What is a wet mount in a Lumbar Puncture (LP) done for?

Wet Mount in Lumbar Puncture: Purpose and Procedure

A wet mount in lumbar puncture (LP) is primarily performed to examine cerebrospinal fluid (CSF) for cellular components and microorganisms in real-time, allowing for immediate assessment of potential central nervous system infections or inflammatory conditions.

Purpose of Wet Mount in LP

• Wet mount examination allows for direct microscopic visualization of CSF to identify white blood cells, red blood cells, and potential pathogens before formal laboratory processing 1
• It provides immediate information that can guide urgent treatment decisions, particularly in suspected meningitis or encephalitis cases 1
• The procedure helps differentiate between viral, bacterial, and fungal causes of meningitis by examining cell types and microorganisms present 1

Clinical Information Obtained from Wet Mount

• Cell count assessment: Identifies presence and type of pleocytosis (elevated white blood cells in CSF) 1

  • Lymphocytic pleocytosis typically suggests viral infection
  • Neutrophilic predominance often indicates bacterial infection
  • Normal CSF contains <5 white blood cells/μL

• Red blood cell evaluation: Helps distinguish between traumatic tap and true subarachnoid hemorrhage 1

  • Approximately 50% of HSV encephalitis cases show elevated CSF red cell count 1
  • In traumatic taps, white blood cell count can be corrected by subtracting 1 white cell for every 7000 red blood cells/μL 1

• Direct visualization of microorganisms: May reveal bacteria, fungi, or other pathogens 1

  • Gram stain can be performed on the wet mount to identify bacterial pathogens 1

Technical Aspects

• The wet mount should be examined promptly after collection to prevent cellular degradation 2
• A small drop of uncentrifuged CSF is placed on a slide with a coverslip for immediate microscopic examination 2, 3
• The procedure requires minimal CSF volume (typically <0.5 mL), preserving fluid for additional testing 4
• For optimal results, the specimen should be examined within 30 minutes of collection 3

Integration with Other CSF Analyses

• Wet mount findings should be interpreted alongside other CSF parameters 1:
  • CSF glucose (with concurrent plasma glucose)
  • CSF protein
  • CSF opening pressure
  • CSF lactate (values <2 mmol/L help rule out bacterial disease) 1

Clinical Significance

• Provides rapid preliminary diagnosis before culture results are available (which may take 24-48 hours) 2, 5
• Particularly valuable in emergency settings for guiding empiric antimicrobial therapy 5
• Helps distinguish between viral and bacterial etiologies, which have different treatment approaches 1
• Can identify cases requiring immediate intervention versus those that can be managed more conservatively 1

Limitations and Considerations

• Wet mount examination has lower sensitivity compared to formal laboratory analysis 5
• Negative findings do not rule out infection, particularly early in disease course 1
• In approximately 5-10% of adults with proven HSV encephalitis, initial CSF findings may be normal 1
• The procedure should be performed by experienced personnel to maximize diagnostic yield 6

Common Pitfalls to Avoid

• Delaying examination of the specimen, which can lead to cellular degradation and false-negative results 2
• Failing to correlate wet mount findings with clinical presentation and other laboratory parameters 1
• Not collecting sufficient CSF volume for all necessary tests (wet mount should not compromise other essential analyses) 1
• Misinterpreting traumatic tap findings as pathological 1