What's the WBC count in a spinal fluid sample with 98 WBCs in 0.9 µL after 1:20 dilution?

Calculation of WBC Count in Cerebrospinal Fluid Sample

The WBC count to report for this spinal fluid sample is 2,178 cells/µL.

Calculation Method

To calculate the WBC count in a cerebrospinal fluid sample, we need to apply the following formula:

WBC count (cells/µL) = (Number of WBCs counted × Dilution factor) ÷ Volume examined

Breaking down the calculation:

• Number of WBCs counted: 98 cells
• Dilution factor: 1:20 (or 20)
• Volume examined: 9 large squares = 0.9 µL (each large square = 0.1 µL)

Therefore: WBC count = (98 × 20) ÷ 0.9 = 1960 ÷ 0.9 = 2,177.78 cells/µL

Rounded to the nearest whole number: 2,178 cells/µL

Clinical Significance

• A WBC count of 2,178 cells/µL in CSF is significantly elevated and strongly suggests meningitis 1
• Normal CSF typically contains <5 WBCs/µL in adults and <20 WBCs/µL in neonates 2
• This elevated count would be classified as pleocytosis and warrants immediate clinical attention

Interpretation in Clinical Context

When interpreting CSF WBC counts:

• The absence of RBCs (0 RBCs counted) indicates this is not a traumatic tap, making the WBC count reliable 2
• If this had been a traumatic tap, correction factors would be needed (typically 1 WBC subtracted for every 500-700 RBCs) 3
• The hazy appearance of the fluid is consistent with elevated WBC count and suggests inflammation 1

Additional Considerations

• In bacterial meningitis, CSF WBC counts are typically >1,000 cells/µL with a predominance of neutrophils 2
• In viral meningitis, CSF WBC counts are typically 10-1,000 cells/µL with a predominance of lymphocytes 2
• Additional CSF parameters that should be assessed include:
  • Protein (typically elevated in meningitis)
  • Glucose (typically decreased in bacterial meningitis)
  • Gram stain and culture to identify pathogens

Common Pitfalls to Avoid

1. Failing to account for the dilution factor when calculating the final WBC count
2. Incorrect identification of the volume examined (number of squares counted)
3. Not considering the clinical context when interpreting the results
4. Failing to report the result promptly when significantly elevated, as this represents a medical emergency

Remember that prompt reporting of this elevated WBC count is critical as it may represent bacterial meningitis requiring immediate antibiotic therapy.