Diagnosis: Acute Infectious Gastroenteritis
This is acute infectious gastroenteritis, and you should initiate symptomatic treatment with oral rehydration, antiemetics if needed, and reassurance while awaiting stool culture results. 1
Clinical Reasoning
The 3-day history of central abdominal pain relieved by defecation, watery stools (4 episodes/day), and acute onset strongly suggests an infectious etiology rather than a functional disorder. 1 The key distinguishing features are:
- Acute onset (3 days) rather than chronic/recurrent pattern required for IBS diagnosis (≥12 weeks over preceding 12 months) 1
- Watery diarrhea with "spotting" of blood suggests infectious colitis rather than functional disease 1
- Normal vital signs and PE exclude severe complications but don't rule out infection 1
- Headache is likely secondary to dehydration or systemic viral illness, not a primary neurologic process 2
Immediate Diagnostic Workup
Obtain the following tests to exclude serious pathology and confirm infectious etiology: 1
- Stool culture and ova/parasites - essential given acute diarrheal illness 1
- Stool for Clostridium difficile toxin - even without antibiotic exposure, given watery diarrhea 1
- Complete blood count - to assess for leukocytosis (infection) or anemia (bleeding) 1
- Stool hemoccult - the "spotting" described warrants formal testing 1
- Serum electrolytes - to assess hydration status given frequent watery stools 1
Do not delay empiric treatment while awaiting stool culture results. 1
Management Plan
Immediate Treatment (Day 1)
- Oral rehydration solution - primary therapy for mild-moderate dehydration 2
- Antiemetics if nausea present (ondansetron 4-8mg) - improves oral tolerance 2
- Acetaminophen for headache - avoid NSAIDs given GI symptoms 2
- Dietary modification - bland diet (BRAT: bananas, rice, applesauce, toast) as tolerated 2
Antidiarrheal Therapy
Hold loperamide initially until infectious colitis with bloody stools is excluded, as antidiarrheals can worsen outcomes in invasive bacterial infections. 1 Once stool cultures are negative or show non-invasive pathogen, loperamide 2-4mg after each loose stool (max 16mg/day) can be added. 1
Antibiotic Consideration
Do not start empiric antibiotics in this immunocompetent patient with mild symptoms and normal vitals. 2 Reserve antibiotics for:
- Severe illness (fever >38.5°C, bloody diarrhea, >6 stools/day) 2
- Immunocompromised state 2
- Positive stool culture showing bacterial pathogen requiring treatment 2
Red Flags Requiring Escalation
Monitor for these warning signs that would require urgent reassessment: 1
- Fever >38.5°C or persistent fever - suggests invasive infection 1
- Increasing bloody stools - may indicate inflammatory bowel disease or severe colitis 1
- Severe abdominal pain or peritoneal signs - concern for perforation or toxic megacolon 1
- Signs of dehydration (tachycardia, orthostasis, decreased urine output) - may require IV fluids 1
- Symptoms persisting >7-10 days - warrants colonoscopy to exclude IBD or post-infectious IBS 1, 2
Follow-Up Plan
Reassess in 3-5 days either by phone or in-person: 1
- If symptoms resolve completely, no further workup needed 2
- If diarrhea persists beyond 7 days, consider giardia testing, celiac serology, and lactose breath test 1
- If symptoms evolve into chronic pattern (>12 weeks), reconsider diagnosis of post-infectious IBS 2, 3
Critical Pitfall to Avoid
Do not diagnose IBS in a patient with acute-onset symptoms. 1 IBS requires at least 12 weeks of symptoms in the preceding 12 months, and acute presentations should always prompt evaluation for infectious or inflammatory causes first. 1 Making a premature functional diagnosis risks missing treatable organic disease and erodes patient trust. 1, 2