PEN-FAST (Penicillin Allergy)

Score the PEN-FAST rule (F, A/S, T) to stratify reported penicillin allergy. Points 0–5; low risk if score is below 3. Educational support for triage, not a substitute for supervised evaluation.

Penicillin Allergy Decision Rule (PEN-FAST)

Stratify reported penicillin allergy using the validated PEN-FAST score. Sum points for timing of the index reaction, severe reaction phenotype, and whether treatment was required. Interpret at score less than 3 versus 3 or higher.

Allergy history

Answer each item for the index penicillin reaction attributed to a penicillin-class antibiotic.

Five years or less since reaction (or timing unknown)

max 2 points

The index penicillin reaction occurred within the past 5 years, or the timing of the reaction is not known.

No (0 points)Yes or unknown (+2 points)

A / S

Anaphylaxis or angioedema, or severe cutaneous adverse reaction

max 2 points

The reaction involved anaphylaxis, angioedema, or a severe cutaneous adverse reaction (for example SJS, TEN, DRESS, or other SCAR).

No (0 points)Yes (+2 points)

Treatment required for the reaction (or unknown)

max 1 point

Medical treatment was needed (for example epinephrine, emergency care, systemic corticosteroids, antihistamines with escalation, or hospitalization for the reaction), or it is unknown whether treatment was required.

No (0 points)Yes or unknown (+1 point)

Disclaimer: PEN-FAST supports risk stratification only. It does not replace specialist drug allergy assessment, institutional protocols, or shared decision-making.

PEN-FAST — Scoring Explained

PEN-FAST mnemonic

PEN-FAST identifies patients with a reported penicillin allergy who are more likely to have IgE-mediated or severe reactions worth specialist evaluation versus lower-risk histories where direct oral or graded challenge may be appropriate. Sum the points below; the maximum score is 5.

• Five years or less since the reaction, or timing unknown — 2 points
• Anaphylaxis or angioedema, or Severe cutaneous adverse reaction — 2 points
• Treatment required for the reaction, or unknown — 1 point

Weights and interpretation follow the primary derivation study and common clinical references such as MDCalc.

Interpretation

Low-risk

Consider a direct oral or graded challenge when appropriate per protocol and shared decision-making.

≥3

Moderate- to high-risk

Refer to an allergist for a more detailed evaluation, which may include skin testing.

Document the PEN-FAST score and any subsequent allergy testing results in the medical record to support future clinical decisions.

Primary reference

Trubiano JA, Vogrin S, Chua KYL, et al. Development and validation of a penicillin allergy clinical decision rule. JAMA Intern Med. 2020;180(5):745-752. doi:10.1001/jamainternmed.2020.0403. PMID: 32176248.

Limitations

• Validation cohorts and pathways may not generalize to every patient or institution.
• The rule does not replace clinical judgment, antibiotic stewardship, or allergy specialist input when indicated.
• Do not use to justify unsupervised rechallenge; management should follow institutional protocols.

What is PEN-FAST?

The Penicillin Allergy Decision Rule (PEN-FAST) is a brief clinical decision aid that helps clinicians categorize adults who carry a penicillin allergy label. Instead of treating every charted allergy the same way, the rule focuses on a small set of historical reaction features that, in validation work, separate lower-risk labels from those more likely to represent true immunologic allergy or severe reactions.

PEN-FAST does not replace skin testing, graded challenge, or allergist consultation when indicated. It is a structured way to document why a patient might be triaged toward expedited delabeling pathways versus more cautious evaluation.

Why penicillin allergy labels matter

A reported penicillin allergy is common in electronic health records. Many labels are inaccurate, outdated, or based on nonspecific symptoms from childhood. Carrying the label often leads to broader-spectrum antibiotics, more resistant organisms, higher costs, and avoidable adverse outcomes. At the same time, some patients truly experienced anaphylaxis or severe cutaneous reactions, where amoxicillin or other beta-lactams should only be reintroduced with appropriate safeguards.

Decision rules such as PEN-FAST aim to identify patients whose reaction history is compatible with a lower-risk phenotype, so that allergy services and inpatient workflows can prioritize supervised testing or challenge for those patients while routing higher-risk histories toward fuller evaluation.

The three PEN-FAST domains

PEN-FAST uses three letters grouped into three scored items: F, A/S, and T. Points are additive. For two items, an answer of “yes” or “unknown” maps to the higher-risk branch because uncertain timing or uncertain treatment need behaves like risk in the rule design.

F: Recency of the index reaction (or unknown timing)

This item captures whether the recorded reaction occurred within the past five years or whether the timing is not known. More recent reactions (or unclear timing) score higher because remote, well-characterized mild reactions are more consistent with low-risk labels, while recent or undocumented timing raises concern for an evolving or poorly defined history.

A/S: Severe reaction phenotype

This item asks whether the reaction involved anaphylaxis, angioedema, or a severe cutaneous adverse reaction (SCAR), such as Stevens-Johnson syndrome, toxic epidermal necrolysis, drug reaction with eosinophilia and systemic symptoms, or similar severe mucocutaneous syndromes. These phenotypes carry different implications than isolated subjective symptoms without objective findings. In PEN-FAST scoring, this item is treated as a yes or no branch (unknown should be resolved clinically rather than assumed benign).

T: Medical treatment for the reaction (or unknown)

This item reflects whether the episode required medical treatment, for example epinephrine, emergency department care, systemic corticosteroids, hospitalization, or other escalation for the reaction. If it is unknown whether treatment occurred, the rule typically counts that uncertainty toward the higher-risk pathway because incomplete histories cannot be assumed to be mild.

How the score is calculated

Points are assigned as follows in this calculator:

F (five years or less since reaction, or timing unknown): 2 points when present.
A/S (anaphylaxis or angioedema, or severe cutaneous adverse reaction): 2 points when present.
T (treatment required for the reaction, or unknown): 1 point when present.

The total score ranges from 0 to 5. The maximum arises when all three domains contribute their full points.

Interpreting the total score

In the operational definition used here, a total score below 3 corresponds to a lower-risk PEN-FAST pattern in validation cohorts. That pattern supports consideration of structured allergy assessment or a supervised oral challenge pathway when appropriate per local protocol and clinician judgment.

A total score of 3 or higher corresponds to a moderate- to higher-risk pattern based on reaction features. These patients are more likely to require specialist evaluation, and management should not be inferred from the numeric score alone. Urgent clinical scenarios, limited antibiotic options, or institutional policies may still alter the practical next step.

How to use this tool responsibly

Apply PEN-FAST only with an accurate history. Clarify which drug was implicated, whether the diagnosis was penicillin versus another beta-lactam, and whether symptoms could be explained by infection itself, concurrent medications, or viral exanthems. Whenever feasible, review outside records rather than relying on a single checkbox carried forward for years.

Do not use PEN-FAST to justify empiric beta-lactam therapy in unstable patients solely because the score is low. Likewise, do not use a high score as an absolute barrier if an allergist determines that testing or supervised reintroduction is still appropriate. Document the score, the questions asked, and the rationale for any challenge or referral.

Limits of any score-based rule

PEN-FAST summarizes selected historical features. It cannot detect silent immune sensitization, partial histories, documentation errors, or reactions mediated by non-IgE mechanisms that do not match the listed severe phenotypes. Institutions differ in challenge protocols, staffing, and monitoring capacity. Always align decisions with current local guidelines, pharmacy policies, and specialist availability.