19.07.08 Screen vs test Flashcards
What is a screen
Process of identifying apparently healthy individuals who may be at increased risk of a disease/condition.
Aim of a screen
Early risk identification, thus save lives or improve quality of life. Risk reduction.
Is a screen considered to provide a diagnostic result
No.
Who does a screen target
Populations. If at-risk individuals are identified, they can then be offered info and further tests/ treatments/ surveillance if appropriate.
What do screening individuals need to understand
Screening does not exclude false positives/negatives. It does not guarantee protection.
Measures of screening performance
Specificity and sensitivity
Specificity is
High specificity means screen has a few false positives as possible. Proportion of those without the condition who return a negative screen result.
Sensitivity is
High Sensitivity means the screen captures as many individuals with the condition as possible. Measured as the proportion of those with the condition who have a positive screen result. Detection rate.
Types of screening
Mass, high risk/selective, multiphasic,
What is mass screening
Screening of a whole population or a subgroup. Offered to all, irrespective of the risk status of the individual.
What is high risk/selective screening
Screening of risk populations only.
What is multiphasic screening
Application of two or more screening tests to a large population at one time in stead of carrying out separate screening tests for single diseases.
Examples of cancer screening programmes
- Pap smear, to detect potentially precancerous lesions and prevent cervical cancer.
- Mammography: to detect breast cancer.
- Colonoscopy and faecal occult blood tests to detect colorectal cancer.
Examples of ultrasound screening programmes
- screening for infectious diseases (Hep B, HIV, syphilis)
- Inherited conditions (sickle cell, thalassaemia)
- Down’s, Edwards’, Patau’s syndrome
- Structural abnormalities (18-21 week scan)
Other screening programs
- Potential sperm bank donors
- Hearing loss in newborns
- Newborn screening for serious health conditions (CF)
Does a test provide a diagnostic result
Yes, unless inconclusive. Will have clinical utility. Establishes presence/absence of disease.
What is clinical utility of a test
A test’s capacity to rule a diagnosis in/out and to make a decision possible.
What factors determine the utility of a diagnostic test
- Association between results and disease
- Pre-test probability of a disease
- demand of test to rule in/out disease with regard to post-test probability.
Difference between a screen and a test with regard to purpose
- Screening: to detect potential disease indicators
- Diagnostic test: to establish presence/absence of disease
Difference between a screen and a test with regard to target population
- Screening: large numbers of asymptomatic but at-risk individuals.
- Diagnostic tests: symptomatic individuals to establish a diagnosis or asymptomatic individuals with a positive screening test.
Difference between a screen and a test with regard to test method
- Screening: simple, acceptable to patients and staff
- Diagnostic tests: can be invasive, expensive but justifiable to establish a diagnosis
Difference between a screen and a test with regard to positive result threshold
- Screening: High sensitivity, to not miss potential disease
- Diagnostic tests: high specificity (true negatives). More weight to accuracy and precision.
Difference between a screen and a test with regard to positive result
- Screening: Indicates suspicion of disease, requires confirmation
- Diagnostic tests: provides a definite diagnosis.
Difference between a screen and a test with regard to cost
- Screening: Cheap, large numbers will be tested to identify a small number of potential cases.
- Diagnostic tests: high costs justified to establish a diagnosis.
