Tool Overview

ProbID turns pretest thinking into a visible workflow by combining setting, findings, and likelihood ratios into an educational post-test estimate, then comparing that probability with a treatment threshold.

Use it for

CAP, VAP, endocarditis, invasive mold, and other syndromes where diagnostic uncertainty matters more than rote recall and where a probability estimate can change what you do next.

Best for

Clinicians who want a structured way to move from suspicion to action without pretending the diagnosis is binary.

Interactive Tool

ProbID

Build the case in three steps: choose the syndrome and setting, add findings and tests, then compare the post-test probability with the treatment threshold. (Educational aid, not a guideline.)

Syndrome

CAP

Setting

Primary Care

Pretest 3.0%

Selected evidence

0 items

0 present • 0 absent

Current direction

More dataPost-test 3.0%

Treat threshold 11%

Step 1

Build the case

Pick the syndrome and setting first, then add the findings or tests you actually have.

Clinical syndrome

Setting

Primary CarePretest 3.0%

First pass

Good CAP starting pattern

Start with the setting, then add the findings that usually move CAP probability the most.

•Choose the outpatient, ward, or ICU-type setting first.
•Add high-yield findings such as infiltrate on chest imaging, hypoxemia, fever, or tachypnea.
•If probability stays below the threshold, keep non-pneumonia causes of respiratory symptoms in play.

Step 2

Selected evidence

Total

Present

Absent

No findings selected yet.

Step 4

See the math

Expand this only when you want to see how each LR changed the probability estimate.

Stepwise update and nomogramv

Choose findings/tests to see stepwise probability updates.

Fagan nomogram

Multiplying LRs assumes conditional independence. Correlated inputs may overestimate certainty.

Step 3

Interpret the result

The post-test probability is the estimate. The threshold tells you when treatment becomes worth it.

Post-test probability

Pretest 3.0%

3.0%

Treatment threshold

11%

Combined LR

1.00

Quick guide: pretest, LR, thresholdv

Pretest

Pretest probability is your starting estimate before applying the selected findings and tests.

Combined LR

The combined likelihood ratio shows how strongly the selected evidence shifts the starting probability.

Threshold

The treatment threshold is the probability at which treatment becomes worth it given the current harm or utility model.

Educational estimate only. Always use clinical context.

At a glance

Setting

Primary Care

Selected

Post-test

3.0%

Treat at

11%

Combined LR 1.00

Current recommendation

More data

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.

Post-test 3.0% versus threshold 11%.

This case setup is kept in the URL.

Patient factors0 selectedv

Toggle factors that make missing CAP more harmful or empiric antibiotics less desirable.

Age 65+ or frailtyRaises the downside of missed CAP more than the downside of treatment.Chronic lung or heart diseaseMakes missed CAP more consequential because decompensation is more likely.Chronic liver diseaseIncreases the danger of untreated CAP while also making antibiotic toxicity and interactions harder to tolerate.Chronic kidney diseaseShifts both disease and treatment utilities because CAP outcomes worsen and some regimens are less forgiving.Immunocompromised hostSubstantially increases the expected harm of missing CAP or undertreating early infection.Prior C. difficile or high microbiome riskMakes unnecessary empiric antibiotics meaningfully less attractive.Severe beta-lactam allergyRaises treatment burden because regimen options narrow and adverse-event concern increases.QT / drug-interaction concernAdds treatment downside when empiric CAP regimens are more likely to cause interactions or arrhythmia risk.High concern about antibiotic side effectsPreference-sensitive penalty for patients who find even mild antibiotic harms very disruptive.

CAP v1 uses transparent structured utilities anchored to published lower-respiratory-infection burden data and CAP severity concepts.

When do antibiotics become worth it?

For CAP, treat empirically when the post-test probability rises above the threshold implied by the selected patient factors.

EU(treat)

0.992

EU(no treat)

0.998

Net utility advantage: -0.005

Antibiotics become worth it at: 11%

0%Below thresholdTreat ≥ 11%100%

Below threshold: keep reassessing

At/above threshold: empiric treatment is justified

Why the threshold movedv

Treat + true CAP0.972 → 0.972

Prompt treatment still carries short-lived illness burden, but expected recovery is substantially better than missed or delayed CAP treatment.

Source: Stets et al. NEJM CABP trial; ATS/IDSA CAP guideline (structured estimate)

No treat + true CAP0.918 → 0.918

Missed CAP has lower expected utility because of prolonged symptoms and higher risk of deterioration, hospitalization, and sepsis before the diagnosis is corrected.

Source: Jose and Corso functional CAP study; ATS/IDSA CAP guideline (structured estimate)

Treat + no CAP0.993 → 0.993

Unnecessary outpatient CAP treatment usually produces a mild, transient utility penalty driven by GI adverse effects, rash, drug interactions, and C. difficile risk.

Source: Cochrane outpatient CAP antibiotics review; CABP trial safety data (structured estimate)

No treat + no CAP1.000 → 1.000

Reference state: no CAP and no unnecessary antibiotic exposure.

Structured CAP utility model anchored to published adult CAP functional-burden data, modern CABP trial response and adverse-event rates, and outpatient CAP antibiotic safety reviews.

Suggested next steps

Reassess trajectory, oxygenation, and chest imaging if CAP remains plausible.
Keep non-pneumonia causes of respiratory symptoms in play before committing to antibiotics.

What is driving the estimate?v

No selected findings yet.

Current result

Not enough probability for antibiotics yet

More data

Post-test 3.0%Treat at 11%

Educational content only. Not medical advice. See references & methodology.