John Hay
John Hay Biomedical
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My ClassGlossary
Genetics of Disease (Medical Interventions)
Unit 1: Clinical Reasoning LadderFeeds no tested domain

Clinical Reasoning 6: Build the differential

The centrepiece of the ladder. Turn a cluster of findings into a short written list of rival explanations, rank it, and let new evidence move the ranks.

Builds on (2 levels back)manual · high confidence
  • Clinical Reasoning 4: Cluster the findings: Rung 4 of the clinical ladder: a differential explains a cluster, so the cluster has to exist first.
  • Clinical Reasoning 5: Rule out the worst first: Rung 5 of the clinical ladder: the dangerous candidate always earns a seat on the list, and the worst-first habit is what remembers to seat it.

Re-learn the skill with worked practice and clear examples.

A working differential holds several candidates that each fit the findings, ranked but none confirmed. The failure modes are certainty, everything, and fashion.

Step 1: See the healthy shape
A few named candidates, each compatible with the cluster, ordered by current support, all still open. That shape keeps every next step honest.
Step 2: Name the three failure shapes
One candidate declared certain on day one. A copied encyclopedia of everything possible. Or whatever is going around this month, adopted by default. Each ends the reasoning instead of starting it.
Step 3: Rank without marrying
The leader is written first and pursued first. It stays beatable, which is the entire point of keeping rivals on the page.
Practice

A 15-year-old has two days of cough, a temperature of 100.4 F, and chest tightness when running. Which written list is a working differential for this case?

Reviewed
  1. A.Pneumonia, recorded as certain so that treatment can begin without further delay
  2. B.Every illness that has ever been recorded as causing a cough, copied out in full from a reference website
  3. C.Whichever illness is circulating at school this month, adopted until someone objects
  4. D.A common cold, an asthma flare, and early pneumonia, each fitting the findings, none yet confirmed
Show the worked solution ▾

Answer: D. A common cold, an asthma flare, and early pneumonia, each fitting the findings, none yet confirmed

  1. Step 1: Check the shape first: A working list is short, named, and open. Certainty on day one, an unfiltered encyclopedia, and a default fashion all fail on shape before content.
  2. Step 2: Check each candidate against the findings: Cough with a low fever fits a cold; cough with exercise tightness fits an asthma flare; cough with fever can open a pneumonia story. Three seats, all earned.

Why it's right: Three named candidates that each account for the findings, ranked and unconfirmed, is exactly the working shape; the rivals are certainty, everything, and fashion, the three classic failures.

Why the others miss:
  • A: Certainty on day one ends reasoning at its first step and bets the patient on it.
  • B: An unfiltered list of everything fails the admission test in bulk and ranks nothing.
  • C: What is going around raises a candidate's rank; it does not hand out the diagnosis.

Aligned to Clinical reasoning: shape of a working differential · reading level ~grade 9

Where you'd see this
  • Hospital notes carry a differential section for every undiagnosed admission, in exactly this shape: named, ranked, open.
Careers that use this
Registered Nurse: Reads the chart's differential before each shift so observations get aimed at the questions still open.
Guided notes

Fill these in as you work through the lesson.

Big idea: A differential is a short written list of rival explanations for one cluster of findings, each candidate fitting the main findings, each held as beatable, ranked and re-ranked as evidence arrives.
Key terms: write the meaning
  • Differential diagnosis (the written rival list):  
  • Candidate (one possible explanation on it):  
  • Rank (current standing, never a verdict):  
  • Promote and demote (what new evidence does to ranks):  
The rule

Every candidate must explain the   findings. The list stays  , usually three to five names. New evidence does not restart the list; it moves the  .

Check yourself
  1. Why is a one-item differential more dangerous than a five-item one? 
  2. What disqualifies a candidate from the list in the first place? 
  3. Give an example of a finding that would promote the last-place candidate to first. 
Work one example

A 13-year-old has three days of sore throat, swollen neck glands, and a 100.9 F fever. Write a three-candidate differential, mark what supports each, and name the single new finding that would most re-order your ranks.