Screening

Sherine Shawky, MD, Dr.PH

Assistant Professor Public Health

King Abdulaziz University College of Medicine

shshawky@hotmail.com

Epidemiology in Medicine

Foundation of medical researchSupport for medical advancementEssential in medical education &

practiceBoundary for biostatistics

Learning ObjectivesUnderstand the concept of

screeningUnderstand the characteristics of

health-related events suitable for screening

Recognize the characteristics of a screening program

Performance Objectives

Apply screening for early detection of risk factors & diseases

Identify the factors suitable for screening

Evaluate the benefit of screening

Screening

Screening is the process to detect among healthy

people disorders or risk factors of which they are

unaware

Flow diagram for a screening program

Population

Test -ve Test +ve

Unaffected Affected

Intervention

Diagnostic procedures

Screening test

Re-screen Re-screen

Types of screening

Mass Multiple or multiphasic Targeted Case-finding or opportunistic

Factors appropriate for screening

Important health problemHigh prevalence Natural history understoodLong latent periodEarly detection improves prognosis

Evaluation ofa screening program

ReliabilityFeasibilityValidityPerformanceEffectiveness

Reliability

Biological variationProgram method Intraobserver variabilityInterobserver variability

Feasibility

AcceptabilityQuickEasySafe

Cost effectivenessScreeningDiagnosisFollow-up Intervention

ValiditySensitivity:

Probability to test positive among truly affected

Specificity: Probability to test negative among

truly unaffected

PerformancePV+:- Probability to be affected

among test positivesPV-:- Probability to be unaffected

among test negativesPCC:- Probability to be correctly

classified

Effectiveness

Outcome measures:MorbidityDisabilityMortality

EffectivenessBias

Patient self-selection Lead timeLength

Study designs for screening 1. Correlation Studies

Use:

Description of populationStrength:

Suggest possibility of benefitLimitation:

Can’t test hypothesis

Study designs for screening 2. Analytical Studies

Types: Case-controlCohorts

Use: Comparison

of rates

Advantage: Test hypothesis

Limitation: Selection Lead time length

Study designs for screening 3. Randomized Trials

Use:

Comparison of ratesStrength:

Most valid test of hypothesisLimitation:

Cost, ethics & feasibility

Evaluation of screeningAffected+ - Total

+ TP FP SP

Scr

een

ing

- FN TN SNTotal TA TH GT

*Sensitivity= TP/TA *PV+ = TP/SP*Specificity= TN/TH *PV-= TN/SN *PCC= TP+TN/GT

ConclusionObjectives of screening

Reduce disease incidenceReduce morbidity, disability &

mortalityCriteria for screening

Appropriate factorBeneficial program

Review Questions (Developed by the Supercourse team)

• What is screening and what types of screening can you name?

• What are the objectives of screening?

• For what type of diseases would it be appropriate to set up screening programs? List characteristics.

• How is screening program evaluated?