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ProbID

Build the case: choose syndrome and setting, add findings and tests, then see how the post-test probability compares to the treatment threshold. (Educational aid, not a guideline.)

Post-test probability
GET MORE DATA
3.0%
Not enough probability for antibiotics yet
The current probability is still below the CAP treatment threshold, so more data or reassessment makes more sense than empiric antibiotics.
Pretest
3.0%
Combined LR
1.00
Treat at
11%
CAP·Primary Care
Case state is in the URL
Build the case

Pick syndrome, choose the setting, then add findings.

Setting
Primary Care
Pretest 3.0%
Setting-only baseline (not symptom-enriched). Add findings such as cough separately.
Evidence: Metlay et al. ATS/IDSA (2019)
Document in order
Keep the graph in view while you open one focused workspace at a time.
0 marked
Live probability trace

ProbID plots the current path from the starting context to the most recent post-test probability, using the same LR sequence shown in the math panel.

Observe below
12%
Treat at
11%
0255075100TreatGet more dataObserveStart
Start
3.0%
Pretest 3.0%
Choose findings above to watch the probability move in real time.
Threshold shading updates live with syndrome-specific modifiers. The trace itself only reflects the exact evidence sequence you selected.
What if you ordered more tests?

These tests could change the decision. Click to add them.

CXR: lobar or multilobar consolidation/infiltrate
High value
0.8% if −20% if +
Tachypnea (RR ≥ 24)
Rule-out
1.5% if −7.2% if +
O2 sat < 95%
Rule-out
1.8% if −6.6% if +
Fever (≥38°C)
Rule-out
2.1% if −5.8% if +
Crackles/rales
Rule-out
2.1% if −5.8% if +
See the math▾
Start: 3.0%Combined LR: 1.00

Choose findings to see stepwise probability updates.

Utility model
Treat + true CAP0.972 → 0.972
No treat + true CAP0.918 → 0.918
Treat + no CAP0.993 → 0.993
No treat + no CAP1.000 → 1.000
Multiplying LRs assumes conditional independence. Correlated inputs may overestimate certainty. Educational content only — not medical advice.
Educational content only. Not medical advice. See references & methodology.

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Created by Alvaro Ayala, MD

Infectious Diseases Fellow at Stanford University, building a clearer, more useful home for case-based learning and clinical reasoning in ID.

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