Information theory and diagnostic reasoning in primary care: a framework for navigating uncertainty.

This is an overview of how information theory offers a rigorous and intuitive framework for understanding diagnostic reasoning in primary care. Primary care clinicians frequently encounter patients early in their illness trajectory, when symptoms are vague, evolving, or non-specific, creating high diagnostic uncertainty. Information theory, through its core concepts of entropy, information content, and information gain, provides a structured way to quantify this uncertainty and understand how it changes as new clinical information is gathered. Entropy reflects the uncertainty associated with the breadth of the differential diagnosis; information content captures how surprising or discriminating a finding is; and information gain describes how much a particular question, sign, or test reduces uncertainty. Applied to the consultation, these principles explain why rare but specific features are disproportionately informative, why sequences of low-yield tests may add little value, and how targeted history-taking can outperform broad investigation. The framework complements Bayesian reasoning, hypothetico-deductive approaches, and intuitive pattern recognition by clarifying the informational value that underpins each. Through clinical examples, the paper illustrates how information theory sharpens diagnostic strategy, supports efficient use of investigations, and helps clinicians navigate uncertainty with greater clarity and safety. Ultimately, it positions diagnosis as a scientific process of uncertainty reduction central to primary care practice.