Screening Chpt5 Flashcards
Are diagnosis and screening the same thing?
why?
Diagnosis does not equal screening
Screening test-often used to diagnose disease generally done to individuals who are not suspected of having disease
Diagnoses-confirmation of the presence or absence of disease in someone who suspected to be at risk for disease
Define screening and explain the purpose of screening
Using a test to detect disease in people who show no symptoms of the disease
The purpose-to classify individuals into categories of likelihood of having a particular disease
List some popular screening tests
Cholesterol-CVD
Mammography-breast cancer
Pap smear-uterine cancer
List in order the natural history of disease to from beginning to end
Explain which parts of that timeline are included in the screening
Stage of susceptibility
Exposure
Stage of subclinical diseases (pathological changes)
Onset of symptoms
Stages of clinical disease (usual time of diagnosis)
Stage of recovery disability or death
Screening can be done;
During stage of susceptibility
Stage of subclinical disease
And the beginning part of stage of clinical disease
List the requirements for a screening test
Should be relatively sensitive and specific
Should be simple and inexpensive
Should be very safe
Must be applicable to subjects and providers
What ethical issues arise when discussing screening?
People who are being screened are generally not patients …..not sick and are expecting treatment
Screening may not benefit the individual, may have a negative impact on the individual
May divert resources away from treatment, how are the resources equitably distributed especially in developing countries
List that in general principles of screening from the world health organization
should be important health problems being addressed
There should be a treatment for the condition
Facilities for diagnosis and treatment should be available
There should be a latent stage of the disease
There should be a test for examination for the condition
The test should be acceptable to the population
The natural history of the disease should be adequately understood
There should be an agreed policy on whom to treat
The total cost of finding a case should be economically balanced in relation to medical expense as a whole
Case findings should be a continuous process, not just a once and for all project
How is the quality of a screening test measured
By assessing validity and reliability
Define validity
Validity is how well does the test measure what it supposed to measure
The extent to which the test distinguishes between people without the disease high validity requires high sensitivity and specificity
Define reliability
Reliability is how well does the test due in different populations?
repeatability
Define false positive
Why are false positives a problem?
People who are not diseased but have a positive test
This is a problem because;
Additional and possibly more invasive screening
Increased burden on healthcare system-money
Psychological impact
Other stigma
Define false negative?
Why are false negatives a problem?
Faults negative-people who really have disease but have a negative test
Problems with false negative:
depend on the severity of the disease
Faults reassurances
We missed an opportunity to change the course of the disease
When setting up a 2 x 2 table as testing screening test what are the titles that should go along the top along the left-hand side and then fill in each box?
Column labels from left to right
Positive for disease negative for disease
Row titles from top to bottom:
Positive test result negative test results
First box true positive alongside faults positive bottom row false-negative negative alongside true negative
Define sensitivity and give the formula
Sensitivity-ability of the test to identify those who are diseased in the screening process
(remember validity of the test is determined by sensitivity and specificity)
Formula= TP/(TP + FN)
The probability of testing positive if the diseases truly present is another definition for what?
Sensitivity
Define specificity and give the formula
Specificity-the ability of the TEST to identify those who do NOT have the disease
Specificity = TN/(TN + FP)
What is the formula for overall accuracy of a test?
TP + TN/TP + TN + FP + FN
BUT remember this is less useful than sensitivity and specificity
The probability that a screening test will be negative if the disease is truly absent defines what?
Specificity
Screening by mammogram will correctly classify 96% of all non-breast cancer patients as being disease-free
What does this measure?
Specificity
Screening by mammograms will identify 77% of all true breast cancer cases
What does this measure?
Sensitivity
Because sensitivity and specificity can’t always be 100% at the same time ….one of those factors must take precedence in varying circumstances
When should sensitivity be more important and when should specificity be more important?
