Can viral gastroenteritis cause leukocytosis?

Leukocytosis in Viral Gastroenteritis

Viral gastroenteritis typically does NOT cause leukocytosis; instead, it commonly presents with normal white blood cell counts or even neutropenia, making leukocytosis a useful clinical marker to suggest bacterial rather than viral etiology. 1, 2

Key Diagnostic Distinctions

Viral Gastroenteritis Patterns

• Neutropenia is common in viral gastroenteritis, particularly with rotavirus and norovirus infections, occurring transiently during acute illness 2
• Total leukocyte counts in viral gastroenteritis typically remain within normal ranges or are decreased 3, 4
• The absence of leukocytosis helps differentiate viral from bacterial causes in the clinical setting 1

When Leukocytosis Suggests Bacterial Etiology

• Elevated total leukocyte and neutrophil counts strongly suggest invasive bacterial pathogens rather than viral causes 1
• Absolute band count ≥1,500 cells/mm³, neutrophil percentage >90%, and left shift ≥16% bands indicate bacterial infection 5
• Shigellosis specifically may produce a leukemoid reaction (extreme leukocytosis) with marked left shift in 71% of cases 1, 3, 4

Important Exception: Hemoconcentration

Leukocytosis can occur in viral gastroenteritis through hemoconcentration from dehydration, not from the viral infection itself. 5

• Volume depletion concentrates both WBC and RBC, creating a falsely elevated count 5
• This mechanism explains why some patients with viral gastroenteritis may present with elevated WBC 6
• Repeat CBC after hydration will demonstrate resolution of the "leukocytosis" if hemoconcentration was the cause 5

Clinical Application Algorithm

When evaluating a patient with gastroenteritis and leukocytosis:

1. Assess hydration status first - Check for signs of volume depletion, elevated BUN (>20 mg/dL occurs in 79.3% of dehydrated viral gastroenteritis patients), and concurrent RBC elevation 5, 6

2. Examine the differential count - True bacterial infection shows absolute band count ≥1,500 cells/mm³ and marked left shift, while viral causes show normal or low counts 1, 5, 4

3. Consider specific bacterial pathogens if leukocytosis persists after hydration:

   • Shigella (leukemoid reaction possible) 1, 3
   • STEC O157 (especially if HUS develops) 1
   • Salmonella species 4

Special Populations

Rotavirus-Specific Findings

• Rotavirus may show leukocytosis more commonly than other viral agents, but this is typically due to associated dehydration 6
• Elevated AST occurs in 11.5% of rotavirus cases (vs. 5.4% norovirus, 0.8% adenovirus) 6
• Fever is more prominent with rotavirus than other viral causes 6

Immunocompromised Patients

• Laboratory tests may not accurately reflect clinical severity in immunocompromised hosts 1
• Neutropenic cancer patients with enterocolitis present with fever, intestinal wall thickening, diarrhea, and abdominal pain 1
• In bacterial sepsis, total leukocyte count may paradoxically be decreased rather than elevated 1

Critical Pitfall to Avoid

Do not assume leukocytosis rules out viral gastroenteritis without first correcting for dehydration. The most common mechanism for elevated WBC in viral gastroenteritis is hemoconcentration, not the infection itself. 5 Rehydrate the patient and repeat the CBC - if the count normalizes, the etiology remains likely viral. If leukocytosis persists with left shift, pursue bacterial causes aggressively. 1, 5, 4