S5) Population-based Screening

• Spontaneous presentation • Opportunistic case finding • Screening

Diagnosis is the definitive identification of a suspected disease or defect by application of tests, examinations or other procedures to definitely label people as either having a disease or not having a disease

Screening is a systematic attempt to detect an unrecognised condition by the application of tests, examinations, or other procedures to identify healthy people who may be at increased risk of disease or condition

To give a better outcome compared with finding something in the usual way (having symptoms and self reporting to health services)

Condition: an important health problem (frequency/severity) with epidemiology, incidence, prevalence and natural history understood

• False positive – refers well people for further investigation, putting them through stress, anxiety and inconvenience - False negative – fails to refer people who have an early form of the disease, inappropriately reasurring them

• Sensitivity • Specificity • Positive predictive value • Negative predictive value

Sensitivity is the proportion of people with the disease who test positive (aka detection rate)

Specificity is the proportion of the people without the disease who test negative

Positive predictive value is the probability that someone who has tested positive actually has the disease

Positive Predictive Value is strongly influenced by the prevalence of the disease

Negative predictive value is the proportion of the people who test negative and actually do not have the disease

• Offered diagnostic testing with all its anxieties and risks for an absent condition • May lead to lower uptake of screening in future and greater risk of interval cancer

• Not offered diagnostic testing when in fact they may have benefited from it • False reassurance – may present late with symptoms as a consequence

• Effective intervention for patients identified through screening - Intervention at a presymptomatic phase leads to better outcomes for the screened individual

• Screening programmes have proven effectiveness in reducing mortality or morbidity • Benefit gained should outweigh any harms e.g. overdiagnosis, overtreatment, false positives, false reassurance, uncertain findings and complications

• Clinical management and patient outcomes should be optimised • Evidence-based information available to potential participants (informed choice) • Public pressure should be anticipated

• Detect many diseases at birth, with no risk to the child • Avoid serious and permanent consequences for the child (presymptomatic treatment)

• Specimens may need to be taken a second time • False positives • False negatives • Complications could still arise

• Lead time bias is when early diagnosis falsely appears to prolong survival • Screened patients appear to survive longer, but only because they were diagnosed earlier

Length time bias involves the easy detection of slow growing, unthreatening cases rather than aggressive, fast-growing ones

• Studies of screening often skewed by ‘healthy volunteer’ effect • Those who have regular screening likely to also do other things that protect them from disease

• NHS breast screening (BSP) programme • NHS cervical screening (CSP) programme • NHS abdominal aortic aneurysm (AAA) programme • NHS bowel cancer screening (BCSP) programme