Antibiotics After Incision and Drainage to Prevent Fistula Formation After First Episode of Perianal Abscess

This is a single-center, randomized, open-label clinical trial evaluating whether a 7-day course of postoperative antibiotics after standard of care incision and drainage of a first-time perianal abscess reduces subsequent anal fistula formation compared with no postoperative antibiotics.

Perianal abscess is a common anorectal condition treated primarily with incision and drainage. After drainage, some patients later develop an anal fistula, which is an abnormal tract between the anal canal and the perianal skin. The role of routine antibiotics after adequate drainage remains uncertain, especially in patients without high-risk clinical features that would otherwise require antibiotics as part of standard care. This study is designed to evaluate whether a short postoperative antibiotic course can reduce fistula formation after a first episode of perianal abscess.

Eligible adult participants with a first-time perianal abscess requiring incision and drainage will be randomized in a 1:1 ratio to either postoperative antibiotics or no postoperative antibiotics. Participants assigned to the antibiotic arm will receive amoxicillin/clavulanate for 7 days. Participants with penicillin allergy may receive ciprofloxacin plus metronidazole instead. Participants assigned to the no-antibiotic arm will not receive postoperative antibiotics unless antibiotics are later determined to be medically necessary by the treating clinician.

All participants will receive standard clinical care for the perianal abscess, including incision and drainage as determined by the treating surgical team. The study does not change the technical approach to drainage or other clinically indicated care. The research intervention is the randomized postoperative antibiotic strategy.

Participants will be followed through routine clinical visits and a 12-month follow-up phone call. The study will collect information from the medical record and follow-up contacts regarding fistula formation, recurrent abscess, additional procedures or treatment, emergency department visits or readmissions, antibiotic use, medication adherence, and antibiotic-related adverse events. The findings may help clarify whether routine postoperative antibiotics should be used after drainage of first-time perianal abscess in patients without other clinical indications for antibiotics.

• Amoxicillin + Clavulanate (Drug): Participants assigned to the antibiotics after incision and drainage arm will receive amoxicillin/clavulanate 875/125 mg by mouth twice daily for 7 days after standard of care incision and drainage.
• Ciprofloxacin + Metronidazole (Drug): Participants assigned to the antibiotics after incision and drainage arm who have a penicillin allergy may receive ciprofloxacin 500 mg by mouth twice daily plus metronidazole 500 mg by mouth three times daily for 7 days after standard of care incision and drainage.

Primary outcomes

• Number of participants who develop fistula in ano within 6 months after incision and drainage [6 months after incision and drainage]: Anal fistula formation will be assessed as a binary outcome for each participant. Fistula will be defined as documentation of a clinically diagnosed anal fistula during follow-up, based on clinical examination, imaging, operative or procedure findings, or medical record documentation.

Secondary outcomes

• Number of participants who develop Fistula in Ano within 12 months after incision and drainage [12 months after incision and drainage]: Anal fistula formation within 12 months will be assessed as a binary outcome for each participant. Participants will be counted as having developed a Fistula in Ano if a clinically diagnosed anal fistula is documented within 12 months after incision and drainage. Documentation may be based on clinical examination, imaging, operative or procedure findings, medical record review, or participant report during the 12-month follow-up phone call and confirmed by clinical documentation when available.
• Number of participants who develop recurrent perianal abscess within 12 months after incision and drainage. [12 months after incision and drainage]: Recurrent perianal abscess will be assessedas a binary outcome for each participant. Recurrent perianal abscess will be defined as documentation of a new or recurrent perianal abscess during follow-up, including clinical diagnosis, emergency department visit, clinic visit, imaging, or need for repeat drainage or procedure.
• Number of participants who require an additional abscess or fistula related procedure or intervention within 12 months after incision and drainage. [12 months after incision and drainage]: The need fo additional procedure or intervention will be assessed as a binary outcome for each participant. Participants will be counted as requiring an addictional procedure or intervention if any abscess or fistula related procedure is documented within 12 months after incision and drainage. Procedures may include repeat incision and drainage, examination under anesthesia, seton placement, fistulotomy, fistula repair, or other abscess or fistula related intervention documented in the medical record.
• Number of participants with an abscess or fistula emergency department visit or hospital readmission within 12 months after incision and drainage. [12 months after incision and drainage]: Emergency department visits or hospital readmissions will be assessed as a composite binary outcome for each participant. Participants will be counted in this outcome if they have at least one emergency department visit or at least one hospital readmission related to perianal abscess, fistula, wound infection, worsening symptoms, recurrent abscess, or complications of the assigned postoperative management strategy within 12 months after incision and drainage. Events will be identified through medical record review and follow-up assessments.
• Number of participants with antibiotic adverse events by event type within 30 days after incision and drainage. [Within 30 days after incision and drainage]: Antibiotic adverse events will include medication side effects or complications, documented during follow-up or through medical record review. These include nausea, vomiting, diarrhea, rash, allergic reaction, yeast infection, clostridioides difficile infection, tendon symptoms, neurologic symptoms, liver-related symptoms, or other adverse events attributed to antibiotic therapy. Adverse event will be summarized by event type. The number of participants with each adverse event type will be reported. A participant may be counted in more than one adverse event type if multiple events occur.

Sex: All, ages: 18 Years to any

Inclusion Criteria:

* Age 18 years or older
* First episode of perianal abscess requiring incision and drainage
* Able to provide informed consent
* English or Spanish speaking patient
* Treating surgical team has determined that incision and drainage is clinically indicated as part of standard care

Exclusion Criteria:

* Known fistula-in-ano at presentation
* Prior perianal abscess within 12 months
* History of inflammatory bowel disease, including Crohn's disease or ulcerative colitis
* Known or suspected anorectal malignancy
* Necrotizing soft tissue infection or Fournier's gangrene
* Systemic infection or sepsis
* Significant surrounding cellulitis or extensive soft-tissue infection for which antibiotics are clinically indicated
* Immunocompromised or immunosuppressed status, including active chemotherapy or radiation therapy, history of transplant, chronic systemic steroids, biologic immunosuppressive therapy, advanced or uncontrolled HIV/AIDS, or other clinically significant immunosuppression
* Cardiac condition requiring antibiotic prophylaxis or antibiotic therapy for procedures involving infected tissue
* Need for non-study antibiotics at the time of enrollment for another infection or medical indication
* Contraindication, allergy, or intolerance to all study antibiotic regimens
* Pregnant
* Prisoner
* Minor
* Adult unable to provide informed consent

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