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.
Example of screen vs test for foetal anomaly screening
- Screen: maternal serum screening or ultrasound scan
- diagnostic test: genetic testing (QF_PCR, microarray, etc), physical examination at birth.
Example of screen vs test for newborn blood spot screening
- Screen: biochemical analysis for analytes, e.g. IRT
- Diagnostic test: Gene mutation analysis of CFTR.
Example of screen vs test for cancer
- Screening: swabs, imaging
- Diagnostic test: biopsy, histopathology, genetic testing
Who advises the government on aspects of screening
UK National Screening Committee (UK NSC)
What does the UK NSC do
- Reviews policies on screening for various conditions based on new research
- Modifying/ implementing/ withdrawing programs.
Main criteria for a new screening programme
- Condition
- Test
- Treatment
- Screening programme
How does the condition affect development of a screening programme
- Is it an important health problem
- Are there detectable risk factors/ disease markers in early symptomatic stage. Is the epidemiology understood.
- Is it cost-effective
- Impact of identifying disease carriers.
How does the test affect development of a screening programme
- Simple, safe, precise and validated.
- Test values should be know, define the cut-offs
- What further investigations required when a positive result is found
How does the test treatment development of a screening programme
- Effective treatments implemented earlier should lead to better outcomes
- Policies of who should be offered the test.
- Clinical management should be optimised and standardised across all health providers.
What other criteria should be considered development of a screening programme
- Evidence from randomised controlled trials show that programme is effective in the reduction of mortality/morbidity.
- Is the test socially and ethically acceptable
- Benefits should outweigh the physical and psychological harm (caused by test/treatment)
- Value for money
- Adequate staffing, equipment, facilities available
- Information available to allow patients to make an informed choice.
- Justify eligibility criteria, there may be public pressure.
How many screening policies does the UK NSC have
~100
How long does it take the UK NSC to review a screening programme
6-24 months
How often does the UK NSC review programmes
Every 3 years.
