Clinical Effectiveness:Clinical Effectiveness:Interpreting test resultsInterpreting test results

Nick Price 17Nick Price 17thth October 2006 October 2006

AimsAims

to reflect on the implications of a to reflect on the implications of a study of health professional's study of health professional's interpretation of a test resultinterpretation of a test result

to develop skills in interpreting test to develop skills in interpreting test resultsresults

ObjectivesObjectives

By the end of the session you should be able to:By the end of the session you should be able to: Define sensitivity in ordinary languageDefine sensitivity in ordinary language Define specificity in ordinary languageDefine specificity in ordinary language Understand how the prevalence of a condition in Understand how the prevalence of a condition in

your test population influences the significance of your test population influences the significance of a positive test result in a particular patient.a positive test result in a particular patient.

Understand how 'testing more patients, just in Understand how 'testing more patients, just in case' will influence the likelihood of a patient with case' will influence the likelihood of a patient with a positive result having the condition.a positive result having the condition.

Understand to term 'positive predictive value'.Understand to term 'positive predictive value'. Have an opportunity to try explaining the result of Have an opportunity to try explaining the result of

a test to your peers. a test to your peers.

SensitivitySensitivity How many true positives in comparison to How many true positives in comparison to

the ‘gold standard’.the ‘gold standard’.Or (most accurately)Or (most accurately) The chance of having a positive test, The chance of having a positive test,

assuming that you do have the condition.assuming that you do have the condition.OrOr So with a very So with a very SSeennsitive Test a sitive Test a NNegative egative

will rule will rule OutOut the condition – the condition – SnNOutSnNOutOrOr So a sensitive test is likely to pick up the So a sensitive test is likely to pick up the

condition.condition.

Sensitivity 2Sensitivity 2

Can you think of some tests with Can you think of some tests with very high sensitivity in very high sensitivity in comparison to a gold standard?comparison to a gold standard?

e.g. D-dimer (99%), Leucocytes on e.g. D-dimer (99%), Leucocytes on Multistix (87%), random blood sugarMultistix (87%), random blood sugar

SpecificitySpecificity

(most accurately)(most accurately) The chance of having a negative test given The chance of having a negative test given

that you do not have the disease. that you do not have the disease. OrOr How many false negatives.How many false negatives.OrOrWith a very With a very SpSpecific test a ecific test a PPositive result ositive result

rules the condition rules the condition IN -SpPinIN -SpPin So with a specific test a positive test is So with a specific test a positive test is

likely to mean you have the condition.likely to mean you have the condition.

Specificity 2Specificity 2

Can you think of some very specific Can you think of some very specific tests?tests?

3+ of glucose and ketones on multistix?3+ of glucose and ketones on multistix?

A hard craggy breast lump?A hard craggy breast lump?

A yes score of 3+ on CAGE (99.8%)A yes score of 3+ on CAGE (99.8%)

Some not very specific ones:Some not very specific ones:

Moderately raised random blood sugar in general Moderately raised random blood sugar in general populationpopulation

The Truth Table

         TRUTH

POSITIVE NEGATIVE

TEST

POSITIVE a b

NEGATIVE c d

Sensitivity is the probability [a / (a + c) in the table] that a true positive has been correctly classified as positive by the test.

Specificity is the probability [d / (b + d)] that a true negative is correctly classified negative by the test

ExampleExample

With leukocyte esterase dipstix (LED) With leukocyte esterase dipstix (LED) for chlamydia vs ‘gold standard’for chlamydia vs ‘gold standard’

In a GUM clinic 500 patients were In a GUM clinic 500 patients were tested, 100 tested positive with gold tested, 100 tested positive with gold standard, 90 tested positive with standard, 90 tested positive with LED. Of these 90, 5 were in fact LED. Of these 90, 5 were in fact negative with the gold standard.negative with the gold standard.

What is the sensitivity and specificity What is the sensitivity and specificity of LEDof LED

Example 2Example 2Sensitivity = 85/100 = 85%Sensitivity = 85/100 = 85%

Specificity = 395/400 = 98%Specificity = 395/400 = 98%

TruthTruth

TestTest

++ -- TotalTotal

++ 8585 55 9090

-- 1515 395395 410410

TotalTotal 100100 400400 500500

So what is the chance that a So what is the chance that a positive LED test means you have positive LED test means you have

chalmydia?chalmydia?

