Download - Interpreting test results –Sensitivity, Specificity, positive and negative predicted values

• Interpreting test results– Sensitivity, Specificity, positive and negative predicted

values

Sensitivity

The chance of having a positive test result given that you have the disease (not to be confused with the positive predicted value which is the other way around.)

REMEMBER SnNOut – If the sensitivity is high and the test negative, the disease (infection) can be ruled out. A very high sensitivity is associated with a very high negative predicted value.

Specificity

The chance of having a negative test result given that you do not have a disease (not to be confused with the negative predicted value which is the other way around.)

REMEMBER SpPIn – If the specificity is high and the test positive, the disease (infection) can be ruled in. A very high specificity is associated with a very high positive predicted

value

The Take home message first – then how we got thereThe Take home message first – then how we got there

Disease present (ICP)

Disease absent

Test positive (no pulsation)

100 A B 200

Test negative (pulsation)

0 C D 1000

SensitivitySensitivity

Retinal vein pulsation Retinal vein pulsation and raised ICPand raised ICP

On fundoscopy, look On fundoscopy, look for vein pulsation. for vein pulsation. Normally the vein Normally the vein pulsates. The test looks pulsates. The test looks for absence of for absence of pulsation. If there is NO pulsation. If there is NO pulsation, the test is pulsation, the test is positivepositive

Sensitivity = Sensitivity = AA//(A+C) (A+C) Sensitivity = 100/100+0 =100%Sensitivity = 100/100+0 =100%


Disease absent


100 A B 200


0 C D 1000


Positive predictive value = Positive predictive value = AA//(A+B) (A+B) Positive predictive value = 100/100+200 = 33.3%Positive predictive value = 100/100+200 = 33.3%

On fundoscopy you cannot see venous pulsation. On fundoscopy you cannot see venous pulsation. How do you interpret the test?How do you interpret the test?

Sensitivity = 100%Sensitivity = 100%

Negative predictive value = Negative predictive value = DD//(D+C) (D+C) Negative predictive value = 1000/1000+0 = 100%Negative predictive value = 1000/1000+0 = 100%


Disease absent


100 A B 200


0 C D 1000



On fundoscopy you cannot see venous pulsation.On fundoscopy you cannot see venous pulsation.

+ve predictive value = 33.3%+ve predictive value = 33.3%

Raised ICP in 33.3% of cases – but with a sensitivity of 100% it Raised ICP in 33.3% of cases – but with a sensitivity of 100% it willwill pick up all cases of those patients with raised ICP,pick up all cases of those patients with raised ICP,

but with such a poor +ve pred value, it is not a very good diagnostic but with such a poor +ve pred value, it is not a very good diagnostic test, i.e. you cannot diagnose raised ICP by absent venous pulsationtest, i.e. you cannot diagnose raised ICP by absent venous pulsation

-ve predictive value = 100%-ve predictive value = 100%


Disease absent


100 A B 200


0 C D 1000



+ve predictive value = 33.3%+ve predictive value = 33.3%

-ve predictive value = 100%-ve predictive value = 100%However, if the test is negative i.e. you see venous pulsation, then However, if the test is negative i.e. you see venous pulsation, then you can be sure the patient you can be sure the patient DOES NOTDOES NOT have raised ICP, i.e., have raised ICP, i.e., raised ICP is raised ICP is excludedexcluded and you can go ahead and do an LP and you can go ahead and do an LP without a CAT scan.without a CAT scan.

If the test is negative, the disease can be ruled outIf the test is negative, the disease can be ruled out

SeNsitivity Negative test means disease ruled OUT ( or SnNOut)SeNsitivity Negative test means disease ruled OUT ( or SnNOut)

Disease present (alcoholism)

Disease absent

Test positive (yes to CAGE )

100 A B 1

Test negative (no to GAGE)

50 C D 99

SpecificitySpecificity

CAGE question for CAGE question for alcoholism.alcoholism.

Cutting down, Cutting down, Annoyance, Guilt, Eye Annoyance, Guilt, Eye opener are the questions opener are the questions asked. asked.

Specificity = Specificity = DD//(D+B) (D+B) Specificity = 99/99+1 =99%Specificity = 99/99+1 =99%


Disease absent


100 A B 1

Test negative (no to CAGE )

50 C D 99


Positive predictive value = Positive predictive value = AA//(A+B) (A+B) Positive predictive value = 100/100+1 = 99%Positive predictive value = 100/100+1 = 99%

The patient scores 0 with the CAGE question.The patient scores 0 with the CAGE question.How do you interpret the test?How do you interpret the test?

Specificity = 99%Specificity = 99%

Negative predictive value = Negative predictive value = DD//(D+C) (D+C) Negative predictive value = 99/99+50 = 66%Negative predictive value = 99/99+50 = 66%


Disease absent


100 A B 1


50 C D 99



The patient scores 0 with the CAGE question.The patient scores 0 with the CAGE question.

+ve predictive value = 99%+ve predictive value = 99%

The patient The patient does not havedoes not have an alcohol problem in 66% of cases – but an alcohol problem in 66% of cases – but with a specificity of 99%, it will be a negative test in virtually all with a specificity of 99%, it will be a negative test in virtually all patients without alcoholism.patients without alcoholism.

but with such a poor -ve predictive value, it is but with such a poor -ve predictive value, it is notnot a very good a very good screening test, i.e. screening test, i.e. you cannot excludeyou cannot exclude alcoholism on the basis of a alcoholism on the basis of a negative CAGE question.negative CAGE question.



Disease absent


100 A B 1


50 C D 99



+ve predictive value = 99%+ve predictive value = 99%


However, if the test is positive i.e. the patient replies in agreement However, if the test is positive i.e. the patient replies in agreement to the CAGE questionnaire, then you can be sure the patient to the CAGE questionnaire, then you can be sure the patient DOES have an alcohol problem.DOES have an alcohol problem.

If the test is positive, the disease/illness can be ruled inIf the test is positive, the disease/illness can be ruled in

SPecificity Positive test means disease ruled IN ( or SpPIN)SPecificity Positive test means disease ruled IN ( or SpPIN)

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