What is the normal range of leukocytes (white blood cells) per microliter (µL) in cerebrospinal fluid (CSF)?

Normal CSF Cell Count

In healthy adults, normal CSF contains ≤5 white blood cells per microliter (µL), with essentially no red blood cells present. 1

Normal CSF Parameters

The American College of Critical Care Medicine and Infectious Diseases Society of America establish that in immunologically normal hosts, the presence of ≤5 white blood cells/µL, normal opening pressure, and normal CSF protein concentration essentially exclude meningitis. 1

Key Normal Values:

• White blood cells (WBCs): ≤5 cells/µL 1
• Red blood cells (RBCs): Essentially 0 cells/µL in non-traumatic taps 1
• CSF:plasma glucose ratio: Approximately 2/3 (or 0.6-0.7) of plasma glucose 1
• Protein: Variable by age, but generally <0.6 g/L suggests against bacterial meningitis 1

Critical Clinical Caveats

Important Exceptions to Normal Cell Counts

Bacterial meningitis can present with normal or minimal CSF pleocytosis, particularly in early disease or immunocompromised patients. 1, 2

• In one study, 10% of bacterial meningitis patients had fewer than 100 cells per mm³ 1
• 2% of bacterial meningitis cases present with ≤5 cells/µL 2
• Patients with normal CSF leukocyte counts and bacterial meningitis often have immunocompromising conditions (49% vs 30% in those with elevated counts) 2

When Normal Cell Count Does NOT Exclude Infection:

In patients with suspected bacterial meningitis but normal CSF cell counts, focus on these diagnostic parameters: 2

• CSF protein elevation: Present in 68% of bacterial meningitis cases with normal cell counts 2
• Gram stain: Positive in 95% of severe pneumococcal meningitis cases despite normal cell counts 2
• Clinical presentation: Severe pneumococcal meningitis (51%), sepsis-predominant (21%), or miscellaneous causes (28%) 2

Diagnostic Considerations for Borderline Counts

The 5-30 Cells/µL Range

Cell counts between 5-30 cells/µL represent a critical diagnostic gray zone requiring manual confirmation. 3

• Automated devices have a lower limit of reliable detection at approximately 20 cells/µL 3
• Manual microscopy remains the gold standard for counts in this clinically important range 3
• The 2024 Microscanner C3 study shows improved sensitivity with detection limits of 3.33 cells/µL for WBCs, but manual confirmation is still recommended for clinical decision-making 4

Traumatic Tap Correction

When traumatic tap occurs, subtract 1 WBC for every 700 RBCs present in CSF. 5

• Alternative formula: True CSF WBC = Actual CSF WBC - [(WBC in blood × RBC in CSF) / RBC in blood] 5
• If corrected WBC count exceeds 10 times the predicted contamination, this strongly suggests true meningitis 5
• CSF lactate >35 mg/dL has 93% sensitivity and 96% specificity for bacterial meningitis 5

Special Populations

Immunocompromised Patients

Maintain high suspicion for infection regardless of cell count until cultures are final in critically ill immunocompromised patients. 1

• Normal CSF parameters do not reliably exclude infection in this population 1
• Additional testing (HSV PCR, CMV, fungal studies) may be required 1

Chronic Inflammatory Conditions

Mild to moderate pleocytosis (up to 10-15 cells/µL) can occur in non-infectious conditions like CIDP. 6

• 6% of CIDP patients had ≥10 cells/µL 6
• Pleocytosis in CIDP often occurs with subacute onset and may spontaneously decrease 6

Technical Considerations

CSF cell counts decrease rapidly after collection—approximately 50% of leukocytes are lost within 1 hour in native CSF. 7

• Addition of serum-containing medium immediately after sampling prevents cell loss 7
• Without preservation, 86% of samples with mild pleocytosis (5-15 cells/µL) may be incorrectly diagnosed as normocellular after 1 hour 7
• This emphasizes the importance of rapid processing or proper sample preservation 7