Trimethoprim-Sulfamethoxazole Allergy Decision Rule (SULF-FAST)

Stratify reported TMP-SMX (Co-trimoxazole) allergy using the SULF-FAST score adapted from PEN-FAST. Intended for adults with nonsevere, labeled sulfa-type allergy when considering allergy testing or supervised oral challenge pathways.

Allergy history criteria

Select each feature that applies to the index reaction attributed to TMP-SMX or a specific sulfa antibiotic consistent with trimethoprim-sulfamethoxazole.

Reaction ≤5 years ago

2 points

The index reaction to trimethoprim-sulfamethoxazole (TMP-SMX) or an equivalent documented sulfa antibiotic label occurred within the past 5 years.

Yes

Anaphylaxis, angioedema, or SCAR

2 points

The reaction involved anaphylaxis, angioedema, or a severe cutaneous adverse reaction (e.g. SJS/TEN, DRESS, other SCAR phenotypes).

Yes

Treatment required for the episode

1 point

Medical treatment was needed (e.g. epinephrine, emergency care, systemic corticosteroids, antihistamines requiring escalation, hospitalization attributable to the reaction).

Yes

Disclaimer: SULF-FAST supports risk stratification only. It does not replace specialist drug allergy assessment, institutional protocols, or shared decision-making. Studies excluded patients with recent anaphylaxis and SCAR from direct challenge pathways.

SULF-FAST — Scoring Explained

Scoring (same weights as PEN-FAST)

SULF-FAST applies the PEN-FAST point system to trimethoprim-sulfamethoxazole (TMP-SMX / Co-trimoxazole) allergy history. Sum the points below; the maximum score is 5.

• ≤5 years since the index reaction — 2 points (the “F” in PEN-FAST: five or fewer years ago)
• Anaphylaxis, angioedema, or SCAR (e.g. SJS/TEN, DRESS) — 2 points (the “A” and “S” domains in PEN-FAST)
• Treatment required for the episode (e.g. epinephrine, ED care, systemic steroids, hospitalization for the reaction) — 1 point (the “T” in PEN-FAST)

Patients should have a documented TMP-SMX–type sulfa allergy label consistent with the cohorts used to validate SULF-FAST.

Interpretation (validated cutoff)

<3

Lower-risk phenotype (rule-in for delabeling pathways)

In validation cohorts, a score below 3 was associated with a low rate of positive allergy testing and high negative predictive value; used to identify candidates for supervised oral challenge or allergy assessment per protocol.

≥3

Higher-risk features

Score of 3 or more indicates features associated with higher probability of confirmed allergy in studied cohorts; specialist drug allergy evaluation is generally preferred before re-exposure.

Sources

Waldron JL, Rose M, Vogrin S, et al. Development and Validation of a Sulfa Antibiotic Allergy Clinical Decision Rule. JAMA Netw Open. 2023;6(6):e2316776. doi:10.1001/jamanetworkopen.2023.16776. PMID: 37273210.

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. (PEN-FAST derivation and point weights.)

Limitations

• Derivation/validation cohorts excluded recent anaphylaxis and SCAR from challenge pathways; this calculator does not replace allergy specialist assessment.
• Performance differs by population (e.g. immediate vs delayed phenotype) and setting.
• Do not use to justify unsupervised rechallenge; management should follow institutional protocols.

Background and clinical need

Trimethoprim-sulfamethoxazole (TMP-SMX, co-trimoxazole) remains first-line therapy for many community and hospital-acquired infections and is a cornerstone of prophylaxis in immunocompromised hosts. Despite favorable pharmacology and spectrum, a documented “sulfa” or TMP-SMX allergy often leads clinicians to avoid the drug entirely. Allergy labels in electronic health records are frequently imprecise, based on remote or mild events, or conflate non–immune-mediated intolerance with true immunoglobulin E–mediated or T cell–mediated hypersensitivity.

When the label is inaccurate, patients receive broader-spectrum or more toxic alternatives, with downstream effects on resistance, Clostridioides difficile infection, cost, and outcomes. Antimicrobial stewardship programs therefore have strong interest in safe pathways to “delabel” low-risk drug allergies. Formal allergy testing and supervised drug challenge are effective but resource-intensive; a brief, point-of-care clinical decision rule can help prioritize who may be appropriate for further evaluation versus who should be referred for specialist assessment before any re-exposure.

Origin of SULF-FAST

SULF-FAST is the application of the PEN-FAST penicillin allergy clinical decision rule—developed and validated in penicillin-labeled cohorts—to patients who report allergy to TMP-SMX or an antibiotic label that clearly refers to this combination. The derivation study for the sulfa adaptation used prospectively and retrospectively assessed cohorts from specialist drug allergy services in Australia and the United States. Patients with recent anaphylaxis or severe cutaneous adverse reactions were generally excluded from the challenge-oriented pathways that motivated the tool; the rule is best understood as stratifying nonsevere, labeled TMP-SMX allergy histories in patients being considered for structured assessment or supervised oral challenge.