Aka what is the ‘positive predictive value’ Aka what is the ‘positive predictive value’ (PPV).(PPV).

This is the true positives / true positives This is the true positives / true positives and the false positivesand the false positives

PPV = a/a+c = 85/90 = 94%.PPV = a/a+c = 85/90 = 94%.

Excellent, so this is a good test to use in GP Excellent, so this is a good test to use in GP e.g. routinely when taking smears!e.g. routinely when taking smears!

PPV 1PPV 1

So the incidence of chlamydia in the general So the incidence of chlamydia in the general population of all women having smears in population of all women having smears in GP is say 5%.GP is say 5%.

We do 500 smears a yearWe do 500 smears a year

We have a test that has sensitivity of 85% We have a test that has sensitivity of 85% and a marvellous specificity of 98%.and a marvellous specificity of 98%.

What chance the patient with a positive test What chance the patient with a positive test actually has chlamydia in this context?actually has chlamydia in this context?

Example 3Example 3Sensitivity = 85%Sensitivity = 85%Specificity = 98%Specificity = 98%

PPV = 21/31 = 67%PPV = 21/31 = 67%

NPV = 465/469 = 99%NPV = 465/469 = 99%

TruthTruth

TestTest

++ -- TotalTotal

++ 2121 1010 3131

-- 44 465465 469469

TotalTotal 500x5% 500x5% = 25= 25

475475 500500

So the incidence of the disease So the incidence of the disease greatly effects the PPV or how greatly effects the PPV or how many patients you will see with many patients you will see with

false positive test resultfalse positive test result

So what about the case in the So what about the case in the experimental study?experimental study?

1% of babies have Down’s1% of babies have Down’s If the baby has Down’s 90% will have If the baby has Down’s 90% will have

+ve test.+ve test. If the baby does not have Down’s 1% If the baby does not have Down’s 1%

chance the result will be positivechance the result will be positive With a +ve result what is the chance With a +ve result what is the chance

baby has Down’s?baby has Down’s?

So what about the case in the So what about the case in the experimental study? 2experimental study? 2

1% of babies have Down’s 1% of babies have Down’s (incidence)(incidence)

If the baby has Down’s 90% will have If the baby has Down’s 90% will have +ve test. (90% sensitivity)+ve test. (90% sensitivity)

If the baby does not have Down’s 1% If the baby does not have Down’s 1% chance the result will be positive chance the result will be positive (99% specificity)(99% specificity)

With a +ve result what is the chance With a +ve result what is the chance baby has Down’s? (PPV)baby has Down’s? (PPV)

Example 4 – Maths solutionExample 4 – Maths solutionSensitivity = 90%Sensitivity = 90%Specificity = 99%Specificity = 99%

PPV = 90/190 = 47%PPV = 90/190 = 47%

NPV = 9800/9810 = 99.9%NPV = 9800/9810 = 99.9%

TruthTruth

TestTest

++ -- TotalTotal

++ 9090 100100 190190

-- 1010 98009800 98109810

TotalTotal 100100 99009900 1000010000

Example 4 – narrative solutionExample 4 – narrative solution

Read the paper!Read the paper!

Now practice explaining one of these Now practice explaining one of these example in trios, then rotate.example in trios, then rotate.

ObjectivesObjectives

By the end of the session you should be able to:By the end of the session you should be able to: Define sensitivity in ordinary languageDefine sensitivity in ordinary language Define specificity in ordinary languageDefine specificity in ordinary language Understand how the prevalence of a condition in Understand how the prevalence of a condition in

your test population influences the significance of your test population influences the significance of a positive test result in a particular patient.a positive test result in a particular patient.

Understand how 'testing more patients, just in Understand how 'testing more patients, just in case' will influence the likelihood of a patient with case' will influence the likelihood of a patient with a positive result having the condition.a positive result having the condition.

Understand to term 'positive predictive value'.Understand to term 'positive predictive value'. Have an opportunity to try explaining the result of Have an opportunity to try explaining the result of

a test to your peers. a test to your peers.