Diagnostic Tests Flashcards
(21 cards)
Why is diagnostic testing important?
Essential to clinical practice
What does the diagnosis part mean?
Separate ‘normal’ form ‘disease’
Give example of diagnostic tests
- Case-finding eye disease
- Blood tests
- Imaging/Radiography
- Scans for breast cancer
- testing for foetal abnormality
- Next generation screening: genetics
When looking at a diagnostic test, what is the main question we are trying to answer?
How accurate is the diagnostic test?
What is the true diagnosis?
The reference standard
For example, with the results of a liver scan and a diagnosis based based on surgical inspection, what is the question we are trying to answer?
How good is the liver scan at predicting the true result?
What is a true positive?
Individual predicted to have disease and they really have it.
What is a true negative?
Predicted to not have the disease and are normal.
What is a false positive?
False alarm- predicted as diseased but are normal.
What is a false negative?
Poor detection- test was normal but the individual has the disease.
When assessing a diagnostic test, what two things would you calculate?
Sensitivity and specificity.
How do you calculate sensitivity?
True positives divided by the number of people in total that have the disease.
How do you calculate specificity?
True negatives divided by the number of patients in total that do not have the disease.
What would a perfect test have?
100% specificity and 100% sensitivity
What does a 90% sensitivity suggest?
Test is good at identifying abnormal and diagnosing patients.
What does a 63% specificity suggest?
Probably too may false alarms - 37% false alarms.
How do you calculate prevalence?
True positives divided by the total number of people that the study was done on.
In screening, what can be a problem when calculating various values?
What do we do to counteract this?
We do not know how the negatives are split.
- Hnece, in screening we use the Positive Predictive Value
How do you calculate PPV?
True positives divided by the total number of true and false positives.
What is a PPV?
Proportion with positive results correctly identified.
What does a PPV value of 33% suggest?
Not so good!
- too many false alarms.
