Principles and Predictive Value of Screening Flashcards
What is screening?
the application of a test to people in order to assist with early identification of a disease.
What is the objective of screening?
to reduce mortality and morbidity with early detection
Good Screening Programs screen for health problems that….(6 things)
- Are important to the individual and community
- Have an acceptable form of treatment
- Has a natural history that is adequately understood
- Has a recognizable latent or early symptomatic stage
- Has a suitable screening test
- Is economically beneficial
3 Requirements of Screening Tests
- Ideally should be inexpensive
- easy to administer (low risk)
- minimal discomfort
What are screening tests based off of?
a Gold Standard based on the evidence
Results of a screening test should be ____ and ____
Accurate / Valid
and
Reliable / Precise
Define Accuracy (Validity)
The degree to which a variable actually represents what it is supposed to represent
What is the best way to assess validity?
compare results to a reference standard
What threatens validity?
Systematic error (aka bias)
Define Precision (Reliability)
The degree to which a variable has nearly the same value when measured several times
What is the best way to assess precision?
repeated measures
What threatens precision?
Random Error
What determines our predictive value?
Sensitivity, Specificity and the Prevalence of the disease
Define the prevalence of a disease
Represented by the number of patients per 100,000 population who have the disease at a given time
How do we determine the validity (accuracy) of a screening test?
Determine its sensitivity and specificity
Define sensitivity
The fraction of those with the disease that are correctly identified as positive by the test
What does sensitivity determine?
Whether or not the test detects true cases of the disease
For example: if you have 100 people with meniscus tears and after performing a meniscal teat test you get a positive meniscus tear 90 times the sensitivity of this screening test is 0.9
Define specificity
The fraction of those without the disease correctly identified as negative by the test.
What does specificity determine?
Whether or not the test excludes those without the disease
How is data estimating sensitivity and specificity typically displayed?
in a 2 x 2 table that classifies people according to their disease status and test results
What are true positives?
people who have the disease and whose test came up positive
What are true negatives?
the people who do not have the disease and whose test came up negative
What are false negatives?
the people who have the disease but their test came up negative
What are false positives?
the people who do not have the disease but their test came up positive
Given true and false positives and true and false negatives how can we determine sensitivity?
= number of true positives / total number of those with the disease
Given true and false positives and true and false negatives how can we determine specificity?
= number of true negatives/ total number of those that do not have the disease
What is Positive predictive value (+PV)?
The fraction of people with positive tests who truly do have the condition.
What is negative predictive value (-PV)?
The fraction of people with negative tests who truly don’t have the condition.
Given true and false positives and true and false negatives how can we determine +PV?
= number of true positives / total number of positive test results (both true and false)
Given true and false positives and true and false negatives how can we determine -PV?
= number of true negatives / total number of negative test results (both true and false)
The sensitivity and specificity are properties of the _____.
test
The positive and negative predictive values are properties of the _____and the _______you test.
test and population you test
If you use a test in two populations with different disease prevalence will the predictive values will be the same or different?
different
When is a screening test most useful?
if directed to a high-risk population (high prevalence and high predictive value)
When you know your patient has the disease and you want to know what is the chance that the test will show that my patient has it what are you trying to determine?
Sensitivity
When you know your patient doesn’t have the disease and you want to know what is the chance that the test will show that my patient doesn’t have it what are you trying to determine?
Specificity
When you receive a positive test result back and you want to know what is the chance that your patient actually has the disease what are you trying to determine?
Positive Predictive Value
When you receive a negative test result back and you want to know what is the chance that your patient actually doesn’t the disease what are you trying to determine?
Negative Predictive Value
What does a high PPV indicate?
There is a strong chance that a person with a positive test has the disease
What does a high NPV indicate?
A negative test will rule out the disease
What can you typically conclude with a low PPV?
There is a low prevalence of the disease in the population being examined
When a sign, test or symptom has an extremely high specificity what does a positive test result tend to do?
rule IN the diagnosis
SpPIn (specificity, positive, in)
When a sign, test or symptom has an extremely high sensitivity what does a negative test result tend to do?
rule OUT the diagnosis
SnNOut (sensitivity, negative, out)
Define systematic review
systematically identifies and appraises all quality research evidence that is relevant to your particular research question
Define Meta Analysis
a quantitative statistical analysis of several separate but similar experiments that tests the pooled data for statistical significance
Meta analysis determines what between treatments?
progress
What does meta analysis use?
effect size to determine the difference among groups
