What are normal microscopic findings in stool?

Normal Microscopic Findings in Stool

In a healthy individual, microscopic examination of stool should reveal minimal to no inflammatory cells, no red blood cells, no white blood cells (fecal leukocytes), and absence of parasites, ova, or cysts. 1

Key Normal Microscopic Features

Cellular Components

• Fecal leukocytes: Should be absent or ≤5 leukocytes per high-power field (hpf) 2
• Red blood cells: Should be absent in normal stool 1
• Epithelial cells: Minimal sloughed epithelial cells may be present as part of normal mucosal turnover 1

Absence of Pathogenic Organisms

When examining stool microscopically for parasites using ova and parasite examination, normal findings include:

• No ova, cysts, or parasites detected 1
• No trophozoites of pathogenic protozoa (such as Giardia lamblia, Entamoeba histolytica) 1
• No helminth eggs or larvae (including Ascaris, Strongyloides, Trichuris, hookworms) 1

The sensitivity of microscopic examination of a single stool specimen for detecting ova, cysts, and parasites generally exceeds 80%, though multiple specimens increase diagnostic yield 1.

Absence of Inflammatory Markers

• No neutrophils or neutrophilic infiltration: The presence of ≥6 fecal leukocytes/hpf suggests bacterial infection with invasive pathogens like Shigella or Salmonella 2
• No eosinophils: Eosinophilia in stool may indicate parasitic infection or allergic/eosinophilic colitis 1

Clinical Context and Pitfalls

Important Caveats

Microscopy has significant limitations for bacterial detection. Histology and direct microscopy are not reliable tools to identify bacterial infections of the intestine, particularly Clostridium difficile infection, which requires specific toxin assays or nucleic acid amplification testing 1.

When Normal Findings Are Misleading

• Intermittent parasite shedding: A single negative stool examination does not exclude parasitic infection, particularly for Strongyloides stercoralis, which has low sensitivity on single examination and often requires serologic testing 1
• Immunosuppressed patients: May have atypical presentations with fewer inflammatory cells despite active infection 1
• Microscopic colitis: By definition presents with normal or near-normal endoscopic and macroscopic appearance but shows distinct histologic patterns (increased intraepithelial lymphocytes in lymphocytic colitis or thickened subepithelial collagen band in collagenous colitis) 1, 3

Technical Considerations

The quality of microscopic examination depends heavily on:

• Specimen collection timing: Three stools collected on three different days are recommended for optimal parasite detection 1
• Technician expertise: Microscopy is labor-intensive and requires skilled personnel 1
• Preparation method: Thick stool smear wet mount methods may be superior to conventional direct wet mount for parasite detection 4

Differential Diagnosis When Abnormalities Are Found

If microscopic examination reveals abnormalities, consider:

• ≥6 fecal leukocytes/hpf: Suggests invasive bacterial pathogens (Shigella, Salmonella, Campylobacter) requiring culture 2
• Parasites detected: Requires species-specific treatment; common pathogens include Giardia, Cryptosporidium, Cyclospora, Entamoeba histolytica 1
• Increased inflammatory cells with normal endoscopy: Consider microscopic colitis, which requires histologic confirmation 1, 3