Screening Tests

Screening Tests Sensitivity & Specificity

Sensitivity & Specificity • They are measures of a Test’s validity, I.e. its ability to correctly detect people with disease and without disease.

Sensitivity • Of a test is its ability to detect people who do have disease. • If a Test is always positive for all diseased persons then sensitivity of the Test will be 100%

Specificity • It is the ability of a Test to detect people who don’t have disease. • Thus a Test which is always negative in non- diseased individuals is called to have 100% specificity

Test

• Sensitivity = a x 100 = 60 x100 = 75% a+c 80 I.e. Test (FNA) is 75% sensitive in detecting disease • Specificity = d x 100 = 70 x100 = 58% d+b 120 I.e. Specificity of (FNA) is 58% to detect non- diseased persons

Positive Predictive Value i.e PPV • PPV = a x 100 = 60 x100 = 55% a+b 110 I.e. 55% persons are actually suffering from disease. PPV ∝ Prevalence Negative Predictive Value i.e NPV • NPV = d x 100 = 70 x100 = 78% c+d 90 I.e. 78% persons are actually free from disease.

Test

Disease Present

Disease Not Present

Total

Positive

True Positive (TP) + +

False Positive (FP) - +

TP + FP

Negative

False Negative True Negative (FN) + (TN) - -

FN + TN

Total

TP +FN

TN + FP

TP+FP+TN+FN

•Sensitivity = TP x 100 TP + FN

•PPV= TP x100 TP + FP

•Specificity = TN x100 TN + FP

•NPV = TN x100 TN + FN