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USMLE Step2 Secrets Biostatistics 1-5


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Sensitivity is defined as ...
The ability of a test to detect DISEASE
How is sensitivity mathematically measured?
The number of true positives divided by the number of people with the disease (meaning true positives and false negatives)
Tests with high sensitivity are used for ...
Disease screening.

False positives (people who don't have the disease, but tested positive) occur, but the test does not miss many people with the disease (low false-negative rate, ie. people who tested negative, but actually have the disease)
Specificity is defined as ...
the ability of a test to detect nondisease (detect health)
Mathematically, how is specificity derived?
The number of true negatives divided by the number of people without the disease (true negatives plus false positives)
Tests with high specificity are used for
disease confirmation.

False negatives (people who tested negative, but actually have the disease), but the test does not call anyone sick who is actually healthy (low false-positive rate).

The ideal confirmatory test must have a high sensitivity AND high specificity, otherwise, people with the disease may be called healthy, meaning there is an unacceptably high false-negative rate.
If the cut-off serum glucose value of a diagnosis of DM is set very low, no cases of diabetes will be missed, but many people withou the diabetes will be mislabeled as diabetes ie...
higher sensitivity
lower specificity
lower PPV
higher NPV
If the cut-off is set very high, the diagnosis of diabetes will not be made in healthy people, but many cases of true diabetes will go undiagnosed ie..
lower sensitivity
higher specificity
higher PPV
lower NPV
When a test is positive for the disease, the PPV measures how likely it is that the patient ...
actually has the disease.

In other words, the probability of having a condition, given a positive test.
PPV is calculated by ...
dividing the number of true positives by the number of people with a positive test.
PPV depends on the ...
prevalence of the disease.
The higher the prevalence of a disease,
the higher the PPV
PPV depends also on the
sensitivity and specificity of the test
An overly sensitive test that gives more false positivies has
a lower PPV

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