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24 Cards in this Set

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What is Sensitivity?
= proportion of patients who have disease who test positive
= true positives / (true positives + false negatives)
What is Specificity?
= proportion of patients who do not have the disease who test negative
= true negatives / (true negatives + false positives)
Sensitive tests...
Rule Out (snOUT)
Specific tests...
Rule In (spIN)
Receiver Operating Curce (ROC)
High Cutoff =
Medium Cutoff =
Low Cutoff =
High = Specific, nonSensitive
Medium = between
Low = non-specific, Sensitive
Positive Predictive Value
= post test probability
Probability of Having the Disease Given a Positive Result
= TP / (FP + TP)
Negative Predictive Value
= post test probability
TN / (FN + TN)
How does the fact that a disease is rare (low pretest probability) affect the interpretation of a positive test?
A positive test may actually be more likely to come from a healthy person
FP > TP !!! Anxiety
What is the Treatment Threshold?
When the post-test probability is high enough, Treat.
What is the Test Threshold?
When the post-test probability is low enough, Stop Testing.
What is likelihood ratio?
probability of result in those who have the disease / probability of result in those who do have the disease
What is likelihood ratio if test is positive?
Sensitivity / (1 - Specificity)
What is likelihood ratio if test is negative?
(1 - sensitivity) / specificity
The Nomogram: why do you use it?
Draw a line from the pre-test probability through the likelihood ratio to get to the post test probability
What magnitudes of LRs are useful?
> 5-10 for rule in
< 0.1 - 0.2 for rule out
What is Efficacy?
= usefulness in ideal circumstances
What is Effectiveness?
= usefulness in usual circumstances
What is a surrogate outcome?
An outcome that stands in for desired clinical outcome, e.g. increased CD4 count in ARVs or decreased cholesterol in preventing MIs
Is fever a good surrogate outcome for predicting death from sepsis?
No. Can give acetominaphen to reduce fever but death will happen regardless.
What is a Type I error?
Probability of finding difference in populations despite absence of difference
What is a Type II error?
Probability of finding no difference when difference exists
= beta. Power = 1 - beta.
What is NNT?
Number needed to treat.
NNT = 1 / ARR
What are good cutoffs for NNT?
Treatment?
Prophylaxis?
Tx = 2 - 5
Prophylaxis = 10 - 100
What is NNH?
NNH = 1 / ARI