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Gastrointestinal · Clinical algorithm

Abdominal Pain: Lower

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General Approach

1 Resuscitation / R/O Critical Diagnosis

  • See Abdominal Pain — General for the initial clinical approach to the undifferentiated abdominal-pain patient.
  • Is the patient crashing / unstable? Begin simultaneous resuscitation and evaluation — ABCs, IV/O2/monitor, fluids or blood as indicated, and focused bedside assessment for the cause.
  • Peritoneal signs / concern for surgical abdomen? Consider perforation, ischemia, obstruction, hemorrhage, or other surgical pathology. If stable → focused US / CT based on suspicion. If unstable → urgent surgical evaluation; do not delay definitive care for imaging.
  • Once immediate threats are addressed, move from “Could this patient die from this?” → “Where is the pain coming from?”

2 R/O Emergent Causes

  • Once the patient is stable and the critical abdominal diagnoses are addressed → localize the pain and narrow the source.
  • RLQ pain / tenderness → R/O appendicitis. Migration of pain, focal RLQ tenderness, peritoneal irritation, and an inflammatory response increase suspicion.
  • LLQ pain / tenderness → R/O diverticulitis. Focal LLQ findings ± fever / inflammatory markers suggest the diagnosis; CT A/P confirms diverticulitis and determines uncomplicated vs complicated disease.
  • Do not stop with GI pathology. Lower abdominal pain frequently overlaps with GU and pelvic disease.
    • GU → nephrolithiasis, pyelonephritis / cystitis; male lower abdominal or groin pain → consider acute scrotum / testicular torsion.
    • GYN → ectopic pregnancy, ovarian torsion, PID / TOA, ovarian cyst. Pregnancy test early → pelvic US based on suspicion.

3 Other GI Causes

  • If appendicitis, diverticulitis, GU, and GYN disease do not explain the presentation → broaden to other inflammatory, infectious, functional, and focal GI causes.
  • Known IBD + abdominal pain → do not anchor on the IBD. First ask: other diagnosis vs active flare vs IBD complication?
    • If flare → determine severity.
    • Severe / focal pain, vomiting / distention, sepsis, or peritoneal findings → R/O obstruction, abscess, perforation, toxic megacolon → CT.
  • Colitis → infectious vs inflammatory vs ischemic. Diarrhea / bloody stool and the clinical context help determine which pathway to pursue.
  • Common recurrent / lower-risk GI syndromes include gastroenteritis, constipation, IBS, and cannabinoid hyperemesis syndrome. These become more likely when the clinical pattern fits and dangerous structural disease has been excluded.

4 Appendicitis / Diverticulitis Mimics

  • Focal GI mimics include mesenteric adenitis, typhlitis, intussusception, epiploic appendagitis, omental infarction, and hernia. Think of these when focal pain resembles appendicitis / diverticulitis but the overall picture or imaging points elsewhere.

01Resuscitation / R/O Critical Diagnosis

  • See Abdominal Pain — General for the initial approach to the undifferentiated abdominal-pain patient.
  • Is the patient crashing / unstable? Begin resuscitation and diagnostic evaluation simultaneously — ABCs, IV/O2/monitor, fluids or blood as indicated, and rapid bedside assessment for the cause.
  • R/O immediately life-threatening causes — perforation, mesenteric ischemia, hemorrhage/AAA, bowel obstruction, ectopic pregnancy when applicable, or other surgical catastrophe.
  • Peritoneal signs / concern for surgical abdomen?
    • Stable → focused US / CT based on the suspected diagnosis.
    • Unstable → urgent surgical evaluation; do not delay definitive care for imaging.
  • Reassess after initial stabilization. Once the immediate threats are addressed, shift from “Could this patient die from this?” → “Where is the pain coming from?”
  • Then use the location, pain pattern, exam, and focused labs/imaging to narrow toward appendiceal, diverticular, GU/GYN, or other GI causes.

