Introduction to screening Flashcards
Screening
Screening is the application of a test to people who are as yet asymptomatic for the purpose of classifying them with respect to their likelihood of having a particular disease.
aka
Screening involves testing people who don’t have symptoms yet to see if they might have a certain disease.”
UK national screening committee
People from a specific group, who might not realise they could get a disease or its complications, are asked questions or offered tests. This helps find out who would benefit more than be harmed by additional tests or treatment.”.
Why do we screen?
To reduce morbidity or mortality from the disease among the people screened
Break the chain of transmission and development of new cases
Early detection can be cost effective
Not intended to be diagnostic
Results of screening trigger diagnostic work-up and preventive interventions
3 types of screening
mass screening:
directed at a whole population or subgroup irrespective of disease (i.e newborn hearing checks, screening in pregnancy etc)
selective screening :
focused on groups at high-risk of developing the disease (i.e gestational DM screening in pregnancy, annual mammography for BRCA carriers)
case finding/ opportunistic screening:
when consult for other reason (i.e check BP when come for flu jab, oral cancer screening by dentist)
Eff
Effects of screening
lead time: time by which the diagnosis is early due to screening
early treatment: resulting in postponing or avoiding the disease
Disadvantages of screening
over- diagnosis (i.e mammogram)
mis-diagnosis (False positives (e.g. Down’s syndrome, psychological harm)
False negatives – (eg false reassurance → late diagnosis in cancer)
psychological harm
criteria for screening (3 considerations)
The disease
- the disease should be serious i.e causes death, disability or discomfort)
- the natural history should be understood
- the disease must have a recognisable latent period before symptoms appear (or early symptomatic stage)
- The latent period between first signs and overt disease should be long enough that screening significantly advances the detection of disease
- prevalence of the disease (screening not rewarding for extremely rare diseases) but there are exceptions (i.e PKU)
Diagnosis and treatment
- Facilities need to be available
- There is an available, effective, acceptable, and safe treatment
- Treatment at early stage better
The screening test
- Should be valid (accurate)
- Reliable
- Simple and cheap
- Safe and acceptable (benefit outweighs harm)
Feasibility of screening
- Screening procedures: convenient and free of discomfort or risk; attractive to target population
- Efficiently and economically
- Treatment available
- High level of case detection (sensitivity) and reasonably low level of false-positive tests (specificity)
Criteria for screening
Common disease
Substantial morbidity/mortality
A preclinical detectable phase
An effective treatment
Improvement in prognosis by treatment in PCDP
A test with good performance
…and which is tolerable (side-effects, acceptable)
…and cost-effective
Measures of screening test performance
Validity
Sensitivity
Validity
The ability of a screening test to accurately identify individuals with and without disease
Validity is measured by sensitivity & specificity
Sensitivity
Ability of the test to identify those with disease correctly
Increase in sensitivity will increase number of true positives.
Sensitivity = true positive / (true positive + false negatives)
Increase in sensitivity will increase false positive (i.e decrease specificity)
Specificity
Ability of test to identify those without disease correctly
Determines whether or not the frequency of false positives will be low enough for a screening programme to be feasible.
Determines the number of false positives.
A test with high specificity will have few false positives
An ideal screening test would have 100% sensitive and 100% specific (ie no false + or false-), but in practice these are usually inversely related. It is possible to vary the sensitivity by changing the specificity
Specificity: true negative/ true negative + false positives
Increased specificity means increase in false negatives (i.e decrease sensitivity)
Receiver Operator Characteristic curves (ROC)
Evaluates test over range of cut-off points
–> performance of a screening test
–> method for comparing screening tests
Plots sensitivity against false-positive rate (1 – specificity) and illustrates the trade-off that exists between them
AUC
Area under the curve is a measure of the test’s accuracy:
AUC= 1 indicates a perfect test
AUC = o.5 indicates no better than chance alone
Summary sensitivity and specificity
Calculated in different populations
Both are needed to understand test performance
Do not add up to one – but gain in one is loss in the other
They are not fixed
Performance (predictive value)
Provides information on how sure we are about a test result (how predictive is the test result itself?)
PPV & NPV
Positive predictive value
Proportion of people with a positive test who have the disease in question.
A high PPV indicates that a reasonably high proportion of the costs of a programme are being expended for the detection in the preclinical phase.
A low PPV indicates that a high proportion of the costs are being wasted on the detection and diagnostic evaluation of false positives.
PPV = TP/(TP+FP)
Negative predictive value
Proportion of people with a negative test who do not have the disease in question.
An NPV close to 1 indicates that testing negative is reassuring as to the absence of disease and that re-screening may not be worthwhile.
NPV = TN/(TN+FN)
Summary PPV and NPV
Determinants of the PPV and NPV for a given screening test are the specificity and sensitivity of the test and the prevalence of disease in the target population
High prevalence = higher the PPV (the more likely a positive result predicts presence of disease)
Low prevalence = PPV will also be low, even when using a test with high sensitivity and specificity wasted resources
Evaluating a screening programme: essential considerations
Study design
- Observational studies (case-control & cohort studies) – commonly used but subject to bias
- Randomised Controlled Trials (RCTs) – ‘gold standard’
Study population & outcome and exposure assessment are also important
Selection bias
People who choose to participate in screening tend to be healthier, have healthier lifestyles, and their outcomes tend to be better because of this
‘Healthy volunteer’ effect, ‘worried-well’
Length time bias
The seemingly improved survival results from finding many slow-growing tumours in diseases that take a long time to develop symptoms.
Lead time bias
The apparent better survival observed for those screened because diagnosis is being made at earlier point in natural history of disease.
RCTs
Main strength of randomization is that the distribution of measured and unmeasured factors that could impact disease outcome is randomly distributed among the screened and unscreened groups
