What are the differential diagnoses and recommended management for an 18‑year‑old patient presenting with acute hematochezia, abdominal cramping, recent constipation, difficulty gaining weight, no arthralgia, and no family history of gastrointestinal disease?

Differential Diagnosis and Management of an 18-Year-Old with Bloody Diarrhea

This 18-year-old with bloody diarrhea, abdominal cramps, recent constipation, and inability to gain weight requires urgent evaluation for inflammatory bowel disease (IBD), infectious colitis, and other structural pathology through colonoscopy with biopsies after initial blood and stool testing.

Differential Diagnosis

High-Priority Diagnoses

Inflammatory Bowel Disease (Ulcerative Colitis or Crohn's Disease)

• The combination of bloody diarrhea, abdominal cramping, difficulty gaining weight, and young age (18 years) strongly suggests IBD, as up to 15% of new IBD diagnoses occur in individuals under 20 years 1
• The recent episode of constipation followed by bloody diarrhea may represent alternating symptoms or initial presentation of Crohn's disease with stricturing behavior 1
• Absence of family history does not exclude IBD, as most cases are sporadic 1

Infectious Colitis

• Bacterial pathogens (Salmonella, Shigella, Campylobacter, Shiga toxin-producing E. coli) must be excluded, particularly given the acute presentation with bloody diarrhea 1
• Clostridioides difficile should be tested even without recent antibiotic exposure, as recommended for all new presentations of diarrhea 1
• Yersinia enterocolitica should be considered given the abdominal cramping in a young patient 1

Other Structural Pathology

• Although colorectal cancer is less likely at age 18, it cannot be completely excluded given hematochezia and constitutional symptoms 2
• Ischemic colitis is unlikely in this age group without vascular risk factors 1
• Microscopic colitis typically presents with watery (not bloody) diarrhea and is more common in older patients 1

Initial Diagnostic Workup

Immediate Laboratory Testing

Blood Tests (Perform in Primary or Secondary Care)

• Complete blood count to assess for anemia and inflammatory markers 1
• C-reactive protein and erythrocyte sedimentation rate for inflammation 1
• Serum albumin to evaluate nutritional status and protein loss 1
• Serum ferritin to assess iron stores 1
• Liver enzymes, urea, and creatinine for baseline assessment and comorbidity screening 1
• Coeliac serology (tissue transglutaminase antibodies) to exclude coeliac disease 1

Stool Testing

• Stool culture for Salmonella, Shigella, Campylobacter, and Yersinia 1
• Shiga toxin testing or culture on sorbitol-MacConkey agar for STEC O157:H7 1
• C. difficile toxin testing 1
• Fecal calprotectin or lactoferrin to distinguish inflammatory from non-inflammatory diarrhea 1

Endoscopic Evaluation

Colonoscopy with Biopsies (Essential for Diagnosis)

• Colonoscopy with histologic confirmation is the cornerstone of diagnosis and should be performed urgently given the presence of alarm features (bloody diarrhea, weight loss, young age with new symptoms) 1
• Biopsies should be obtained from multiple sites throughout the colon and terminal ileum, even from normal-appearing mucosa, to distinguish between ulcerative colitis and Crohn's disease 1
• The presence of blood in stool, weight loss, and inability to gain weight constitute "red flag symptoms" requiring urgent gastroenterology referral 1, 3

Timing of Colonoscopy

• For hemodynamically stable patients with bloody diarrhea and alarm features, colonoscopy should be scheduled on the next available inpatient list after adequate bowel preparation 2
• Urgent colonoscopy within 24 hours is not necessary unless the patient becomes hemodynamically unstable 2

Cross-Sectional Imaging

CT Enterography or MR Enterography

• Consider if Crohn's disease is suspected to evaluate small bowel involvement, which cannot be assessed by colonoscopy alone 1
• CT imaging is appropriate when abdominal pain is prominent to exclude complications such as abscess or perforation 1

Management Plan

Immediate Management

Hemodynamic Assessment

• Calculate shock index (heart rate ÷ systolic blood pressure); if >1, this indicates hemodynamic instability requiring immediate resuscitation 2
• For stable patients, calculate the Oakland score to guide admission decisions 2

Resuscitation (If Needed)

• Initiate IV crystalloid resuscitation if hemodynamically compromised 2
• Transfuse packed red blood cells using restrictive thresholds: hemoglobin trigger 70 g/L (target 70-90 g/L) for patients without cardiovascular disease 2

Empiric Treatment Considerations

Avoid Premature Corticosteroid Use

• Do not start systemic corticosteroids before establishing a diagnosis, as this may mask infectious colitis and complicate histologic interpretation 1
• If IBD is confirmed and corticosteroids are needed for induction, prefer non-systemic options like budesonide when appropriate for disease phenotype 1

Symptomatic Management

• Antidiarrheal agents (loperamide) should be avoided in the setting of bloody diarrhea until infectious causes are excluded 1
• Maintain adequate hydration and nutrition 3

Antibiotic Therapy

Indications for Empiric Antibiotics

• Antibiotics are generally not recommended for acute bloody diarrhea unless the patient has signs of septicemia, is immunocompromised, or has specific risk factors 1
• If Shiga toxin-producing E. coli is suspected, antibiotics are contraindicated as they may increase risk of hemolytic uremic syndrome 1
• Empiric antibiotics may be considered for severe illness while awaiting culture results, but should be tailored once pathogen is identified 1

Definitive Management Based on Diagnosis

If IBD is Confirmed

• Risk-stratify based on disease severity, extent, and phenotype (perianal disease, stricturing, penetrating) 1
• For moderate-to-severe disease, early biologic therapy may be preferred over step-up approach with immunomodulators 1
• Avoid systemic corticosteroids for maintenance therapy 1

If Infectious Colitis is Confirmed

• Treat with pathogen-specific antibiotics as indicated 1
• Repeat stool testing to confirm clearance if symptoms persist 1

Critical Pitfalls to Avoid

• Do not delay colonoscopy in young patients with bloody diarrhea and weight loss—these are alarm features requiring urgent evaluation regardless of age 1
• Do not assume functional bowel disease (IBS) in the presence of bloody diarrhea—IBS does not cause hematochezia 1, 4
• Do not start empiric IBD treatment without histologic confirmation—other diagnoses (infection, ischemia) require different management 1
• Do not forget to test for C. difficile even without antibiotic exposure—it should be tested in all new presentations of diarrhea 1
• Do not overlook the possibility of Crohn's disease despite absence of family history—most IBD cases are sporadic 1