John Hay
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Principles of Biomedical Technology (Principles of Biomedical Science)
Unit 2: Unit 2.3 New to the PracticePBS 2.3Biotechnology Research and Experiments

Form A Differential Diagnosis

Use patient evidence to form a differential diagnosis without overclaiming.

Builds on (2 levels back)inferred · high confidence
  • Sign vs. symptom: Clinical data mixes measured findings with patient-reported history.
  • Normal range comparison: Students need a reference range or baseline to tell whether a value is concerning.

Prerequisites are inferred: pending teacher review.

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

To pick the most likely diagnosis, choose the row whose evidence FOR fits the patient and whose evidence AGAINST does NOT appear.

Step 1: Rule in
A diagnosis is 'ruled in' when the patient shows the evidence listed FOR it.
Possible diagnosisEvidence forEvidence against
Flufever 39C, body achesno stomach upset
Food poisoningvomiting, crampsno fever
Migraineone-sided headacheno fever, no vomiting
Differential diagnosis table matching symptoms to three named diagnoses
Step 2: Rule out
A diagnosis is 'ruled out' when the patient shows something listed as evidence AGAINST it.
Step 3: Pick the best fit
The most likely diagnosis is the one with matching evidence FOR and no matching evidence AGAINST.
Practice

A patient has vomiting and cramps but NO fever. Using the table, which diagnosis fits best?

Reviewed
Possible diagnosisEvidence forEvidence against
Flufever 39C, body achesno stomach upset
Food poisoningvomiting, crampsno fever
Migraineone-sided headacheno fever, no vomiting
Differential diagnosis table matching symptoms to three named diagnoses
  1. A.Food poisoning
  2. B.Flu
  3. C.Migraine
  4. D.All three fit equally
Show the worked solution ▾

Answer: A. Food poisoning

  1. Step 1: List the patient's evidence: Vomiting, cramps, and no fever.
  2. Step 2: Match to a row: Food poisoning lists 'vomiting, cramps' as evidence for and 'no fever' is not a problem for it. Flu needs fever; Migraine needs a headache.

Why it's right: Vomiting and cramps are the evidence FOR food poisoning, and the absence of fever rules out Flu, which requires it.

Why the others miss:
  • B: Flu lists fever as evidence for it, but the patient has no fever.
  • C: Migraine's evidence for is a one-sided headache, which the patient does not have.
  • D: Only one row matches both the present and absent evidence.

Aligned to Biotechnology Research and Experiments · reading level ~grade 9

Where you'd see this
  • A clinician crosses off diagnoses whose 'evidence against' shows up in the patient's chart.
Careers that use this
Physician Assistant: Lists possible diagnoses, then rules each in or out using the patient's evidence.Registered Nurse: Matches a patient's symptoms to the most likely cause before calling the provider.
Video library
Watch: Form A Differential Diagnosis
Clinical Reasoning 03a: Making a Differential Diagnosis
Rahul Patwari (Rahul's EM) · 10 min
Guided notes

Fill these in as you work through the lesson.

Big idea: A differential diagnosis is a list of possible causes; you use the patient's evidence to rule each one in or out and name the most likely.
Key terms: write the meaning
  • Differential diagnosis (list of possible diseases for a patient):  
  • Rule in (evidence supports this diagnosis):  
  • Rule out (evidence argues against this diagnosis):  
  • Most likely (the best-supported diagnosis so far):  
The rule

A diagnosis is ruled   when the patient shows its 'evidence for', and ruled   when the patient shows its 'evidence against'.

Check yourself
  1. Which symptom is the patient asking you to match? 
  2. Which named diagnosis lists that symptom as evidence FOR it? 
  3. Does the patient show any evidence AGAINST that diagnosis? 
Work one example

A patient has a fever and body aches but no vomiting. Use the table to rule out food poisoning (needs vomiting) and name Flu as the most likely diagnosis.