructurethatenablethemtofunctiontogether• Classificationsandcategories• Principlesandgeneralisations• Theories,modelsandstructures

C.Proceduralknowledge Howtodosomething;methodsofinquiry,andcriteriaforusingskills,algorithms,techniquesandmethods• Subject-specific skillsandalgorithms• Subject-specifictechniquesandmethods• Criteriafordeterminingwhentouseappropriateprocedures

D.Metacognitive knowledge Knowledgeofcognitioningeneralaswellasawarenessandknowledgeofone’sowncognition• Strategic knowledge• Knowledgeaboutcognitivetasks,includingappropriatecontextualand

conditionalknowledge• Self-knowledge

Krathwohl DR.ArevisionofBloom’sTaxonomy:anoverview.(2002).Theoryintopractice41(4):212-218.

Usingandinterpretingdiagnostictests

Ina60-yearoldheavysmokerwhopresentswithpersistentbreathlessnessonexertionandwheeze,whatistheprobabilityofhimhavingemphysemawithanormal*spirometryresult?

*FEV1/FVCratio>70%:GOLDGuide2017

Usingandinterpretingtests

Testslie!– testsgiveustestprobabilitiesnotrealprobabilities

Alltestresultsareaffectedbythefollowing:• How‘normal’isdefined

• Factorsotherthandiseasethatinfluencetestresults

• Operatingcharacteristics

• Sensitivityandspecificity

• Prevalenceofdiseaseinthepopulation

Sensitivityandspecificity

• Sensitivityistheabilitytodetecttruepositives

• Specificityistheabilitytodetecttruenegatives

• Notesthas100%sensitivityandspecificity

Disease Nodisease

Positivetest A(Truepos)

B(Falsepos)

Negativetest C(Falseneg)

D(Trueneg)

Sensitivity=A/(A+C)x100

Specificity=D/(D+B)x100

Theprobabilityofadiseasedependsontheclinical(pre-test)probabilityplusthesensitivityandspecificityofthetest

Anelderlyladyisadmittedfollowingafall.Shehadhurtherlefthipandwasunabletoweightbear.Onexamination,thelefthipwasextremelypainfultomove.HerX-ray(shown)isnormal.Isthereafracture?

Example:

Themostfundamentalprincipleinclinicaldecisionmakingisthattheinterpretationofnewinformationdependsonwhatyoubelieved*beforehand

Understandingnewinformation:Bayes’theorem.In:Sox HC,HigginsMC&OwensDK.Medicaldecisionmaking2nd Ed.Wiley-Blackwell,2013

Ina60-yearoldheavysmokerwhopresentswithpersistentbreathlessnessonexertionandwheeze,whatistheprobabilityofhimhavingemphysemawithanormal*spirometryresult?

*FEV1/FVCratio>70%:GOLDGuide2017

Probability of having a disease

http://vassarstats.net/clin2.html

Probability of having a disease

http://vassarstats.net/clin2.html

Understanding basic science

Van der Lee, I et al. (2002). Nitrous oxide diffusing capacity versus spirometry in the early diagnosis of emphysema in smokers. Respiratory Medicine; 103: 1892-1897.

Prevalenceofdisease

Ifatesttodetectadiseasewhoseprevalenceis1:1000hasafalsepositiverateof5%,whatisthechancethatapersonfoundtohaveapositiveresultactuallyhasthedisease,assumingyouknownothingabouttheperson’ssymptomsandsigns?

(45%ofHarvarddoctorssaid95%)

KahnemanD.Thinking,fastandslow.AllenLane,2011.

Answer

Present Absent TotalActualdisease 1 999 1000Test+ 1 50 51Test- 0 949 949

Falsepositives50/1000Truepositives1/1000Meansthechanceofapositiveresultwithdisease=1outof51or2%

The importance of understanding prevalence

Predictivevalues

• Predictivevaluesarethecombinationofsensitivity,specificityandprevalence

• Sensitivityandspecificityarecharacteristicsofthetest–thepopulationdoesnotchangethis

• ButweareinterestedintheQ,‘Whatarethechancesthatapersonwithapositivetestresulttrulyhasadisease?’– thepositivepredictivevalueofatest

Disease Nodisease

Positivetest A(Truepos)

B(Falsepos)

Negativetest C(Falseneg)

D(Trueneg)

PositivePV=A/(A+B)x100

NegativePV=D/(D+C)x100

An imperfect test for something that is not very common …

Acabwasinvolvedinahit-and-runatnight.Twocabcompaniesoperateinthecity,theGreenandtheBlue.85%ofthecabsinthecityareGreenand15%areBlue.AwitnessidentifiedthecabasBlue.TheCourttestedthewitnessunderthecircumstancesthatexistedonthenightoftheaccidentandconcludedthatthewitnesscorrectlyidentifiedthecolour80%ofthetime.WhatistheprobabilitythatthecabwasactuallyBlue?

