What determines whether you should perform a diagnostic test?

What determines whether you should perform a diagnostic test?

• A. Posttest probability
• B. Pretest probability
• C. LR+
• D. Sensitivity

Answer: (B)

Starting point is the estimate of how likely the disease is before testing. This pretest probability comes from history, exam findings, and risk factors. It matters because it determines whether a diagnostic test will meaningfully change your assessment and management. If pretest probability is very low, even a positive result may not raise the probability enough to justify testing; if it’s very high, a negative result may not rule out disease. The test then updates that initial estimate to a posttest probability, using how much the test shifts probability (likelihood ratios). Test characteristics like sensitivity or the actual posttest probability after testing aren’t the initial trigger for ordering the test. However, they describe how informative the test is once you’ve decided to test.

Starting point is the estimate of how likely the disease is before testing. This pretest probability comes from history, exam findings, and risk factors. It matters because it determines whether a diagnostic test will meaningfully change your assessment and management. If pretest probability is very low, even a positive result may not raise the probability enough to justify testing; if it’s very high, a negative result may not rule out disease. The test then updates that initial estimate to a posttest probability, using how much the test shifts probability (likelihood ratios). Test characteristics like sensitivity or the actual posttest probability after testing aren’t the initial trigger for ordering the test. However, they describe how informative the test is once you’ve decided to test.