Clinical Reasoning--Diagnosis (Stevens)

Question 1

Sensitivity

Answer 1

Percent of patients with disease who are correctly identified

Sn_N_Out: want a SeNsitive test to rule Out a disease if Negative result

(Few false negatives, many false positives)

a/(a+c) = Disease positive/total disease

High sensitivity means if they have the disease they’ll be positive

Ex: D-dimer test

Question 2

Specificity

Answer 2

Percent of patients without disease who are correctly identified

Sp_P_In: want a SPecific test with Positive result to rule disease In

(Few false positives, many false negatives)

d/(b+d) = No disease and negative/total no disease

High specificity means if they don’t have the disease they’ll be negative

Question 3

Confidence Interval

Answer 3

Plausible range of results from the study

Use diagnosis calculator online to calculate

For 95% confidence interval that INCLUDES 0, means that if the two treatments are truly the same, a difference as large (or larger) than found in this study occurs by chance in greater than 5% of studies like this

Question 4

Positive likelihood ratio

Answer 4

Sensitivity/(1 - Specificity)

Likelihood of + test in diseased/Likelihood of + test in undiseased

(don’t need to memorize)

Question 5

Negative likelihood ratio

Answer 5

(1 - Sensitivity)/Specificity

Likelihood of - test in diseased/Likelihood of - test in undiseased

(don’t need to memorize)

Question 6

Positive predictive value

Answer 6

Percent with positive test who in fact have disease

a/(a+b) = Disease positive/total positive

Question 7

Negative predictive value

Answer 7

Percent with negative test who don’t have disease

d/(c+d) = No disease negative/total negative

Question 8

Verification bias

Answer 8

When you decide to do the reference test ONLY if your study test is positive

You’ll miss a lot of false negatives (that are actually positive, but you’d never know)

Question 9

When do you use a sensitive versus specific test?

Answer 9

Sensitive test first for screening, to get rid of the people you know don’t have the disease

Specific test second to confirm people who definitely do have the disease

Question 10

Prevalence

Answer 10

People in the population who have the disease

(a+c)/(a+b+c+d)