PEN-FAST Components for Penicillin Allergy Delabeling
The PEN-FAST tool consists of three specific components with weighted scoring: (1) timing of the reaction (≤5 years ago = 2 points), (2) type of reaction (anaphylaxis/angioedema OR severe cutaneous adverse reaction = 2 points), and (3) treatment requirement for the allergy episode (1 point). 1 Any answer option that does not match these three elements is not part of the PEN-FAST decision tool.
Understanding the PEN-FAST Scoring System
The PEN-FAST mnemonic breaks down as follows 1:
- Penicillin allergy (the label itself - this is the prerequisite, not a scored component)
- EN = Five or fewer years ago (2 points - Major criterion)
- FAST = Anaphylaxis/angioedema (2 points - Major criterion)
- S = Severe cutaneous adverse reaction (SCAR) (2 points - Major criterion, alternative to anaphylaxis)
- T = Treatment required for the allergy episode (1 point - Minor criterion)
Risk Stratification Threshold
A score of <3 points identifies low-risk penicillin allergies suitable for direct oral challenge without skin testing, with a negative predictive value of 96.3% (95% CI, 94.1%-97.8%). 1 This threshold was validated across multiple international cohorts totaling 1,567 patients. 1
Clinical Application
The tool enables point-of-care assessment by non-allergists 1:
- Low-risk patients (PEN-FAST <3): Can proceed directly to oral penicillin challenge without formal skin testing 1, 2
- Higher-risk patients (PEN-FAST ≥3): Should undergo formal allergy evaluation with skin testing before challenge 1
In prospective validation, only 3.7% of low-risk patients (17 of 460) had positive allergy testing results 1, and in real-world implementation, the negative predictive value remained 95.8% even in populations with high prevalence of true allergies. 2
Key Exclusions from PEN-FAST
Elements that are NOT part of the three-component PEN-FAST scoring system include 1:
- Specific drug involved (though aminopenicillins account for >70% of cases, this is not scored) 3
- Age of the patient 1
- Documentation by healthcare personnel 3
- Number of previous reactions 1
- Family history of penicillin allergy 1
Common pitfall: Clinicians may confuse risk stratification criteria from other guidelines (such as recent reaction, mucosal involvement, or healthcare documentation) 3 with the actual PEN-FAST components. The tool deliberately uses only three weighted elements to maintain simplicity and point-of-care usability. 1