Diagnostic Assessment and Investigation
Core concepts
Understand test characteristics and appropriate ordering. Know the difference between screening and diagnostic tests, and when a test changes management. Familiarize yourself with common laboratory patterns (anion gap, liver enzyme patterns, urinalysis, coagulation), imaging principles (when CT versus ultrasound versus plain film is appropriate, radiation and contrast risks), and monitoring (ECG interpretation basics, pulse oximetry limitations). Recognize false positives from low-prevalence settings and the value of confirmatory testing.
Traps
Questions frequently test the concept that a highly sensitive test is best for ruling out (SnNout: sensitive test, negative result rules out) and a highly specific test for ruling in (SpPin). Watch for options that order the most advanced test first when a simpler test answers the question. Also watch for misinterpretation of normal ranges in special populations (pregnancy, elderly, athletes) and for ignoring pre-analytical errors such as hemolysis or improper collection.
Rule of thumb
Before ordering, ask: will the result change what I do? If not, do not order it. For interpretation, always anchor a result to the clinical picture; treat isolated abnormal values with caution and repeat when plausibly spurious.
Worked example
A patient with low pre-test probability for a condition undergoes a highly sensitive screening test that returns positive. Because prevalence is low, the positive predictive value is modest; the correct next step is a confirmatory, more specific test rather than immediate definitive treatment.