02R/O Emergent Causes

Acute Appendicitis

Clinical Presentation

  • Periumbilical / poorly localized pain → migrates to RLQ → constant / sharper
  • RLQ tenderness / localized peritoneal irritation
    • McBurney point tenderness
    • Guarding / rebound
  • Anorexia, N/V ± constipation / diarrhea
  • Fever / tachycardia ± ↑WBC / CRP
  • Rovsing sign → LLQ palpation produces RLQ pain
  • Psoas sign → pain with hip extension / flexion; may suggest retrocecal appendix
  • Obturator sign → pain with flexion / internal rotation of hip
  • Rovsing / psoas / obturator → supportive findings; absence does not exclude appendicitis

Red Flags — What Moves The Needle?

  • Migration of pain → RLQ
  • Focal RLQ tenderness / peritoneal irritation
  • Anorexia / N/V
  • Inflammatory response → fever, ↑WBC / CRP
  • Combination of migration + localized peritoneal findings + inflammatory response → ↑ suspicion
  • Extremes of age / pregnancy / immunocompromised → may have atypical or subtle presentation

Diagnosis

Imaging Based On Patient Population

Treatment / Disposition

  • Analgesia
  • Antibiotics
  • Surgery consultation
  • Laparoscopic appendectomy → standard definitive treatment
  • Complicated appendicitis — perforation / diffuse peritonitis / abscess / phlegmon / sepsis → source-control pathway

Nonoperative Treatment

  • Selected uncomplicated appendicitis → antibiotics alone may be considered with surgery involvement + shared decision-making.
CODA Trial
  • Antibiotics noninferior to appendectomy for 30-day health status
  • 29% → appendectomy by 90 days
  • Appendicolith:
  • Appendicolith → higher risk of antibiotics-first failure / complications
  • Nonoperative treatment applies to selected uncomplicated disease; complicated appendicitis requires a separate source-control strategy.

In The Pit...

  • RLQ pain / tenderness +
  • Migration / peritoneal findings / inflammatory response → Image
  • Adult → CT
  • Child → US → MRI / CT if equivocal
  • Pregnant → US → MRI if equivocal
  • Nondiagnostic US ≠ negative study
  • Confirmed appendicitis → surgery consultation
  • Can't Miss Diagnosis: Any right lower quadrant abdominal pain or tenderness needs a workup for appendicitis

Diverticulitis

Clinical Presentation

  • Acute / subacute LLQ pain + focal tenderness
  • Fever ± ↑WBC / CRP
  • Change in bowel habits ± nausea
  • Clinical findings supportive but nonspecific — H&P alone accurately predicts diverticulitis in only ~40–65%
    AGA — Clinical Presentation

Red Flags — What Moves The Needle?

  • Focal LLQ pain / tenderness + inflammatory response
  • Peritoneal signs / severe pain → perforation?
  • Fever / sepsis / toxic appearance → complicated disease
  • Persistent vomiting / PO intolerance
  • Immunocompromised / frail / significant comorbidity
  • Failure to improve → abscess / other complication?
  • Blood in stool / anemia / weight loss → alarm feature / alternate diagnosis

Diagnosis

  • CT A/P → confirm diagnosis + uncomplicated vs complicated
  • First episode / uncertain diagnosis / severe presentation / failure to improve → CT
  • Known recurrent, classic mild disease → repeat imaging not always necessary
    AGA — CT Indications
  • CT unavailable / contraindicated → US or MRI
  • Pregnant → US or MRI
    IDSA 2024 — Pregnancy

Uncomplicated Vs Complicated

  • Uncomplicated → localized diverticular inflammation
  • Complicated → abscess / phlegmon / perforation / fistula / obstruction ± systemic illness
    ACP 2022

