Form A Differential Diagnosis
Use patient evidence to form a differential diagnosis without overclaiming.
- 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.
| Possible diagnosis | Evidence for | Evidence against (if present) |
|---|---|---|
| Flu | fever 39C, body aches | vomiting as the main problem |
| Food poisoning | vomiting, cramps | fever of 39C |
| Migraine | one-sided headache | fever |
A patient has vomiting and cramps but NO fever. Using the table, which diagnosis fits best?
Reviewed| Possible diagnosis | Evidence for | Evidence against (if present) |
|---|---|---|
| Flu | fever 39C, body aches | vomiting as the main problem |
| Food poisoning | vomiting, cramps | fever of 39C |
| Migraine | one-sided headache | fever |
- A.Food poisoning
- B.Migraine
- C.Flu
- D.All three fit equally
Show the worked solution ▾
Answer: A. Food poisoning
- Step 1: List the patient's evidence: Vomiting, cramps, and no fever.
- Step 2: Match to a row: Food poisoning lists 'vomiting, cramps' as evidence for it, and its against entry, a fever of 39C, is absent here. 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.
- B: Migraine's evidence for is a one-sided headache, which the patient does not have.
- C: Flu lists fever as evidence for it, but the patient has no fever.
- D: Only one row matches both the present and absent evidence.
Aligned to Biotechnology Research and Experiments · reading level ~grade 9
A patient reports body aches and a fever of 39 C, with no vomiting and no one-sided headache. Which row does that evidence rule IN?
Reviewed- A.Migraine, because a fever can bring on a headache later
- B.Food poisoning, because body aches include stomach cramps
- C.Flu, because the patient shows both findings listed for that row
- D.None of the rows, because a patient can match only one finding
Show the worked solution ▾
Answer: C. Flu, because the patient shows both findings listed for that row
- Step 1: Read the patient's findings: Fever of 39 C and body aches, with no vomiting and no one-sided headache.
- Step 2: Check each row: Flu lists fever 39 C and body aches. Food poisoning needs vomiting and cramps. Migraine needs a one-sided headache.
Why it's right: The patient shows both findings listed for the flu row and neither of the findings the other two rows need.
- A: Migraine is ruled in by a one-sided headache, which this patient does not have.
- B: Body aches are not stomach cramps, and there is no vomiting either.
- D: This patient matches two findings in a single row, which is exactly what ruling in means.
Aligned to Biotechnology Research and Experiments · reading level ~grade 9
- A clinician crosses off diagnoses whose 'evidence against' shows up in the patient's chart.
Fill these in as you work through the lesson.
- 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):
A diagnosis is ruled when the patient shows its 'evidence for', and ruled when the patient shows its 'evidence against'.
- Which symptom is the patient asking you to match?
- Which named diagnosis lists that symptom as evidence FOR it?
- Does the patient show any evidence AGAINST that diagnosis?
A patient has a fever and body aches but no vomiting. Use the table to rule out food poisoning (its against entry, fever, is present, and its vomiting evidence is missing) and name Flu as the most likely diagnosis.