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Sc re eni ng a ndDia gnos ticTest
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Definition of Screening:
Th e pre sump tiv e ide ntificatio n ofunrecogniz ed disease or
defect byth e app lica tion of te sts,exa mina tions or other
procedure swhich ca n be ap plie d rapi dly.
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Screening program
Goa l: secon da ry p reven tion det ec t di sea se befo re clini
calpoi nt for cure or improvedoutcome ge t peop le with di sease
int oappropriate treatment
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1. Design of screening test
gol d stan dard Se lect ion of objecti ves Samp le siz e
cas es:
controls
Asse ssing th e validity an dreliability
2
2
1
)1(
d
SeSezn =
2
2
2
)1(
d
SpSpz
n
=
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2. Assessment of Screening Test valid ity reliabil ity
predictive
valu e
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(1) Validity of screening test
The abi lity of a tes t todisting uish bet ween who has adisease
an d wh o does not haveth e di sease.
Two m ain comp on en ts:sen sitiv ity an d sp ecif icity
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Sensitivity (Se):Pro portion of positiv e testwh o have th e
disease.
Se = a / (a+c ) Specificity (Sp) :
Pro portion of neg ative testwh o do n ot h ave th e diseas e.
Sp = d / (b+ d)
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False positive rate (FP):
FP = b / (b +d) = (1 - S p) False negative rate (FN):
FN = c / (a+c) = (1 -Se )
Youdens index (YI):
YI = (Se + Sp) - 1 Likelihood ratio (LR):
Sp
Se
dbb
caaLR
=
+
+=+
1)/(
)/(
Sp
Se
dbd
cacLR
=
+
+=
1
)/(
)/(
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Agreement rate:
Diagnostic index (DI):
Diagnostic availability (DA):
db
d
ca
aSpSeDI
+
+
+
=+=
1)(2 22 += SpSeDA
%100rateAgreement +++
+=
dcba
da
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ROC c urve a nd i tsapplications ROC cur ve(Recei ve Oper at
or
Char act er isti c cu rve) can show therel ati ons hi p between
sens it ivi tyand spec ifi city i nt ui ti vel y.
Ap pli cation s: Se lect ion of cut off value (cu t poi nt )
Diagno st ic val ue co mpar is on amo ngsever al di agno st ic tes
ts
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(2) reliability ( Repeatability )
The test is reli abl e orrep ea table.
Intra-subject variation (var iati onwi thin i ndivi du al sub
jec ts) Variance , sta nda rd dev iati on ,C.V. etc .
Inter-observer variation (var iati onbetween th ose rea di ng th
e testresu lts) agreement, Kap pa val ue
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Dr. APositive Negative
Dr. B Positive aagreement
b
Negative c dagreement
Agreement = (a+d) / (a+b+c+d)
Kappa va lue: (sel f readi ng ) Kappa =
2(ad-bc)/(a+b)(b+d)(a+c)(c+d)
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(3) predictive value (PV)
Positive PV (+ PV):
The likel ih ood th at a pati entwi th a pos itive test resu lt
reallyhas th e diseas e.
+ PV = a / (a+b) Negative PV (- PV) :
The likel ih ood th at a pati entwi th a nega ti ve test result
reallydoes not ha ve th e di sea se.- PV = d / (c +d)
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PV and disease prevalence
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P : disease prevalence
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3.Two-stage screening
To increase sen siti vity orspecificity:Two-stage screening
Series Test:mo re sp ec ifi ci ty( eg. di abe tes, HIV
inf ect ion ) Parallel Test: mor e sensi tivi ty
( eg. blood donorsc ree ni ng )
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Multiple test
multiple test Test 1 Test 2 Result+ + +
in parallel + - +- + +- - -+ + +
in series + - -- Not necessary -
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