Differences between screening and diagnostic tests and case finding

Screening tests are not diagnostic tests

The primary purpose of screening tests is to detect early disease or risk factors for disease in large numbers of apparently healthy individuals.

The purpose of a diagnostic test is to establish the presence (or absence) of disease as a basis for treatment decisions in symptomatic or screen positive individuals (confirmatory test). Some of the key differences are tabled below:

Table 3.3.1 Differences between screening and diagnostic tests

	Screening tests	Diagnostic tests
Purpose	To detect potential disease indicators	To establish presence/absence of disease
Target population	Large numbers of asymptomatic, but potentially at risk individuals	Symptomatic individuals to establish diagnosis, or asymptomatic individuals with a positive screening test
Test method	Simple, acceptable to patients and staff	maybe invasive, expensive but justifiable as necessary to establish diagnosis
Positive result threshold	Generally chosen towards high sensitivity not to miss potential disease	Chosen towards high specificity (true negatives). More weight given to accuracy and precision than to patient acceptability
Positive result	Essentially indicates suspicion of disease (often used in combination with other risk factors) that warrants confirmation	Result provides a definite diagnosis
Cost	Cheap, benefits should justify the costs since large numbers of people will need to be screened to identify a small number of potential cases	Higher costs associated with diagnostic test maybe justified to establish diagnosis.

Case finding

Case finding is a strategy for targeting resources at individuals or groups who are suspected to be at risk for a particular disease. It involves actively searching systematically for at risk people, rather than waiting for them to present with symptoms or signs of active disease. Note the similarities to screening - both seek to risk stratify the population for further investigation. Examples of case-finding strategies include:

Communicable disease control

Case finding is a key strategy in communicable disease outbreak management (e.g. sexual partner ascertainment in syphilis outbreaks; household/work contacts in food-borne outbreaks). The purpose is to identify at-risk individuals and offer them screening and treatment if necessary.

Health systems data

Can be used to identify 'missed' risk groups (e.g. registered GP patients over 50 years of age with a BMI>30 who may not be on the register of people at risk for coronary heart disease)
Using population based data such as the Index of Multiple Deprivation to target interventions at disadvantaged populations (https://www.nice.org.uk/guidance/ph15/evidence/statins-report-371204173)
The King's Fund software for 'patients at risk of re- hospitalisation' (PARR) use patterns in routinely collected data to forecast which individuals are at higher risk of emergency hospital admission in the forthcoming year (www.kingsfund.org.uk/parr).

Opportunistic screening

Screening activities can be organised in formal screening programmes (as described above, e.g. breast cancer screening) or opportunistically (e.g. assessment of smoking status during primary care visits or chlamydia screening to men and women aged under 25 years). Opportunistic screening is a process by which eligible individuals are able to access a screening test, without the presence of a formal register where invitations are sent to the eligible population.