What the score measures

SULF-FAST does not diagnose allergy in isolation. It summarizes three history-based features that, in multivariable models for penicillin allergy testing, carried the most stable incremental association with a positive allergy workup. Those same weighted items were applied to TMP-SMX–labeled patients when validating the sulfa rule. The total score ranges from 0 to 5.

Component 1: Recency of the index reaction (2 points)

Assign 2 points if the patient’s defining reaction to TMP-SMX (or equivalent documented sulfa antibiotic consistent with this agent) occurred within the past five years. More remote reactions are associated with lower probability of persistent sensitization and positive testing in studied populations, whereas recent events increase concern for ongoing hypersensitivity. Clinicians should anchor this item to the index episode—the reaction that led to the allergy label—rather than to unrelated symptoms or uncertain timing.

Component 2: Anaphylaxis, angioedema, or severe cutaneous adverse reaction (2 points)

Assign 2 points if the history describes anaphylaxis, angioedema, or a severe cutaneous adverse reaction (SCAR) phenotype such as Stevens-Johnson syndrome, toxic epidermal necrolysis, drug reaction with eosinophilia and systemic symptoms (DRESS), or other serious immune-mediated cutaneous syndromes attributed to the drug. This domain captures severity of phenotype rather than every benign exanthem or isolated pruritus. Mild morbilliform eruptions without mucosal involvement, when clearly documented as such, typically do not satisfy this major criterion; when documentation is ambiguous, conservative interpretation favors specialist input.

Component 3: Treatment required for the episode (1 point)

Assign 1 point if medical treatment was required because of the reaction—for example epinephrine administration, emergency department evaluation, systemic corticosteroids, intravenous antihistamines, or hospitalization attributable to the drug event. The intent is to identify reactions that prompted an escalated medical response beyond self-limited home care. Self-treated symptoms with over-the-counter antihistamines alone may or may not meet this threshold depending on documentation and clinical judgment; the rule was operationalized in research cohorts with explicit phenotype definitions.

Interpretation of the total score

The validated operational cutoff mirrors PEN-FAST: a total score strictly less than 3 identifies a lower-risk phenotype in the studied TMP-SMX cohorts, associated with a high negative predictive value for confirmed allergy on testing and a low observed rate of positive challenge or patch-test outcomes at that threshold. A score of 3 or higher aggregates features associated with a materially higher probability of confirmed allergy in those same development and validation analyses and should generally prompt specialist drug allergy evaluation before any attempt at reintroduction outside protocol.

Subgroup performance differs by allergy phenotype (immediate versus delayed versus unclear timing). The tool showed stronger discrimination in some cohorts and phenotypes than others; clinicians should not treat the score as a substitute for shared decision-making, institutional pathways, or access to allergy expertise when the clinical stakes are high.

Role in antimicrobial stewardship and patient safety

Where TMP-SMX is preferred for efficacy, narrow spectrum, or guideline-based prophylaxis, an overly broad allergy label directly conflicts with stewardship goals. SULF-FAST offers a structured way to triage history at the bedside or in pre-clinic review: patients scoring below the cutoff may be candidates for referral to an antibiotic allergy clinic, stewardship-led delabeling programs, or supervised oral challenge according to local policy. Patients scoring at or above the cutoff remain candidates for expert assessment but are less likely to be appropriate for abbreviated or non-specialist pathways.

The rule must never be used to justify unsupervised rechallenge or empiric full-dose treatment in an unmonitored setting. Even low-risk scores reflect population probabilities, not a guarantee of tolerance in an individual patient. Facilities should align use of the score with observation protocols, rescue medications, and documentation standards used for other drug challenges.

Important limitations

Validation cohorts were specialty-referred and protocol-driven; performance may differ in primary care, urgent care, or populations with sparse documentation.
Single-dose or short challenge protocols may not capture every delayed hypersensitivity pattern; extended prescribing observation has been reported in follow-on work but remains context-dependent.
The score does not replace history-taking, review of prior records, or evaluation for coexisting conditions that modify risk.
“Sulfa” allergy labels sometimes refer to non-antibiotic sulfonamides or other agents; the rule was developed in patients with TMP-SMX–specific or equivalent antibiotic labels.

Clinical pearl: Use SULF-FAST as a triage adjunct—not as permission to administer TMP-SMX. Pair scores below 3 with a formal delabeling or challenge pathway; pair scores of 3 or higher with allergy specialist consultation when TMP-SMX is clinically essential.

CalcMD.

SULF-FAST (TMP-SMX Allergy)

Free SULF-FAST calculator: score TMP-SMX allergy history (≤5 years, anaphylaxis/angioedema/SCAR, treatment required). Interpret at cutoff <3 vs ≥3 for delabeling and stewardship pathways.