Lecture 16 Flashcards
Even though it is necessary, the necessary cause may not
result in the disease if the other stuff is there
Define screening (in health)
involves identifying risk factors for disease or unrecognised disease by applying tests on a large scale to a population.
Is screening primary, secondary or tertiary prevention?
It can be either one of the three depending on what they are screening for
When is screening a primary prevention
Before the biological onset, when they are screening for a risk factor (alcohol in women)
When is screening a secondary prevention
After the biological onset, when they are looking early features of disease before the disease gets worse (mammograms)
When is screening a tertiary prevention
Following the diagnosis and treatment of the disease, when they are screening for a complication of a diagnosed disease (bone density)
What is the flow diagram for a screening program?
First they take the total population and find an eligible population without symptoms. Then subject them to a screening test that is less expensive and can identify probable cases of the disease. Test positives are then subjected to invasive and expensive gold standard test. Then the disease positive from this group receive intervention/treatment. The disease negative will be rescreened after a specified amount of time.
How do they find the eligible population group
through descriptive epidemiological studies
What groups does the screening test identify
test positive and test negative
Define gold standard test
The best diagnostic test they have to offer- the ideal test.
Why can’t we subject large populations to gold standard tests?
They are often invasive and expensive.
For a successful screening initiative (criteria)
suitable disease, test, treatment
The objective of screening is to
To improve health outcome for a population
What is a suitable disease
It has to be an important health problem. The natural history of the disease has to be known/ or relationship of risk factors known.
Can diseases chosen be common or uncommon ?
Both. For uncommon diseases that are easily detectable and easily intervened, they are screened as well
For natural history of disease what about the timing of the disease?
It needs to be detectable early. The pre-clinical phase (early features to diagnosis) has to be long enough for the screening program to identify early feature and treat it before it gets worse
What are features of a Suitable Test
It has to be an easy test that can be applied to a large population. It has to be reliable- giving consistent results. It has to safe, simple, effective and clearly accurate.
To be an effective test it has to be
affordable and acceptable to the population you’re delivering it so they all participate in the screening programme
What is measures of accuracy
Specificity and sensitivity
When you want to test the accuracy of a screening test, you compare it against a
gold standard test
What is sensitivity
The likelihood of a positive test in those who actually have the disease. How many people did the test identify had the disease out of the people who actually have the disease?
How do you calculate sensitivity
(true positives/ all those who have the disease (EG) ) x 100
What is specificity
The likelihood of a negative test in those who actually did Not have the disease. How many people did the test say did not have the disease that actually didn’t have the disease?
How do you calculate specificity
(true negatives/ all those who don’t have the disease (CG) ) x 100
What units are specificity / sensitivity given in
percentage
Sensitivity of a screening test is high if
the proportion of true positives is high
Specificity of a screening test is high if
the proportion of true negatives is high
Wherever we apply the test, the results of specificity and sensitivity are a
fixed value characteristic of the test.
If you have the same population size, sensitivity and specificity values, why can they pick up different positive tests
They is a difference in the prevalence of the disease in the populations (EG and CG)
What are predictive values
Another measure of accuracy of the screening test in practice. This looks at the number of people that are testing positive and testing negative
Positive predictive value (PPV) is
the probability of having the disease if the test is positive
What is the main difference between predictive values and specificity/sensitivity
predictive values look across the outcome grid whereas specificity/sensitivity go vertically up.
How do you calculate PPV
((true positives/ all those who test positive)) x 100
Negative predictive value (NPV) is
The probability of being free from the disease if the test is negative
How do you calculate NPV
((true negatives/ all those who test negative)) x 100
Are predictive values fixed values of the test?
No, they are dependent on the prevalence of disease in the population. So they reflect both test accuracy and the prevalence of the disease in the population
What are the requirements of suitable treatment?
You have to have evidence that early treatment leads to better health outcomes. The treatment has to be effective, acceptable and accessible. There has to be evidence based policies that regulate who is offered treatment and what treatment is offered so as not to cause disparities in healthcare utilisation- therefore health outcomes for population groups
What are the requirements of a suitable screening programme
The benefits must outweigh the harm. There must be RCT evidence that the screening programme reduces mortality or increases survival time.
There must be adequate resourcing, agreed policy for testing, diagnosis, treatment and programme management with health care support throughout. Cost effective. Needs to reach all those who are likely to benefit from it.
What are the two biases involved in calculating survival time
lead time bias and length time bias
Define lead bias
This happens when the time of survival appears to be longer even though the time of death is the same for both when there was screening and when there was not, because the time it was measured from is earlier (screening) than it would’ve been without screening (diagnosis)
Define length time bias
If there is variation in the speed of progression of the disease, a mean survival time can be taken but this may lead to a bias towards longer survival times because the people with faster progressing diseases may have already died (so not counted) or have yet to start.
