How to Calculate SULF-FAST Score

The SULF-FAST is a clinical decision rule adapted from PEN-FAST for assessing the risk of true allergy in patients with reported trimethoprim-sulfamethoxazole (sulfa) allergy. It helps determine if delabeling or rechallenge is safe.

FORMULA

The SULF-FAST Score is calculated by adding the selected points:

Variable	Points
Five years or less since reaction	No: 0 Yes/unknown: 2
Anaphylaxis or angioedema OR Severe cutaneous adverse reaction*	No: 0 Yes: 2
Treatment required for reaction	No: 0 Yes/unknown: 1

*Severe cutaneous reactions may include Stevens-Johnson syndrome, toxic epidermal necrolysis, drug reaction with eosinophilia and systemic symptoms, and acute generalized exanthematous pustulosis. Patients with a severe delayed rash with mucosal involvement should be considered to have a severe cutaneous adverse reaction.

Total score ranges from 0 to 7. Select Yes/No for each criterion and calculate.

Note: Used for adults with reported sulfa allergy; consult specialist for complex cases.

Using the SULF-FAST Calculator

This calculator assists clinicians in risk-stratifying patients with labeled sulfa allergies to guide safe delabeling or rechallenge without specialist referral.

Select presence of each criterion. The calculator outputs the score and risk interpretation.

Example 1: >5 years ago, no anaphylaxis, no SCAR, no treatment.

1. ≤5 years: No
2. Anaphylaxis/angioedema: No
3. SCAR: No
4. Treatment: No
5. Score: 0
6. Interpretation: Low risk; consider delabeling

Example 2: ≤5 years, anaphylaxis, SCAR yes, treatment yes.

1. ≤5 years: Yes
2. Anaphylaxis/angioedema: Yes
3. SCAR: Yes
4. Treatment: Yes
5. Score: 7
6. Interpretation: High risk; avoid rechallenge

Use in conjunction with clinical judgment.

Common FAQ

Below are frequently asked questions about SULF-FAST Score:

• Q: What is the SULF-FAST Score?
  A: A tool to predict the risk of true sulfa allergy in patients with reported history, adapted from PEN-FAST.
• Q: How does it differ from PEN-FAST?
  A: It's specifically for trimethoprim-sulfamethoxazole (sulfa) allergies, but uses similar criteria.
• Q: What are typical risk levels?
  A: Score <3: <5% risk, >=3: >20% risk of hypersensitivity.
• Q: Is it validated?
  A: Yes, cohort studies show good negative predictive value for low scores.
• Q: Can it be used for all patients?
  A: Primarily for adults with reported sulfa allergy; use caution in immunocompromised or complex cases.

Advice on Interpreting Results

The SULF-FAST helps identify low-risk patients for safe delabeling but should not replace clinical judgment. For low scores (<3), direct oral challenge may be appropriate. For higher scores, refer to allergy specialist.

Facts & Figures

Interpretation:

SULF-FAST Score	Risk of True Antimicrobial Sulfonamide Allergy
<3	<5% (low risk)
≥3	>20% (elevated risk)

Evidence from validation studies shows high NPV for low-risk classification.

Consider patient history and comorbidities.