SULF-FAST Calculator
Trimethoprim-Sulfamethoxazole Allergy Decision Rule for identifying low-risk antimicrobial sulfonamide allergy.
< 3 points = low risk, published risk <5%,
and ≥ 3 points = elevated risk, published risk >20%.
It does not provide a validated exact percentage for every individual score from 0 to 5.Point assignment
- Reaction within 5 years: 2 points
- Anaphylaxis, angioedema, or SCAR: 2 points
- Treatment required: 1 point
SULF-FAST is a clinical decision rule used to identify people with a reported trimethoprim-sulfamethoxazole (TMP-SMX, “sulfa antibiotic”) allergy who are at low risk of having a true allergy. It was adapted from the PEN-FAST rule to help clinicians decide which patients may be suitable for further evaluation, often including a supervised direct oral challenge in the right setting.
The score uses three features from the past reaction:
- Reaction within 5 years = 2 points
- Anaphylaxis, angioedema, or severe cutaneous adverse reaction (SCAR) = 2 points
- Treatment required for the reaction = 1 point
That gives a total score from 0 to 5.
How it is interpreted:
- Score < 3: low-risk group, with published risk under 5%
- Score ≥ 3: higher-risk group, with published risk over 20%
It is meant to help with risk stratification, not to replace a clinician’s judgment. In practice, higher scores generally mean the patient should have more cautious specialist evaluation, while lower scores may support supervised delabeling pathways in appropriate patients. A later international validation study also evaluated the tool in broader settings.
