Diagnostic Tests & Screening? Flashcards
Define Validity?
Validity = truth of result, lack of bias
What is Reliability?
Reliability = consistency of results
* repeatability
* reproducibility
How do we test reliability
Repeat measurements improve reliability
Within-observer repeatability
- take several BP readings
- duplicate or triplicate readings in biochemical assays
- tests repeated on more than one occasion Between-observer repeatability
- X-rays or histology slides read by two observers
Nevertheless, a reliable test might not give valid results
Clasification of a test
TP, true positive
FP, false positive
FN, false negative
TN, true negative
Whats the PPV?
PPV = TP / (TP+FP)
Proportion with positive test who have disease
If someone has a positive test, what is the probability that they will have the disease?
POSITIVE PREDICTIVE VALUE
Whats Specificity?
Specificity = TN / (TN+FP)
Proportion without disease who test negative
If someone does not have the disease, what is the probability that the test result will be negative?
SPECIFICITY
Whats Sensitivity?
Sensitivity = TP / (TP+FN)
Proportion with disease who test positive
If someone has the disease, what is the probability that the test result will be positive?
SENSITIVITY
If there are 1000 subjects, what is PPV? Prevelance? Sensitivity? Specificity?
What is screening?
- Actively identifying disease, or pre-disease, in apparently healthy subjects who may benefit from early treatment
- Testing, case-finding and ‘opportunistic screening’ are not screening
- Population screening, an entire population at risk is called to be screened
What is the natural history of disease?
When does screening take place?
Criteria for implementing a screening programme
- Condition
- Test
- Treatment
- Screening programme
- Role of UK National Screening Committee (NSC)
What is Criterion 1?
Criterion 1: the condition is important in terms of frequency, impact on affected individuals
What is Criterion 2?
Criterion 2: Primary prevention is not effective or feasible
What is Criterion 3?
Criterion 3: There is an identifiable pre-clinical stage of disease
What is Criterion 4?
Criterion 4: There is a test that can separate those with a high probability of the disease from those with a low probability
What are the charectrisitics of the test?
- The test should be simple, safe, precise and have been validated
- There should be a clear distinction between ‘normal’ and ‘abnormal’ results
- The test should be acceptable to subjects, and costs should be ‘reasonable’
- There should be an agreed policy on the further diagnostic investigation of individuals with a positive test result and on the choices available
What is Criterion 5?
Criterion 5: Effective treatment is available; the outcome of early treatment is better than later treatment after clinical diagnosis
What is lead time bias?
Lead time bias
Screen-detected cases appear to survive longer
Lead time: interval between the diagnosis of a disease at screening and the usual time of diagnosis (by symptoms)
What is length bias?
Length bias
Slow growing tumours may be more often detected through screening
What are the Biases associated with screening?
- Lead time (interval between the time of detection by screening and the time at which the disease would have been diagnosed)
- Length-biased sampling (rapidly progressive disease cause the individual to consult, but less rapidly progressing cases are likely to remain for screening detection)
- Selection bias (those who enter screening almost invariably are more conscious than those who decline)
- Overdiagnosis bias (some lesions identified as disease in a screening programme would not have presented clinically during the individual’s life time)
Solution to biases
- Randomised control trials when mortality rather than survival is used as the outcome
- Survival can be used only if there is no evidence of overdiagnosis bias and observation period is from randomisation date.
- Individual and community trials
Advantages of screening?
- Improved prognosis for true positives
- Less radical treatment required
- May be resource savings
- Reassurance for those with true negative test results
Disadvantages of screening
- Longer period of awareness for true positives whose prognosis is unaltered
- Over-treatment of borderline abnormalities * False reassurance of false negatives
- Anxiety and hazard for false positives
- Hazard of screening test to all recipients