Treatment / Disposition

  • Uncomplicated / immunocompetent / mild
  • Outpatient treatment if well appearing, stable, tolerating PO, pain controlled + reliable follow-up
  • Antibiotics selective — not routine
  • Au & Aly 2019 meta-analysis: 9 studies / 2,505 patients → no significant difference in additional treatment, readmission, surgery / radiologic intervention, recurrence, or complications without antibiotics
    DOI | PubMed
  • Clear liquids for comfort → advance as tolerated
    AGA — Diet
  • Antibiotics / admission more likely
  • Complicated disease
  • Sepsis / systemic illness
  • Immunocompromised / frail / significant comorbidity
  • Refractory symptoms / persistent vomiting
  • Unable to tolerate PO
  • Failed outpatient treatment
  • If antibiotics → gram-negative + anaerobic coverage
    AGA — Higher-Risk Disease

Complications

  • Abscess → antibiotics ± drainage
  • Perforation / generalized peritonitis → surgery / source control
  • Fistula
  • Obstruction

Follow-Up

  • Colonoscopy after complicated diverticulitis and generally after first uncomplicated episode, depending on prior colonoscopy / disease course
  • Usually delay 6–8 weeks after resolution
    AGA — Colonoscopy After Diverticulitis

In The Pit...

  • LLQ pain / tenderness + inflammatory findings → consider diverticulitis
  • CT → confirm + uncomplicated vs complicated
  • Uncomplicated / healthy / tolerating PO → outpatient ± no antibiotics
  • Complicated / high-risk / septic → antibiotics + admission / source control
  • Failure to improve → reassess / image for complication

Genitourinary Causes

  • Lower abdominal pain may originate from the urinary tract or scrotum.

  • Pyelonephritis / cystitis → dysuria, frequency, fever, flank / CVA tenderness
    See Low Back Pain Algorithm — Coming soon

  • Nephrolithiasis → colicky flank / lower abdominal pain ± hematuria, N/V
    See Low Back Pain Algorithm — Coming soon

  • Testicular torsion / epididymitis / other acute scrotal pathology → lower abdominal or groin pain may precede or accompany scrotal symptoms
    See Acute Scrotum Algorithm

Gynecologic Causes

  • In reproductive-age females, always consider a pelvic source of lower abdominal pain.

  • Ectopic pregnancy

  • Ovarian torsion

  • PID / tubo-ovarian abscess

  • Ovarian / hemorrhagic cyst

  • Fibroids / other pelvic pathology

  • Pregnancy test early → pelvic US based on suspicion
    See Pelvic Pain Algorithm — Coming soon

03Other GI Causes

Inflammatory Bowel Disease

  • Main ED question → uncomplicated flare vs acute complication?

Clinical Presentation

  • Crohn disease → abdominal pain ± diarrhea; RLQ pain with terminal ileitis

  • Ulcerative colitis → abdominal pain + bloody diarrhea / urgency

  • Fever / tachycardia ± ↑WBC / CRP → active / severe inflammation

  • New focal / severe pain or systemic illness → look for complication rather than assuming “flare”

Red Flags — What Moves The Needle?

  • Pain + vomiting / distention → obstruction / stricture?

  • Fever + focal tenderness / sepsis → abscess?

  • Peritoneal signs → perforation?

  • Severe / ongoing hematochezia → hemorrhage?

  • Bloody diarrhea + systemic toxicity → acute severe UC / toxic megacolon?

  • Steroids / immunosuppression → may blunt peritoneal findings

Diagnosis

Acute Severe Ulcerative Colitis

  • Frequent bloody diarrhea + systemic toxicity → ASUC

  • Admit + GI

  • C. difficile testing

  • Assess for toxic megacolon

  • IV corticosteroids

  • No routine broad-spectrum antibiotics unless infection / sepsis / perforation concern
    ACG 2025 UC — Antibiotics

  • Avoid NSAIDs / opioids / anticholinergics → may worsen colonic dilation / toxicity
    ACG 2025 UC — Medications To Avoid

Toxic Megacolon

  • Colonic dilation + systemic toxicity → life-threatening complication of severe inflammatory / infectious colitis

