How do you correct the White Cell Count (WCC) for a bloody Cerebrospinal Fluid (CSF) sample?

Correcting White Cell Count in Bloody CSF Samples

The standard correction method for white blood cell count in a bloody CSF sample is to subtract 1 white cell for every 700 red blood cells. 1

Understanding CSF Contamination with Blood

When a lumbar puncture results in a traumatic tap, blood from the procedure contaminates the CSF sample, artificially increasing the white blood cell (WBC) count. This makes interpretation challenging, particularly when trying to diagnose conditions like meningitis or encephalitis.

Correction Methods

Several approaches exist for correcting WBC counts in bloody CSF:

1. Standard Approximation (Most Widely Accepted):

   • Subtract 1 WBC for every 700 RBCs 1
   • This is the most commonly used formula in clinical practice
   • Sufficient for most clinical scenarios

2. Alternative Correction Ratios:

   • Some guidelines suggest subtracting 1 WBC for every 1000 RBCs 2
   • UK Joint Specialist Societies recommend subtracting 1 WBC for every 1000 RBCs 1
   • Some sources suggest a ratio of 1:700-1000 RBCs

3. Peripheral Blood Ratio Method:

   • Formula: CSF WBC (predicted) = CSF RBC × (blood WBC/blood RBC)
   • More complex but theoretically accounts for the patient's specific blood counts
   • Less commonly used in routine practice

Limitations of Correction Methods

It's important to recognize that all correction methods have limitations:

• Recent research suggests that correction formulas may be inaccurate when RBC counts exceed 100,000/μL 2
• Correction can result in loss of sensitivity with only marginal gains in specificity 3
• In some cases, there may be fewer WBCs in the CSF than the ratio would predict (38% in one study) 4

Special Considerations

HSV Encephalitis

Blood-stained CSF samples may reflect the hemorrhagic pathophysiology of HSV encephalitis rather than traumatic tap, especially if serial CSF specimens remain blood-stained 1

Pediatric Patients

• In neonates and infants, adjustment of WBC counts may not improve diagnostic utility 3
• For infants aged 29-60 days, correction using a ratio of 877:1 substantially reduced false positives while misclassifying very few cases of bacterial meningitis 5

Protein Correction

For protein levels, subtract 0.1 g/dL protein for every 1000 red blood cells 1

Clinical Decision Making

When interpreting a bloody CSF sample:

1. Apply the standard correction (1 WBC per 700 RBCs)
2. Consider the clinical context and other CSF parameters:
   • CSF glucose/plasma glucose ratio
   • CSF protein level
   • CSF lactate (if available)
3. Remember that a CSF lactate <2 mmol/L helps rule out bacterial disease 1
4. Consider that the presence of more than 10 times the number of WBCs than allowed by the adjustment is a sensitive and specific indicator of meningitis 4

Pitfalls to Avoid

• Don't rely solely on corrected WBC counts for clinical decisions
• Don't rule out infection based solely on corrected counts, especially in immunocompromised patients
• Be aware that HSV encephalitis can present with hemorrhagic CSF, which may be mistaken for a traumatic tap
• Remember that initial CSF findings may be normal in approximately 5-10% of adults with proven HSV encephalitis 1

By applying these correction methods while considering their limitations and the overall clinical picture, you can more accurately interpret CSF findings in the context of a bloody sample.