Sensitivity should be increased when the penalty associated with missing a case is high… When the diseases spread quickly and easily… When subsequent diagnostic evaluations are associated with minimal cost and risk
Specificity should be increased when the cost or risk associated with further diagnostic test techniques are substantial… Minimize false positives… Positive screen requires a biopsy to be performed
Sensitivity and specificity are not able to predict the performance of the screening test in a population
Therefore what value do we use to assess the performance of the test in a population?
Positive and negative predictive value
Define positive predictive value
The probability that a person actually has the disease given a positive test
Define negative predictive value
The probability that a person is disease-free given a negative test
What does predictive value measure?
and what is predictive value affected by?
Predictive value is measured whether or not individual actually has the disease, given the results of the screening test
Predictive value is affected by specificity, prevalence of pre-clinical disease, sensitivity
Give the formula for positive predictive value
PPV = TP/TP + FP
IT’S THE PROBABILITY THAT A PERSON ACTUALLY HAS THE DISEASE WHEN THEY RECEIVE A POSITIVE TEST RESULT
Get the formula through negative predictive value
NPV = TN/FN + TN
PROBABILITY THAT A PERSON IS DISEASE-FREE WHEN RECEIVING A NEGATIVE TEST RESULTS
Which predictive value is the following sample;
Among persons who test -99% disease-free
Negative predictive value
Which predictive value is the following example:
Among persons who test positive, 40% are found to have heart disease
(in other words given a positive test there’s only a 40% chance of having the disease)
Positive predictive value
the formula for percent agreement
A + D / A+B+C+D all multiplied by 100
The denominator should be total everyone
The formula for percent agreement expected by chance alone ?
In a 2 x 2
A + D/the total sum of all
Sequential screening
If you take all the people who screen positive on the first test and give them a second test (of higher sensitivity and specificity) what happens to those values??
The net sensitivity decreases
The net specificity increases
What is the relationship of disease prevalence to predictive value
As the disease prevalence increases the positive predictive value increases
What is the relationship of specificity to positive predictive value
As the specificity of a test increases the positive predictive value increases
When is the positive predictive value maximized? Why?
PPV is maximized when used in high risk populations since the prevalence of pre-clinical disease is higher than in the general population
Screening the general population for a relatively infrequent disease can be very wasteful of resources and may you a few undetected cases
Why is reliability of screening test important
The extent to which the screening test will produce the same or similar results each time it is administered is crucial
I test must be reliable before it can be valid
However and in valid test can demonstrate high reliability
What are the sources of variation for reliability of a test
Instrument variation-need for calibration or standardization
Intrasubject variation-changes in repeated blood pressure measurements overtime
Inter-observe her variation-inconsistency of interpretation by two or more diagnostic workers
Define bias in relation to a screening program
The deviation of the results from the truth
Define survival time
The average length of time a person lives after the diagnosis with the disease or condition
A measure of prognosis
What is lead time bias?
Leadtime by S-the interval between diagnosis of the disease at the screening and when it would have been detected by clinical symptoms
Survival me appear to increase among screen detected cases simply because diagnosis was made earlier in the course of the disease
Explain self selection bias
Volunteers may be healthier than people who don’t volunteer
Worried well-increased participation due to family history or lifestyle characteristics
Explain overdiagnosis bias
Person to screen positive who are really disease-free (FP) can be erroneously diagnosed with the disease resulting in a more favorable long-term outcome. Result in the appearance of effective screening
What is the formula for inter-observer variation a.k.a. percent agreement
Percent agreement = a + F + K + P/all readings (Times 100)
Diagonal line from top left corner to bottom right corner all of the agreements
What are the risks of screening?
Faults positives:
Anxiety
Fear future test
Money of future testing
False negatives:
Delayed intervention
Disregard of symptoms could lead to delayed diagnosis
What does the Kappa statistic question?
what is the formula?
The Kappa statistic-are the readers better than chance?
Numerator:
% agreement observed - % agreement expected by chance alone
Denominator:
100% - % agreement expected by chance alone