  • Ill appearing, abdominal distention, fever / tachycardia, bloody diarrhea

  • ASUC → assess specifically for toxic megacolon
    ACG 2025 UC — Toxic Megacolon

  • Resuscitation + GI / surgery

  • Toxic megacolon / perforation / severe refractory hemorrhage / refractory disease → surgical pathway

Crohn Complications

  • Stricture / obstruction

  • Abscess

  • Fistula / penetrating disease

  • Perforation

  • Severe hemorrhage

  • Suspected complication → CT + GI / surgery ± IR

  • Abscess → antibiotics + drainage / source control as appropriate
    ACG 2025 Crohn Disease — Original Guideline

In The Pit...

  • Other diagnosis? → flare? → how sick? → complication?
  • Known IBD + abdominal pain...
  • 1. Is this actually the IBD?
  • Do not anchor on IBD
  • Consider usual abdominal-pain differential → appendicitis, diverticulitis, GU/GYN, gastroenteritis, dyspepsia, etc.
  • 2. Does this fit an IBD flare?
  • UC → ↑ stool frequency + hematochezia / urgency / tenesmus
  • Crohn → abdominal pain + diarrhea ± weight loss / inflammatory findings
  • Suspected flare with diarrhea → exclude infection, especially C. difficile
  • 3. If this is IBD, how sick?
  • Mild / uncomplicated
    • Well appearing, stable, tolerating PO
    • No significant bleeding / anemia / sepsis
    • No peritoneal signs / obstruction
    • Supportive care + outpatient GI follow-up / disease-directed plan
  • Moderate / severe
    • Significant bloody diarrhea, dehydration / PO intolerance, anemia, fever / tachycardia, systemic toxicity
    • Admit + GI
  • ASUC
    • ≥6 bloody stools/day + systemic toxicity
    • Admit + GI
    • C. difficile testing
    • IV corticosteroids
    • Assess for toxic megacolon
  • 4. Could there be an IBD complication?
    • Focal / severe pain
    • Persistent vomiting / distention → obstruction / stricture?
    • Fever / sepsis / focal tenderness → abscess?
    • Peritoneal signs → perforation?
    • Toxic megacolon / major hemorrhage?
    • → CT A/P with IV contrast + admit + GI / surgery ± IR
    • Antibiotics → infection / abscess / sepsis / perforation concern; not routine uncomplicated flare

Colitis

  • Infectious colitis → diarrhea ± fever / blood → See Diarrhea — Coming soon
  • Inflammatory colitis → See IBD
  • Ischemic colitis → abdominal pain + hematochezia, especially older / vascular-risk patient → consider CT See Mesenteric Ischemia

Other Common GI Causes

  • Gastroenteritis → N/V/D + benign abdominal exam → See Diarrhea — Coming soon
  • Constipation / fecal loading → abdominal discomfort / distention + ↓ stool output
  • IBS → recurrent abdominal pain + altered bowel habits; no alarm features
  • Cannabinoid hyperemesis syndrome → chronic cannabis use + recurrent severe N/V ± abdominal pain; consider after excluding important alternate causes

04Appendicitis / Diverticulitis Mimics

  • Mesenteric adenitis → RLQ pain, often after viral illness; diagnosis of exclusion
  • Typhlitis → RLQ pain + fever in neutropenic / immunocompromised patient → potentially serious
  • Intussusception → episodic / colicky abdominal pain ± vomiting; especially pediatrics
  • Epiploic appendagitis → focal LLQ or RLQ pain; CT mimic of diverticulitis / appendicitis; usually supportive treatment
  • Omental infarction → focal abdominal pain; CT diagnosis; usually supportive treatment
  • Hernia → focal pain / mass; incarceration / strangulation → surgical emergency

Cases

Level: MS3 CC: Right Lower Quadrant Abdominal Pain Open case~5 min Case 03 CC: My belly hurts. Walking makes it worse Open case8–10 min

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Additional Cited Guidance

Table of contentsAbdominal Pain: General

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