Lecture 10: Diagnostic Tests and Screening

Screening

Identification of disease or defect

Selective screening

Test for disease in high-risk group

Mass screening

Test for disease with no reference to risk

Opportunistic screening

Screening during general practice/checkups

Purpose of screening

Detect disease/health problems early → treated early

Criteria for screening: Disease

• Severity and Frequency

• Natural history of disease

• Recognizable early symptomatic/latent stage

Criteria for screening: Population

• Identification of high-risk group

• Population attitude to test

Criteria for screening: Test

• The accuracy and availability of the test

• Effectiveness, availability, acceptability of treatment

• Cost-effectiveness

• Sustainability

Evaluation of Screening Programme

• reduction of incidence/mortality

• prolonged survival time

• improved quality of life

Bias in Screening programme evaluation

Lead time bias, length time bias

Lead time bias

Survival time appears longer because diagnosis was done earlier, irrespective of whether patient lived longer

Length time bias

Screening preferentially detects slow-growing disease over aggressive/rapidly progressing diseases

Sensitivity

Ability to correctly identify someone with disease

Sensitivity formula

True Positive / (True Positive +False Negative)

Specificity

Ability to correctly identify someone without disease

Specificity

True Negative / (True Negative + False Positive)

Positive Predictive Value (PPV)

Probability that an individual has the disease given positive result

PPV formula

True positive / (true positive + false positive)

Negative predictive value (NPV)

Probability than an individual does not have the disease given negative result

NPV formula

True negative / (true negative + false negative)

Dependency of PPV/NPV

Frequency dependent → affected by prevalence of condition

Accuracy

(TP + TN) / (TP + TN + FP + FN)

Cut-off point

Determines sensitivity and specificity

Tests to increase sensitivity/specificity

Parallel testing, sequence testing

Parallel testing

• Uses multiple tests

• Positive result in any one test = test positive

Effect of parallel testing on sensitivity/specificity

Increase sensitivity and decrease specificity

Sequence Testing

• Use one test

• Positive in all tests = “test positive”

Effect of sequence testing on sensitivity/specificity

Increase specificity and decrease sensitivity