Background: Clinicians rely on a correction formula (Predicted CSFWBC=CSFRBC×BloodWBC/ BloodRBC) to determine if a true CSF leukocytosis exists. This formula may overestimate true CSF leukocytosis leading to delayed diagnosis and treatment of meningitis. Methods: A retrospective review of CSF data of 105 patients registered at 3 hospitals (Saskatoon, Canada) between 2011-2013 who met the following criteria: 1) CSF samples from lumbar puncture (LP) contained≥1000 RBC/mm3; 2) a complete blood count (CBC) performed within 24 hours of LP; and 3) CSF not obtained due to high clinical suspicion of meningitis and was negative for microbial staining and culture. Regression analysis was performed to determine the relationship between actual and predicted CSF WBC values. Results: Mean adult age was 48.9 years; CSF profile (mean WBC 146.3×106/L; RBC 17374×106/L; glucose 4.1 mmol/L; protein 1.4 g/L); mean peripheral WBC 8.2×109/L; RBC 3.9×109/L. Mean pediatric age was 1.4 years; CSF profile (mean WBC 171.8; RBC 41763; glucose 2.7; protein 1.7); mean peripheral WBC 12; RBC 7.2. The observed LP CSF WBC value was 47% of predicted (r2=0.54 pediatric cohort; r2=0.91 adult cohort). Conclusions: True CSF leukocytosis in both pediatric and adult patients could be missed in a traumatic CSF sample if correction is based on current formulas. We propose a modifcation: ObservedCSFWBC=0.5×[CSFRBC×BloodWBC/BloodRBC].