Screening Flashcards
Define diagnosis.
Definitive identification of a suspected disease or defect by application of tests, examination, or other procedures to definitively label people as either having a disease or not
Patient then prepared to accept the (reasonable) risks & side-effects associated with the treatment in order to get well
Define screening.
Systematic attempt to detect an unrecognised condition by the application of tests, examinations, or other procedures which can be applied rapidly (& cheaply) to distinguish between apparently well persons who probably have a disease (or its precursor) and those who probably do not
NOT A DIAGNOSIS!!!
Can also be defined as: public health service in which members of a defined population who do not necessarily perceive they are at risk of, or are already affected by a disease or its complication, are asked a question or offered a test, to identify those individuals more likely to be helped than harmed by further tests or treatment to reduce the risk of a disease or its complications
What occurs after someone is picked up by screening?
Person is screen-positive (does not mean that they definitively have the disease)
—> further testing is required before diagnosis of disease can be made
—> treatment follows once definitive diagnosis is made
What is the purpose of screening?
Give a better outcome
Therefore if treatment can wait until there are symptoms, then there is no point in screening
i.e. finding something earlier is NOT the primary objective
What are the NHS population screening programmes active at present?
- abdominal aortic aneurysm
- bowel cancer
- breast cancer
- cervical cancer
- diabetic retinopathy
- Down’s syndrome
- foetal abnormalities
- phenylketonuria
- sickle cell anaemia & thalassaemia
- inherited metabolic diseases
What are the criteria related to the disease or condition for a screening programme?
Must be an important health problem
Epidemiology & pathophysiology must be well understood
Must have an early detectable stage
Cost-effective primary prevention interventions must have been considered and, where possible, implemented
What are the criteria related to the test of a screening programme?
Must be:
- simple & safe
- precise & valid
- acceptable
Distribution of test values in population must be known (i.e. proportion who test positive and negative)
Agreed cut-off level must be defined
Agreed policy on who to investigate further
Describe the different features of the effectiveness of a screening test.
SENSITIVITY = proportion of people with disease who test positive (a)
SPECIFICITY = proportion of people without disease who test negative (d)
b = false-positive c = false negative
note: when the SAME test is applied in the SAME way in DIFFERENT populations, the test will always have the SAME sensitivity and specificity
Contrast positive and negative predictive values of a screening test.
POSITIVE PREDICTIVE VALUE =
probability that someone who has tested positive actually has the disease (strongly influenced by prevalence - low prevalence conditions have a low PPV even if the sensitivity and specificity of the test is high)
= true positives (a) / true positives (a) + false positives (b)
NEGATIVE PREDICTIVE VALUE =
proportion of people who test negative who actually do not have the disease
What are the implications of false results from screening?
False positives =
- offered (invasive) diagnostic tests for a condition they do not have
- lower uptake of screening in future & greater risk of interval disease
- increased stress of diagnosis
- increased cost
False negatives =
- not be offered tests which they may have benefitted from
- disease is not diagnosed
- falsely reassured —> may present late with symptoms
What are the criteria related to the treatment of a screening programme?
Effective evidence-based treatment must be available
Early treatment must be advantageous
Policy on who to treat must be agreed upon
Clinical management of condition & patient outcomes should be optimised before participating in screening programme
What are the criteria of the screening programme itself which must be fulfilled?
Effectiveness must be proven (by RCTs)
Quality assurance for whole programme (not just data)
Facilities for diagnosis, treatment, and counselling required
Other options should be considered e.g. improving treatment
Think about opportunity costs
Decisions about parameters should be scientifically justifiable to the public
Benefit should outweigh physical & psychological harm
Give some examples of issues raised by screening.
Alteration of usual doctor-patient contract =
in clinical practice, people self-define as patients but in screening apparently healthy people who have not sought the help of the health service are offered treatment for something they may not have ever thought about
Complexity of screening programmes = e.g. cervical cancer
what age should people be screened for cervical cancer?
- under 25s = cervical cancer extremely rare, but benign changes in the cervix are common
- over 60s = unlikely to develop cervical cancer
How can screening programmes produce biased results?
LEAD TIME BIAS = screening patients appear to survive longer but only because they were diagnosed earlier (patients live same length of time, but live longer knowing they have the disease)
LENGTH TIME BIAS = screening better at picking up slow growing, unthreatening cases
- diseases detectable through screening are more likely to have a favourable prognosis (may have never caused a problem)
- false conclusion that screening is beneficial in lengthening the live of those found positive
SELECTION BIAS = healthy volunteer effect - those who have regular screening are likely to also do other things that protect them from disease (RCT would help but may be unethical)
Give some examples of the limitations of screening.
Need for informed choice as screening has the potential to harm
GPs incentivised for certain screenings
Over-diagnosis e.g. breast cancer screening
Difficulty communicating benefits, harms, risks
