Flashcards in Lecture 21-23 - Screenings in Medicine Deck (21)
What two questions are important to ask of medical screening tests?
-how ACCURATE is the test
-how CONFIDENT can you be about the prediction of disease status from the results
What are the four outcomes from a screening test and what differentiates them?
True positive (A):
-CORRECT reporting of a POSITIVE result and patient actually HAS the disease
True negative (D):
-CORRECT reporting of NEGATIVE result and patient actually DOES NOT have disease
False positive (B):
-INCORRECT reporting of POSITIVE result and patient DOES NOT have disease
False negative (C):
-INCORRECT reporting of NEGATIVE result and patient DOES have disease
What two factors can be used to assess how accurate a screening test is in determining disease status?
How are sensitivity and specificity evaluated after they have been calculated?
-comparison to known disease status using a “gold standard”
What does sensitivity represent?
What does a high sensitivity indicate about a test?
-how well a test can detect disease pressence when the disease is actually present
-sensitivity = TP / (TP + FN) or A / (A + C)
-high sensitivity indicates a low false negative rate
What does specificity represent?
What does a high specificity indicate about a test?
-how well a test can detect disease absence when the disease is actually absent
-specificity = TN / (TN + FP) or D / (B + D)
-high specificity indicates a low false positive rate
What two factors can be used to assess confidence in prediction of disease presence or absence from a screening test?
-positive predictive value
-negative predictive value
What does positive predictive value (PPV) represent?
-how accurately a positive test predicts the presence of disease
-PPV = TP / (TP + FP)
-PPV = A / (A + B)
What does negative predictive value (NPV) represent?
-how accurately a negative test predicts the absence of disease
-NPV = TN / (TN + FN)
-NPV = D / (C + D)
Prevalence of a disease in a community is a factor in __________ and __________ but is not a factor in __________ and __________.
PPV; NPV; sensitivity, specificity
The lower the specificity, the lower the __________.
What does diagnostic accuracy/precision represent?
-proportion of screenings where a patient is correctly identified
- DA/DP = (TP + TN) / (all patients)
- DA/DP = (A + D) / (A + B + C + D)
What does likelihood ratio represent?
-ratio of the probability of a person with disease receiving a test result and the probability of somebody without disease getting that same result
-can be calculated for both a positive result (LR+) or a negative result (LR-)
What is likelihood ratio positive (LR+) and how is it calculated?
-ratio of the probabilities of a positive test in those with disease and the probability of a positive test of those without disease
-sensitivity / (1 - specificity)
-(A / (A + C)) / (B / (B + D))
What is likelihood ratio negative (LR-) and how is it calculated?
-ratio of the probabilities of a negative test in those with disease and the probability of a negative test of those without disease
-(1- sensitivity) / specificity
-(C / (A + C)) / (D / (B + D))
To be considered useful, a test should have a LR+ of __________ and a LR- of __________.
What is validity?
-ability to accurately identify those with disease and those without disease
-precision of results
Differentiate between internal validity and external validity.
-how accurately a studies results reflect the true conditions of a study population
-how accurately a studies results reflect true conditions of a population other than the one that was studied
What is reliability?
-the ability of a test to give the same results with repeated used
-degree of reproducibility or consistency
What relationship do validity and reliability have?
-valid tests are always reliable
-reliable tests are not always valid