Differential Diagnosis: Loose Stools + Abdominal Pain

Loose stools with abdominal pain is a common clinical presentation with a broad differential. The key first step is to categorize the diarrhea by duration, stool character, associated features, and exposures.

Step 1 - Define the Diarrhea

Duration	Category
< 14 days	Acute
14-29 days	Persistent
≥ 30 days	Chronic

Diarrhea = at least 3 loose bowel movements per day. Duration is the most important factor shaping the differential.

Step 2 - Broad Differential Diagnosis

A. INFECTIOUS CAUSES (account for ~87% of acute cases)

1. Viral Gastroenteritis (most common overall)

Norovirus / Calicivirus - watery, self-limited; outbreaks in groups
Rotavirus - especially in children < 5 years
Adenovirus, Cytomegalovirus - more in immunocompromised

2. Bacterial - Gastroenteritis pattern (watery diarrhea, no blood)

Vibrio cholerae - profuse rice-water stools, no fever
Escherichia coli (ETEC) - traveler's diarrhea
Staphylococcus aureus - preformed toxin, rapid onset (1-6 hrs), vomiting dominant
Bacillus cereus - reheated rice; preformed toxin
Clostridium perfringens - meat/poultry, onset 8-16 hrs

3. Bacterial - Colitis pattern (bloody/mucoid stools, fever, tenesmus)

Shigella spp. - dysentery with high fever
Salmonella spp. - poultry/eggs; can cause bacteremia
Campylobacter jejuni - undercooked poultry; periumbilical cramps
Yersinia enterocolitica - can mimic appendicitis (right iliac fossa pain)
E. coli O157:H7 (EHEC) - bloody diarrhea, risk of HUS; antibiotics avoided
Clostridioides difficile - recent antibiotic/hospital exposure; pseudomembranous colitis

4. Parasitic

Giardia lamblia - bloating, fatty stools, travel/camping/well water
Entamoeba histolytica - amoebic dysentery; liver abscess risk
Cryptosporidium parvum - self-limited in immunocompetent; severe in HIV
Cyclospora, Cystoisospora - travel-related, longer course

B. NON-INFECTIOUS CAUSES

5. Inflammatory Bowel Disease (IBD)

Crohn's disease - can affect any part of GI tract; right lower quadrant pain, skip lesions, perianal disease
Ulcerative colitis - bloody diarrhea, crampy left lower quadrant pain, tenesmus

6. Irritable Bowel Syndrome (IBS)

Altered bowel habit without structural pathology; pain relieved by defecation; no alarm features

7. Malabsorption syndromes

Lactose intolerance - bloating, cramps, watery stools after dairy
Celiac disease - fatty/bulky stools, weight loss
Pancreatic insufficiency - steatorrhea, chronic pancreatitis history

8. Drug/medication-induced

Antibiotics, metformin, colchicine, digoxin, SSRIs, laxatives, antacids (Mg-containing), chemotherapy

9. Surgical / Structural causes

Appendicitis - periumbilical pain shifting to right iliac fossa, loose stools possible early
Mesenteric ischemia - severe pain out of proportion to exam, elderly/vascular risk
Bowel obstruction - overflow diarrhea with partial obstruction
Diverticulitis - left lower quadrant pain, fever, change in bowel habit

10. Endocrine/Systemic

Hyperthyroidism - frequent loose stools, weight loss, palpitations
Carcinoid syndrome - secretory diarrhea + flushing
VIPoma - profuse watery diarrhea (Verner-Morrison syndrome)
Diabetic autonomic neuropathy - nocturnal diarrhea

Step 3 - Diagnostic Approach

Use this flowchart framework (from Symptom to Diagnosis, 4th Ed.):

Key history questions:

Duration of symptoms
Character of stool: watery vs. bloody/mucoid vs. fatty
Fever, nausea, vomiting, tenesmus
Travel history (traveler's diarrhea, parasites)
Recent antibiotics or hospitalization (think C. difficile)
Dietary exposures - raw seafood, unpasteurized dairy, undercooked poultry
Immunocompromise (HIV, transplant, steroids)
Medications - metformin, antibiotics, laxatives
Sick contacts or outbreak

Key differentiating features:

Feature	Likely Cause
Watery stools, no fever, vomiting	Viral gastroenteritis, toxin-mediated bacterial
Bloody mucoid stools + fever + tenesmus	Bacterial colitis (Shigella, Campylobacter, EHEC)
Recent antibiotics/hospitalization	C. difficile colitis
Travel to endemic area	ETEC, Giardia, Entamoeba, Cyclospora
Right iliac fossa pain mimicking appendicitis	Yersinia, Crohn's disease
Nocturnal diarrhea	IBD, secretory diarrhea (not IBS)
Chronic + bloating + fatty stools	Malabsorption (celiac, pancreatic)
Rapid onset after specific meal	Food poisoning (Staph, Bacillus)
Flushing + watery diarrhea	Carcinoid, VIPoma

Step 4 - Initial Investigations

Test	Purpose
Stool routine + microscopy	Ova, cysts, pus cells, RBCs
Stool culture	Bacterial pathogens (Salmonella, Shigella, Campylobacter)
C. difficile toxin PCR	If recent antibiotics/hospitalization
Stool Shiga toxin + E. coli O157:H7	If bloody diarrhea
Molecular multipathogen stool panel	Broad infectious workup
CBC, CRP, ESR	Inflammatory markers
Electrolytes, renal function	Assess dehydration
Colonoscopy/sigmoidoscopy	If IBD/colitis suspected
Imaging (CT abdomen)	If surgical cause (appendicitis, ischemia) suspected

Must-not-miss diagnoses to always consider:

E. coli O157:H7 (risk of HUS - do NOT give antibiotics)
Mesenteric ischemia (can present acutely with pain and loose stools)
Appendicitis (especially with loose stools early in the course)
C. difficile colitis (can rapidly progress to toxic megacolon)

Sources: Symptom to Diagnosis, 4th Ed. | Sleisenger & Fordtran's Gastrointestinal and Liver Disease | Rosen's Emergency Medicine

This is a shared conversation. Sign in to Orris to start your own chat.