(Themostcommonansweris80%)

KahnemanD.Thinking,fastandslow.AllenLane,2011.

Answer

Green BlueActual 85 15

Witness+ 68(80%of85) 12(80%of15)

Witness- 17 3

• 12/100 times the witness will correctly identify a Blue cab as Blue• 17/100 times the witness will incorrectly identify a Green cab as Blue• There is therefore a 12+17=29% chance the witness will identify the

cab as Blue• This results in a 12/29 or 41% chance that the cab identified as Blue

is actually Blue.

Base rate neglect (prevalence neglect)

Understandingprevalence

A30-yearoldwomancomplainedofadullleft-sidedheadache

Onexaminationshewastenderoverherlefttemple

AjuniordoctorrememberedlearningabouttemporalarteritisandrequestedanESR(atestfortemporalarteritis)whichwasabnormal

Thejuniordoctordiagnosedtemporalarteritis

Temporalarteritisdoesnotexistinpeopleaged<50

Smallgroupwork

Youseetwopatientswithchestpainanddecidetosendthembothforanimagingstresstesttoseewhethertheyhaveangina(seeWORKSHEET)Stresstestinghasasensitivityof90%andspecificityof85%WeknowtheactualprevalenceofIHDinthepopulationbasedonangiographyandPMstudiesWhatisthechanceofapositivestresstestbeingcorrectineachofyourtwopatients?

(Theanswerisnot90%)

65yearoldmanwithtypicalanginahistory:results

IHD NoIHD

Actual 94 6

PosST

NegST

84.6 0.9

9.4 5.1

PPV =truepos/(truepos+falsepos)x100=84.6/(84.6+0.9)x100=99%

Theabovearetheactualresultsyouwouldget.

Sensitivity = true pos

Specificity = true neg

35yearoldwomanwithatypicalchestpainhistory:results

IHD NoIHD

Actual 1 99

PosST

NegST

0.9 14.9

0.1 84.1

Theabovearetheactualresultsyouwouldget.

PPV =truepos/(truepos+falsepos)x100=0.9/(0.9+14.9)x100=5.7%

Atestresultbyitselfisnottheanswer

• TestsmustbeinterpretedinthelightofCLINICALPROBABILITY

• YoumustalsoknowsomethingabouttheCHARACTERISTICSofthetestinquestion

• AndifthePREVALENCEofthediseaseisveryhighorverylowinthepatient’sgroup– thisaffectsthepredictivevalueofthetest

Conclusions:‘Commonlyusedmeasuresoftestaccuracyarepoorlyunderstoodbyhealthprofessionals’

2015

Therapeuticthreshold

• Itisnotnecessarytoknowthetruestateofthepatientbeforedecidingwhethertoact

•Thetherapeuticthresholdcombinesfactorssuchastestcharacteristics,risksofthetest,andtherisksandbenefitsoftreatment

•Thepointatwhichallfactorsareevenlyweighedisthethreshold

• Ifatestortreatmentiseffectiveandlowriskwewouldhavealowerthresholdforgoingahead…

Therearetwoclinicalreasoningworkshopsinthefirstyear.

Bytheendofthis sessionyoushould:

• Understandwhatismeantby‘clinicalreasoning’

• Knowthedifferentcomponentsofclinicalreasoning

• Understandtheprinciplesofusingandinterpretingdiagnostictests

• Nexttime– reasoning/rationality!

Learningobjectives

1.Gotothewebaddressbelow:https://bluecastle.nottingham.ac.uk

OnlineSETforA12M1G

OrifyouhaveaQRcodescanner,scaninthecodeontheright

2.EnteryourUniversityusernameandpassword.3.Clickon‘MySurvey’.4.ClickontheCompleteSurveybutton.5.CompletetheSETsurvey.6.ClickontheSubmitbutton.

Further resources

Actual IHD on angiography/PMs (%)

Non-anginalCP Atypicalangina TypicalanginaAge Men Women Men Women Men Women30-39 5.2 0.8 21.8 4.2 68.7 25.840-49 14.1 2.8 46.1 13.3 87.3 55.250-59 21.4 8.4 58.9 32.4 92 79.460-69 28.1 18.6 67.1 54.4 94.3 90.6