Screening Flashcards
What is screening
Screening is the practice of investigating apparently healthy individuals with the object of detecting unrecognised disease
or its precursors in order that measures can be taken to prevent or delay the development of disease or improve
Screening test and diagnostic test difference
Diagnostic tests normally give you more definite answers of whether or not you have the disease. Results from screening normally only give you a ‘risk’ e.g. high levels of x means your more likely to get a disease
What does validity mean for a test?
ability to distinguish between who has a disease and who does not and has 2 components: sensitivity and specificity
What is sensitivity of a screening test
Ability to pick up true positives
Of those who have the disease, how many of them are picked up by the test
What is specificity of a screening test
ability to exclude true negatives
Of those who do not have the disease, how many of them were actually excluded by the screening test
What is positive predictive value
Of those who tested positive, how many of them were true positives
What is negative predictive value
Of those test negative, how many of them are true negatives
examples of diseases screened during antenatal screening?
Split into 4 groups:
Chromosomal, Downs Pataus and Edwards
Infections: syphilis, HIV, Hep B
Physical abnormalities (anomaly scan)
Metabolic: phenylketonuria (PKU) and hypothyroidism
Haemoglobinopathies: Sickle cell disease andthalassaemia
Examples of newborn screening
Physical exam (includes eyes, heart, hips, testes)
• Hearing test
• Blood spot (for rare metabolic conditions) - also called heel prick test
Examples of adult cancer screening in UK
Breast (women 50 – 70)
• Cervical (women 25 – 64)
• Bowel (men and women 60 – 74)
What is lead time bias?
Early diagnosis results in increase in time from diagnosis to death even if natural history of disease is unaltered by intervention. This may be result in an apparent improvement in length of survival when it is due to the fact that it was diagnosed early
What is selection bias in terms of screening
Those who attend screening are likely to be different to those who dont attend screening
What is length bias
Length bias exists as some conditions may be slower in developing to a health threatening stage, that is, they have a longer preclinical stage. This means they are more likely to be detected at that stage but they may also have a more favourable prognosis compared to fast developing diseases.This may lead to the false conclusion that screening is beneficial due to the fact that there is a disproportionately larger proportion of slow developing cases captured by the screening compared to fast growing cases
Three types of biases that can arise in screening
length bias
lead time bias
selection bias
What are predictive values dependent on?
dependent on sensitivity, specificity, and the prevalence of disease ( see example in iBOOK)
What curves are used to determine cut off values in a diagnostic test? What are the labels on the axis ?
ROC curves
used in medicine to determine a cut-off value for a diagnostic or screening test. The ROC curve is a graph of sensitivity (y-axis) vs. 1-specificity (x-axis). The choice of a particular cut-off value for a test is informed by the attempt to maximize sensitivity and specificity.
Screening can be targeted to the whole population or just to a specific group of people. What do you call these ?
Mass screening- whole population
targeted screening- specific group
Within each of these types of screening, how can they be further subdivided?
Mass and targeted screening can each be subdivided into systematic and opportunistic
Systematic= when people are called for screening in a systematic way
opportunistic = eg chlamydia screening in the young people where patients present to doctors and then doctor offer them screening
What is the criteria for screening to be offered for a disease? Consider factors relating to the disease itself, the diagnostic test and the potential treatment options
Disease:
- important health problem
- well recognised and detectable pre-clinical stage
- natural history of disease understood
- long period between first signs and proper presentation of disease
Diagnostic test: valid- sensitive and specific simple + cheap safe reliable
Diagnosis and treatment: adequate facilities effective, acceptable and safe treatment available cost effective sustainable
What are the ethical problems surrounding screening?
Screening itself and the diagnostic tests following it may have risks attached
False positive may give unnecessary anxiety
false negative may give unnecessary reassurance
What does ‘gold standard’ mean?
a recognised way of determining who really has the disease
