ass screening test

(1)

Assessment of Screening

Test

Epidem iology in Medical

Pr act ice

By DJP Bar ker and G Rose

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Two basic components

Repeat abilit y / Reliabilit y

Whet her single m easur em ent can guide act ions

Poor r epeat abilit y _Æ r epeat ed m easur em ent s

^{Validit y}

Scr eening t est usually cheap, sim ple, and quick

Ther efor e quest ionable validit y

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Validity: two types of errors

False posit ive

Needless alar m

Wast ed inv est igat ion

F + is high w hen pr evalence is low

Br east cancer : Se 80% , Sp 96% , F+ : 75%

False negat ive

Neglect t he needy

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Diagnostic criteria to assess screening

test

Repeat abilit y / r eliabilit y

^{Validit y}

Pr edict ive values

Posit ive pr edict ive value

Negat ive pr edict ive value

Sensit ivit y / specificit y

Refer ence t est

+ -

Scr eening Test

+ Tr ue + ( a)

False+ ( b)

PV+ a/ a+ b _ False –

( c)

Tr ue – ( d)

PV- d/ c+ d Se

a/ a+ c

Sp d/ C+ d

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ROC

Pr evalence %

Predictive value

A scr eening t est w it h 95% se and sp.

Relat ionship bet w een disease

pr evalence and t he pr edict ive values

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Sensitivity and specificity

^Se

Pr opor t ion of t r ue posit ive cor r ect ly ident ified

Hi Se is necessar y w hen false – is ser ious.

^Sp

Pr opor t ion of t r ue negat ive cor r ect ly ident ified

Hi specificit y is necessar y w hen false + is undesir able.

^Se _Æ ^{r educe F-} _Æ incr ease F+

^Sp_Æ r educe F+ _Æ incr ease F-

Balance: a m at t er of j udgem ent

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Evaluation of a screening tes

^Cheap

^Safe

Cost of t he t est ( consider ing t he t hr eat of t he disease)

^{Tim e lost}

Discom for t

^{Anxiet y}

Com pet e r esour ces

^Yield

The num ber of t est s done t o ident ify one case

Low pr evalence _Æ poor yield _Æ scr een hi r isk gr oup

Balance above fact or s