Principles of Screening for malignancy

Screening of a population is used to detect disease in individuals without signs & symptoms of that disease.
The aim is early detection thus enabling early intervention & management in the hope to reduce mortality & morbidity,

Issues

Overdiagnosis – PSA – more men die with prostate cancer than of it
False positive
Misdiagnosis
False sense of security – false negative
Cost & use of medical resources
Adverse effects from screening – stress & anxiety, discomfort, radiation exposure, chemical exposure
Adverse effects from further Ix in false positive (ie: biopsy)

Lead-time bias
- Two people, one screened, one not, the non-screened one picked up on symptoms 6 months later. Both die same day, it could look like screened patient had survival advantage as had longer survival from diagnosis
Length bias – screening more likely to pick up slower growing tumours
Selection bias – there will be differences in those that choose to take up screening

Universal screening – screening of all individuals in a certain category
Case finding – screening a smaller group based on the presence of risk factors (family member been diagnosed with a hereditary disease)

Important health problem
Recognisable latent or early symptomatic stage
The natural history is understood
Accepted treatment
Suitable test or examination that has a high level of accuracy
Test should be acceptable to the population
Agreed policy on whom to treat as patients
Facilities for diagnosis and treatment should be available
Cost of screening should be economically balanced in relation to possible expenditure on medical care as a whole
Screening should be a continuing process

sensitivity = proportion of people with the disease who test positive (a/a+c)
specificity = proportion of people without the disease who test negative (d/d+b)
+ve predictive value = chance that people have the disease if they test positive (a/a+b)
- increases when the prevalence in a population is high
-ve predictive value = chance that people don’t have the disease if they test -ve (c/c+d)
- increases when the prevalence in a population is low