Diagnostics: Laboratory and Imaging
Diagnostics: Laboratory and Imaging tests test interpretation, pretest probability, sensitivity and specificity, and safe use of imaging. The exam rewards choosing the right test for the right patient, not ordering the most tests.
Core rules
- Sensitivity: probability of a positive test in disease; high sensitivity helps rule out disease when negative.
- Specificity: probability of a negative test in health; high specificity helps rule in disease when positive.
- Positive predictive value depends on prevalence; a good test in a low-prevalence population produces more false positives.
- D-dimer: useful in low or intermediate pretest probability to exclude venous thromboembolism; not useful to rule in disease and not for high pretest probability.
Worked example
A young patient with pleuritic chest pain and no leg swelling has low pretest probability for pulmonary embolism. A negative D-dimer can exclude pulmonary embolism. If the same patient were postoperative with tachycardia and hypoxemia, pretest probability would be high, and CT pulmonary angiography would be needed rather than D-dimer. Reasoning: pretest probability determines test interpretation.
Traps
- Ordering D-dimer in high pretest probability and being reassured by a negative result.
- Ignoring renal function before contrast imaging.
- Treating incidental findings without clinical context.
- Confusing sensitivity with positive predictive value.