A patient with four days of diarrhea and indigestion has an isolated monocytosis (1,310/µL; normal 200‑950) with a normal complete blood count; what is the most likely diagnosis and recommended management?

Acute Gastroenteritis with Isolated Monocytosis

This presentation is most consistent with acute infectious gastroenteritis, likely viral or early bacterial infection, and should be managed conservatively with aggressive oral rehydration, symptomatic treatment, and close monitoring for 24-48 hours with clear return precautions. 1

Most Likely Diagnosis

The isolated monocytosis (1,310/µL) in the context of 4 days of diarrhea and indigestion, with an otherwise normal CBC including normal total WBC, strongly suggests:

• Viral gastroenteritis is the most likely diagnosis, as monocyte predominance typically occurs with viral etiologies rather than bacterial infections 2
• Early bacterial gastroenteritis remains possible, as monocyte predominance may suggest the presence of an intracellular pathogen such as Salmonella 2
• Inflammatory bowel disease (IBD) should be considered if symptoms persist beyond 7-10 days, as monocytosis is significantly elevated in active ulcerative colitis and correlates with disease activity 3, 4

Why This is NOT Concerning for Serious Pathology

• The normal total WBC count argues strongly against severe bacterial infection or systemic inflammatory process 2
• Absence of fever makes invasive bacterial pathogens (Salmonella, Campylobacter, Shigella) much less likely, as these typically present with fever in 58-100% of cases 2
• The short duration (4 days) and lack of alarm features make chronic conditions like CMML extremely unlikely, as sustained monocytosis over months is required for concern about hematological malignancy 5
• Stool studies, CBC with differential, and serologic assays should NOT be performed to establish an etiology in uncomplicated acute diarrhea without fever, bloody stools, or severe illness 2, 1

Immediate Management Protocol

Hydration Strategy

• Aggressive oral rehydration with 8-10 large glasses of clear liquids daily, with electrolyte solutions like Gatorade or broth preferred over plain water 1
• Monitor for orthostatic symptoms (dizziness upon standing) to indicate significant dehydration requiring IV fluids 1

Symptomatic Treatment

• Ondansetron for nausea and vomiting to improve oral intake tolerance 1
• Loperamide 4mg initially, then 2mg after each unformed stool (maximum 16mg/day) to control diarrhea 1

Dietary Modifications

• Stop all lactose-containing products immediately, as acute gastroenteritis causes temporary lactose intolerance 1
• Frequent small meals with bland foods (bananas, rice, applesauce, toast, plain pasta) 1

Critical Red Flags Requiring Immediate Escalation

Return immediately or seek urgent care if any of the following develop:

• Fever ≥38°C (100.4°F), which suggests bacterial superinfection and warrants stool culture and possible empiric antibiotics 2, 1
• Bloody diarrhea or severe abdominal pain with peritoneal signs 2, 1
• Inability to tolerate oral fluids, persistent vomiting, orthostatic hypotension, or confusion indicating severe dehydration 1
• Symptoms persisting beyond 7-10 days, which should prompt stool studies and consideration of endoscopy for IBD evaluation 2, 6

When Diagnostic Testing IS Indicated

Testing should be pursued only if:

• Fever develops (stool culture for Salmonella, Shigella, Campylobacter, and C. difficile testing) 2, 6
• Bloody stools appear (stool culture including STEC, C. difficile testing) 2
• Symptoms persist >10-14 days (stool culture, C. difficile, fecal calprotectin, and consider endoscopy) 2, 6
• Severe illness with systemic toxicity (blood cultures, stool studies, inflammatory markers) 6

Follow-Up Plan

• Recheck in 24-48 hours by phone or in-person to assess symptom trajectory 1
• Document stool frequency and character to monitor response to treatment 1
• If monocytosis persists after complete resolution of GI symptoms, repeat CBC in 3 months to ensure it normalizes 5

Critical Pitfalls to Avoid

• Do NOT prescribe empiric antibiotics for acute watery diarrhea without fever, bloody stools, or severe illness, as this increases C. difficile risk and antibiotic resistance 1
• Do NOT order stool studies, fecal leukocytes, or extensive workup in uncomplicated acute diarrhea, as these have poor predictive value and do not change management 2, 6
• Do NOT dismiss the isolated monocytosis as requiring immediate hematologic workup, as reactive monocytosis from infection is far more common than CMML, which requires sustained monocytosis and has an extremely low absolute risk (0.1% even with sustained monocytosis in primary care) 5