Clinical Reasoning

Clinical reasoning is one of the core skills of a physician, used to diagnose the cause of new symptoms, to guide the choice of tests and treatment, to assess response to therapy, and to recognize patients’ strengths and address barriers to health.

Diagnostic reasoning can be thought of as a process that begins with the patient’s history and leads to a prioritized differential diagnosis, the list of most likely and most serious diagnoses that might explain a patient’s symptoms.  This prioritized differential guides diagnostic testing, allowing the physician to focus testing on the illness that are most likely or most urgent.

Establishing a diagnosis allows us to provide an explanation of a patient’s health problem and to predict its course.

We’ll talk more about tests and treatment later in the course, and you will build your clinical reasoning each time you see a patient.

judgment under uncertainty, with a consideration of possible diagnoses that might explain symptoms and signs, the harms and benefits of diagnostic testing and treatment for each of those diagnoses, and patient preferences and values.

Diagnostic reasoning begins with gathering information. As an experienced clinician takes a history, the patient’s story activates illness scripts in their memory.  An illness script is an organized mental summary of that clinician’s understanding of an illness, based on both their clinical experience and theoretical knowledge. An illness script may be activated almost automatically by pattern recognition, or it may require conscious effort and analysis. Illnesses whose scripts match the patient’s clinical picture are added to the differential diagnosis. By the time an experienced clinician has finished an HPI, they usually have a differential in mind.

Dual process theory

Humans use two cognitive systems make sense of the world around them.  System 1 is based on pattern recognition – intuitive, efficient, and so quick that the process of reasoning is subconscious. System 2 is slow and deliberate, consciously and explicitly analyzing information to arrive at a conclusion. Physicians rely on both systems to make their diagnoses.

When a patient’s problem is familiar, system 1 uses pattern recognition to suggest a diagnosis quickly and efficiently. To avoid errors, this first impression can be double checked to make sure everything fits and to exclude more dangerous causes. When a problem is less familiar, system 2 must consciously and deliberately analyze the history to arrive at a differential.

System 1 and system 2 each have advantages and disadvantages.  System 1 is quick, requires less cognitive effort, and is generally accurate for familiar problems.  System 2 is slower and requires more cognitive energy but can reason through a problem in the face of complexity and uncertainty. Experienced doctors fall back on system 2 when seeing a less familiar problem or one that is more complex or potentially high stakes.

The majority of your reasoning will be System 2 until you gain more knowledge and expertise.

Testing diagnostic hypotheses

Diagnostic possibilities can be tested with focused questions, physical exam, and diagnostic tests.  The clinician actively looks for findings that support or argue against the illnesses on their differential. A prioritized differential diagnosis – the most likely or most lethal causes of a patient’s problem, ranked by likelihood – then guides decisions about further testing and treatment.

Practicing clinical reasoning

Clinical reasoning is dependent on medical knowledge. You will build your diagnostic skill through practice with cases – written cases, small group cases, and simulated and real life patients.

FCM simulation workshops are a structured and fun chance to practice clinical reasoning. In each sim, you’ll see several different patients with a chief concern relevant to your block.  You’ll practice each step in the process as you manage a simulated patient, get feedback on how you’ve done, and build your approach to these common symptoms.

Medical students tend to use more system 2 thinking, since they have too little knowledge and experience to rely on system 1.  You may intuitively come up with diagnostic hypotheses for some of your patients’ problems, like ‘heart attack’ for chest pain or ‘meningitis’ for fever and headache. But as first year students, we expect most of your reasoning to be system 2 – analytic and slow – and it will often happen after you are done with your H&P.  Reasoning in real time will become easier as you learn more about medicine, see more patients, and develop your own mental frameworks for common problems.

Learning from patients is another important way to build your reasoning skills, every time you are in the hospital or clinic.  If the patient’s diagnosis is known, you’ll begin to recognize the pattern of that illness. You can also build and practice using diagnostic schema, a cognitive tool to help you systematically approach a clinical problem by providing an organizing scaffold. Building these scaffolds for one patient will give you an approach that you can then use with future patients who have the same chief concern.

Diagnostic schema for many problems can be found in:

• The Patient History: An Evidence Based Approach to Differential Diagnosis.  A basic overview, great for first years.
• Clinical Problem Solvers, a site devoted to diagnostic reasoning with many contributions from senior medical students. Easier to understand after you’ve finished the relevant block!

Written patient cases and small groups in the block will also help you grow the medical knowledge needed to be a good diagnostician.  For more practice, check out the New England Journal of Medicine’s Clinical Problem-Solving series.  

A Universal Model of Diagnostic Reasoning : Academic Medicine (lww.com)