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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 = nonspecific, 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 posttest probability is high enough, Treat.


What is the Test Threshold?

When the posttest 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 pretest probability through the likelihood ratio to get to the post test probability


What magnitudes of LRs are useful?

> 510 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
