Diagnostic & Screening Tests in Respiratory Diseases Flashcards
What is the purpose of a diagnostic test?
To confirm the presence of disease
Who do you give diagnostic tests to?
Patients who already have a clinical suspicion of disease
What is the purpose of a screening test?
To identify patients who may have a disease or an early form of disease or a risk factor for a disease to allow for early intervention
Who do you give screening tests to?
Patients who have had no clinical assessment before hand
What is a true negative?
People who don’t have the disease and the test shows they don’t have the disease
What is a false negative?
People who have the disease and the test shows they don’t have the disease
What is a false positive?
People who don’t have the disease and the test shows the do have the disease
What is a true positive?
People who have the disease and the test shows they have the disease
What is sensitivity?
The percentage of people with the disease that test positive
How is sensitivity calculated?
TP/(TP+FN)
What is specificity?
The percentage of people without the disease that test negative
How is specificity calculated?
TN/(TN+FP)
What is the positive predictive value?
The percentage of positive tests that are actually positive
How is the positive predictive value calculated?
TP/(TP+FP)
What is the negative predictive value?
The percentage of negative tests that are actually negative
How is the negative predictive value calculated?
TN/(TN+FN)
Are sensitivity, specificity, PPV and NPV constant for a test?
Sensitvity and specificity are but PPV and NPV are not
What are PPV and NPV dependent on?
The prevalence of disease
What will the PPV be if the prevalence is low?
Low
What will the NPV be if the prevalence is low?
High
If a test has a high threshold for the definition of the disease what will the sensitivity and specificity be?
Low sensitivity but high specificity
What are the axes in a ROC curve?
1-specificity vs sensitivity
What does a linear graph on a ROC curve show?
That the test doesn’t discriminate between people with the disease who test positive and people without the disease who test positive
What does the area under the curve measure?
The discriminating ability of the test
What biases are there in screening tests?
Selection bias, lead time bias and length time bias
What is the selection bias in a screening test?
Healthy people are more likely to be screened because they are more motivated to undertake healthy behaviour
What is the lead time bias in a screening test?
A screening test might show a longer survival time than a diagnostic test - but this is only because the test is done earlier not that the test makes them live longer - the time of death could potentially be the same - early detection not prolonged survival
What is the length time bias in a screening test?
Diseases that have a slow enough progression to allow for a screening test have better outcomes than faster diseases - detecting of a non aggressive disease
