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Evidence Based Practice

Summary:

Here you can find questions that you can use to critically evaluate a diagnosis study.

Remember that you are looking to appraise the study's:

  • Validity
  • Impact
  • Applicability

Diagnosis: Statistical Terms & Concepts

Gold Standard Positive

(condition is present)

Gold Standard Negative

(condition is not present)
Test Result Positive      True Positive    False Positive
Test Result Negative False Negative True Negative

Stable Properties:

  • Sensitivity = True Positives/(True Positives + False Negatives) = the proportion of truly diseased persons, as measured by the gold standard, who are identified as diseased by the test under study 
  • Specificity = True Negatives/(False Positive + True Negative) = the proportion of persons who truly do not have the disease, as measured by the gold standard, who are identified by the diagnostic test under study as not having the disease

Frequency Dependent Properties:

  • Predictive value: In screening and diagnostic tests, the probability that a person with a positive test is a true positive (i.e., does have the disease), or that a person with a negative test truly does not have the disease. The predictive value of a screening test is determined by the sensitivity and specificity of the test, and by the prevalence of the condition for which the test is used. 
  • Positive Predictive Value = True Positive/(True Positive + False Positive) = probability that a person with a positive test is a true positive (i.e., does have the disease) 
  • Negative Predictive Value = True Negative/(True Negative + False Negative) = the probability that a person with a negative test truly does not have the disease.

Likelihood Ratios (LR)
The likelihood ratio for a test result compares the likelihood of that result in patients with diseaseto the likelihood of that result in patients without disease:

Condition Present Condition Not Present
Test Reult Positive a b
Test Result Negative c d

Positive LR = (a/a+c)/(b/b+d)
Negative LR = (c/a+c)/(d/b+d)

How much do Likelihood Ratios change disease likelihood?
LRs >10 or <0.1 cause large changes in likelihood.
LRs 5-10 or 0.1-0.2 cause moderate changes.
LRs 2-5 or 0.2-0.5 cause small changes.
LRs between <2 and 0.5 cause little or no change.

Appraising/Evaluating a DIAGNOSIS Study

Validity: Is this evidence about the accuracy of a diagnostic test valid?

Impact: Does this (valid) evidence show that this test can accurately distinguish patients who do and do not have a specific disorder?

Applicability: How can I apply this valid, accurate diagnostic test to a specific patient?

Back to Appraise.