After obtaining new data, what should you do with probabilities in diagnostic reasoning?

• A. Probabilities should be kept fixed regardless of new data
• B. After data is gathered, update probabilities to reflect new information
• C. Only update probabilities after imaging
• D. Discard prior probabilities if the data contradicts them

Answer: (C)

When new data arrives, you should revise your diagnostic probabilities to reflect the new information. This is the essence of rational diagnostic reasoning: start with a prior belief about how likely each diagnosis is, then adjust those beliefs in light of what the data says (the likelihood), and renormalize to get updated, or posterior, probabilities. This updating should happen with any new evidence, whether it comes from history, physical exam, labs, imaging, or other tests. If the data challenges your initial probability, you don’t discard the prior entirely; you shift toward what the new evidence supports, with the influence of the prior diminishing as more informative data accumulates. Keeping probabilities fixed ignores new information and delays arriving at a more accurate conclusion.