APPRAISE: Diagnosis Part 2
A reference standard is the best available method or criterion for definitively establishing the true presence or absence of a condition (diagnosis) or the true effect of a treatment. Compare the reference standard to study results.
Sensitivity = true positive / all disease positives
measures the proportion of patients with the disease who also test positive for the disease in this study. It is the probability that a person with the disease will have a positive test result.
Specificity = true negative / all disease negatives
measures the proportion of patients without the disease who also test negative for the disease in this study. It is the probability that a person without the disease will have a negative test result.
Sensitivity and specificity are characteristics of the test but do not provide enough information for the clinician to act on the test results. Likelihood ratios can be used to help adapt the results of a study to specific patients. They help determine the probability of disease in a patient.
Likelihood ratios (LR):
- LR + = positive test in patients with disease / positive test in patients without disease
- LR - = negative test in patients with disease / negative test in patients without disease
Likelihood ratios indicate the likelihood that a given test result would be expected in a patient with the target disorder compared to the likelihood that the same result would be expected in a patient without that disorder.
- Likelihood ratio of a positive test result (LR+) increases the odds of having the disease after a positive test result.
- Likelihood ratio of a negative test result (LR-) decreases the odds of having the disease after a negative test result.
Knowledge Check
Read the information on this page then answer the following questions.
Question 1) A new diagnostic test for Disease X shows high sensitivity but low specificity. What does this imply about how the test performs?
A) It correctly identifies most people with the disease but falsely labels some healthy people as diseased.
B) It correctly identifies most people without the disease but misses many who have it.
C) It has an equal likelihood of false positives and false negatives.
Question 2) Which situation best demonstrates the use of likelihood ratios (LRs) in clinical decision-making?
A) A clinician compares the average sensitivity of multiple tests for the same condition.
B) A clinician uses a test’s LR+ to estimate how much a positive result changes the probability of disease for their specific patient.
C) A clinician uses the test’s reproducibility to assess observer agreement.
Question 3) A diagnostic test yields an LR+ of 8 and an LR− of 0.3. How should these values be interpreted in terms of clinical impact?
A) Both values indicate large changes in disease probability.
B) The test moderately increases the likelihood of disease with a positive result and causes a small decrease with a negative result.
C) The test provides little to no meaningful change in disease probability.