Screening Flashcards
What is screening?
A process which sorts out apparently “well” people who
probably have a disease (or precursors or susceptibility to a disease) from those who probably do not.
It is not a diagnosis
What is the point of screening?
It is a type of secondary prevention, which aims to detect early disease in order to alter its course.
Reduce the risk of developing disease • Provide treatment • Provide information (prenatal screening)
What is a true positive?
False positive?
True negative?
False negative?
True = test is correct False = test is incorrect
Positive = test labels you as having the disease/condition
Negative = the test labels you as NOT having the disease/condition
So true positive means the test correctly labels you as having the condition
False positive = test incorrectly labels the individual as having the disease/condition/higher risk
Etc…
What is sensitivity what is the equation?
Sensitivity – the proportion of people with the disease who
are correctly identified by the screening test
a / a+c
A = True positive
C = false negative
What is specificity?
Specificity – the proportion of people without the disease
who are correctly excluded by the screening test
d / b+d
Where d = true negative and b = false positive
What is PPV?
Positive predictive value – the proportion of people with a
positive test result who actually have the disease
a / a+b
A = true positive B = false positive
What is NPV?
Negative predictive value – the proportion of people with a
negative test result who do not have the disease
d / c+d
C = false negative
D = true negative
The affect of higher prevalence on sensitivity, specificity, prevalence, PPV and NPV?
Will increase the prevalence and increase the NPV, but decrease the PPV.
The rest will stay constant
What are the criteria for screening based on?
The Wilson and Jungner criteria
See PowerPoint on screening for all the points if needed?
Is screening biased?
Yes
Sampling bias:
People who choose to participate in screening programmes may be different from those who do not. May be at higher risk e.g. women with family history of breast cancer more likely to attend.
May be at lower risk e.g. women in higher socioeconomic groups (lower risk of cervical
cancer) more likely to attend. Same with well educated people.
Lead-time bias:
Even though disease developed at the same time as screened patient diagnosed earlier, on records their survival time = longer.
Length-time bias:
Less aggressive slow disease more likely to be picked up at screening, where as faster more aggressive diseases are picked up between screening. Therefore survival rates look better but only due to those being piked up by screening being more treatable.
Screening examples:
X6
AAA screening
Bowel cancer screening
Breast screening
Cervical screening
Diabetic eye screening
Newborn screening
Types of screening:
X 5
Population-based screening programmes (“mass” screening)
• Opportunistic screening
• Screening for communicable diseases
• Pre-employment and occupational medicals
• Commercially provided screening
See sheets printed off for more info and for questions
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