How is infant diarrhea defined?

Definition of Infant Diarrhea

Infant diarrhea is defined as the passage of ≥3 loose or liquid stools per day, where stool consistency—not just frequency—is the critical diagnostic criterion. 1, 2

Key Diagnostic Criteria

Stool Consistency Requirements

• True diarrhea requires liquid or watery stools that take the shape of the container, corresponding to Bristol Stool Scale types 5-7 (loose to watery). 1
• Frequent passage of formed stools is NOT diarrhea, regardless of frequency—even if an infant passes 6 formed stools daily, this does not meet the clinical definition. 2
• The distinction between soft-but-formed stools versus truly loose/liquid stools is clinically essential and directly impacts testing and treatment decisions. 1

Frequency Component

• The standard definition requires ≥3 unformed bowel movements per 24 hours when combined with the consistency criterion. 1, 2
• Increased frequency alone, without abnormal consistency, does not constitute diarrhea. 2

Duration-Based Classification

Acute diarrhea lasts <14 days and typically represents viral gastroenteritis in infants. 3

Persistent diarrhea is defined as diarrhea lasting ≥14 days (2 weeks). 4, 5, 6, 7

Chronic diarrhea extends beyond 14 days and may indicate underlying pathology requiring systematic evaluation. 8, 6

Age-Specific Considerations

Infants ≤12 Months

• Testing for C. difficile should NEVER be routinely performed in neonates or infants ≤12 months with diarrhea due to high rates (>40%) of asymptomatic colonization. 4
• The absence of a validated definition of clinically significant diarrhea in this age group creates diagnostic challenges, as frequent loose stools are common in normal infants. 4
• Testing should only occur if pseudomembranous colitis, toxic megacolon, or other causes have been excluded. 4

Children 1-2 Years

• C. difficile testing should not be routinely performed unless other infectious and noninfectious causes have been excluded, as asymptomatic colonization remains elevated. 4

Children ≥2 Years

• Testing is recommended for prolonged or worsening diarrhea with risk factors (inflammatory bowel disease, immunocompromise) or relevant exposures (healthcare contact, recent antibiotics). 4

Clinical Assessment Framework

Severity Classification by Functional Impact

Rather than counting stools, classify by impact on the infant's function: 4

• Mild: Tolerable, not distressing, does not interfere with activities
• Moderate: Distressing or interferes with planned activities
• Severe: Incapacitating or completely prevents activities; all dysentery (grossly bloody stools) is severe 4

Dehydration Assessment

Accurate assessment of fluid deficit guides therapy: 4

• Mild (3-5% deficit): Increased thirst, slightly dry mucous membranes
• Moderate (6-9% deficit): Loss of skin turgor, skin tenting, dry mucous membranes
• Severe (≥10% deficit): Severe lethargy, prolonged skin tenting >2 seconds, cool/poorly perfused extremities, decreased capillary refill 4

Critical Pitfalls to Avoid

Do not confuse increased stool frequency with diarrhea if stools remain formed—this leads to unnecessary testing and inappropriate treatment. 1, 2

Do not order stool testing on formed or mildly soft specimens—laboratories should reject specimens that are not liquid or soft enough to take the shape of the container. 1

Do not assume all loose stools require antibiotics—most acute watery diarrhea in immunocompetent infants is viral and self-limited. 1

Avoid testing infants <12 months for C. difficile based solely on diarrhea, as this results in false positives and unnecessary treatment due to high colonization rates. 4