Diagnostic ThinkingDiagnostic Thinking

StepsSteps

Start with pre-test probabilityStart with pre-test probability Add Ruling-in power/Ruling-out power of the Add Ruling-in power/Ruling-out power of the

testtest Get new, post-test probabilityGet new, post-test probability

0.1Normal V/QPE

0.520>40 pack years

COPD

0.240Compression USG

DVT in hospital patient

LR -LR +TestTarget Disorder

Fagan NomogramFagan Nomogram

Likelihood ratiosLikelihood ratios

LR around 2 are minimally powerfulLR around 2 are minimally powerful LR around 5 are moderately powerfulLR around 5 are moderately powerful LR above 10 are highly powerfulLR above 10 are highly powerful LR- of 0.5 minimally powerfulLR- of 0.5 minimally powerful LR- of 0.2 is moderately powerfulLR- of 0.2 is moderately powerful LR- below 0.1 are highly powerfulLR- below 0.1 are highly powerful

Ruling-in PowerRuling-in Power

If a minimally powerful test is positive, adds If a minimally powerful test is positive, adds 15% to pre-test probability15% to pre-test probability

If a moderately powerful test is positive, adds If a moderately powerful test is positive, adds 30% to pre-test probability30% to pre-test probability

If a highly powerful test is positive, adds 45% to If a highly powerful test is positive, adds 45% to pre-test probabilitypre-test probability

NOTE THAT ITS MULTIPLES OF 15NOTE THAT ITS MULTIPLES OF 15

Ruling-out PowerRuling-out Power

If a minimally powerful test is negative, subtracts If a minimally powerful test is negative, subtracts 15% from pre-test probability15% from pre-test probability

If a moderately powerful test is negative, If a moderately powerful test is negative, substracts 30% from pre-test probabilitysubstracts 30% from pre-test probability

If a highly powerful test is positive, subtracts If a highly powerful test is positive, subtracts 45% from pre-test probability45% from pre-test probability

NOTE THAT ITS MULTIPLES OF 15NOTE THAT ITS MULTIPLES OF 15

Tips on testingTips on testing

Ordering tests when pretest prob is high or low Ordering tests when pretest prob is high or low unlikely to lead to changed post test prob, often unlikely to lead to changed post test prob, often confuses picture, unexpected results usually are confuses picture, unexpected results usually are false pos/neg.false pos/neg.

Tests are most useful when the pretest prob is Tests are most useful when the pretest prob is 50%50%

Use tests in sequence so the posttest prob of the Use tests in sequence so the posttest prob of the first test becomes the pretest prob for the next first test becomes the pretest prob for the next one: tests build on each other this wayone: tests build on each other this way