What a differential is, and what it isn't

A differential diagnosis is the list of conditions that could explain what you are seeing. Physicians use it to reach a final diagnosis. EMTs use it to reach a field impression: the most likely problem, plus the dangerous ones you can't yet set aside, so you choose the right treatment, the right destination, and the right urgency.

You will rarely prove anything in the field. The goal is to narrow the list with each question, vital sign, and exam finding, and to never let a deadly condition slip off the list without a reason.

An EMT never "rules out" a condition. Findings make it more likely or less likely. Say it that way in your report.

The four moves

Open wide. Start from the chief complaint and list everything that could cause it. General complaints like "weak," "dizzy," or "not acting right" start with the widest lists.
Worst first. Put the conditions that can kill or disable in the next hour at the top. These are the "must not miss" conditions in this lab.
Narrow with findings. Each finding pushes conditions up or down. A pertinent positive is a finding that's present and points toward something. A pertinent negative is a finding that's absent and points away from something ("no fever," "lungs clear," "glucose normal").
Commit and reassess. Choose a working field impression, treat for it, and keep reassessing. If the patient doesn't respond as expected, re-open the list.

Where findings come from

Source	What it narrows	Examples
Scene size-up	Environment, mechanism, toxic exposures	Generator in the garage, pill bottles, hot gym
Chief complaint + OPQRST	Which organ system	Tearing to the back, worse with a breath, worst ever
SAMPLE history	Risk factors and triggers	Diabetic who skipped a meal, recent long trip, blood thinner
Vital signs	Severity and physiology	Pulse 48, SpO2 86%, temp 105°F
Devices	Specific causes	Blood glucose, SpO2, CO-oximetry, 12-lead per protocol
Physical exam	Confirms or refutes	Crackles, one-sided weakness, pinpoint pupils, stiff neck

Tools that keep the list open

AEIOU-TIPS for altered mental status: Alcohol, Epilepsy/Endocrine/Electrolytes, Insulin, Overdose/Oxygen, Uremia, Trauma/Temperature, Infection, Psychiatric/Poisoning, Stroke/Shock.
"Check the sugar." Hypoglycemia can mimic stroke, seizure, intoxication, and psychiatric illness. Every altered patient gets a glucose check.
"What's different today?" Chronic conditions have a baseline. The change from baseline is the clue.
Treat the patient, not the label. "Frequent flyer," "drunk," "anxious," and "psych patient" are the labels most often behind missed diagnoses.

Traps

Anchoring: sticking with the first impression (often the dispatch info) even as new findings argue against it.
Premature closure: stopping the search once something fits. An intoxicated patient can also be hypoglycemic and have a head bleed.
Diagnosis of exclusion first: calling something anxiety, a stomach bug, or a psychiatric crisis before the dangerous causes are less likely.
Trusting one number: a normal SpO2 in CO poisoning, a normal temperature in an older septic patient, a normal BP in a young patient compensating for blood loss.

How to use this lab

Narrow it down: pick a starting complaint. The candidate list appears with must-not-miss conditions on top. Tap findings to mark them present (tap once) or absent (tap twice). Watch the list re-rank. The Check next box suggests the findings that would best separate the top candidates.
Guided cases: findings come out one step at a time. Answer each question before revealing, then commit to a field impression.
Conditions: a reference sheet for every condition, with key clues, red flags, EMT care, transport, and the common trap.

All treatments listed are within the EMT scope of practice under the National EMS Scope of Practice Model. Doses, thresholds, and destination decisions follow local protocol and medical direction. The scoring in this lab is a teaching aid that shows how findings shift likelihood. It is not a clinical decision tool.