The records of two cohorts of children who received lumbar puncture (LP) for suspected meningitis at Children’s Memorial Hospital, Chicago, IL, were reviewed retrospectively to determine the value of a ratio of observed to predicted (O:P) cerebrospinal fluid (CSF) white blood cells (WBC) after a traumatic LP in the differentiation of children with and without meningitis. The predicted CSF WBC count was calculated using the formula CSF RBC x blood WBC/blood RBC (blood counts had been obtained within 6 hours of LP). All 57 children in the study were older than 1 month and the CSF had >500/mm3 red blood cells (RBC). Cohort 1, consisting of 12 (21%) patients examined in 1990 through 1999, had CSF cultures positive for a bacterial pathogen. Cohort 2 patients (n=45 [79%]) were tested during Jan-Dec 1999, in the post-H influenzae b vaccine era, and had a CSF culture negative for bacterial pathogens. Patients receiving antibiotics within 72 hours before LP, and those with a previous neurosurgical procedure or CNS bleed were excluded.
Patients with meningitis were significantly older (median 7.8 months; range 1-106 months) than those without meningitis (median 1.3 months; range 1-139 months). The O:P and WBC:RBC ratios were significantly lower for patients without, compared to those with meningitis (medians 0.064 and 0.001 cf 1.26 and 1.98, respectively). An O:P ratio <0.01 and WBC:RBC ratio <1:100 (0.01) were each 100% predictive of the absence of meningitis. The absence of pleocytosis was also highly specific for a negative culture, whereas a negative Gram stain, lack of polymorphonuclear cell predominance, CSF glucose, and protein were less predictive. When the LP is traumatic, both the WBC:RBC ratio and the O:P ratio may be helpful in the identification of patients without meningitis, but the former calculation is less cumbersome and is recommended for an initial screening. All clinical and laboratory data should be evaluated before deciding not to treat for meningitis after a traumatic tap. 
COMMENT. Traumatic lumbar punctures (LP) are difficult to interpret, and apart from CSF culture, methods used to differentiate patients with or without meningitis must be viewed with caution. The CSF leukocyte count in bacterial meningitis is usually greater than 1000/mm3. CSF becomes turbid when the leukocyte count exceeds 200-400/mm3. A CSF leukocyte count of less than 250 mm3 may be present in 20% of patients with acute bacterial meningitis, and pleocytosis may be absent in patients with severe sepsis and meningitis. A traumatic LP will alter the CSF leukocyte count and protein concentration, while the gram stain, culture and glucose level may not be influenced. 
Methods proposed to correct for the presence of red blood cells have previously focused on a test using the observed:predicted (O:P) ratio of CSF WBCs for the diagnosis of meningitis. The present investigators were interested in a highly specific test and the identification of patients without meningitis by a method that would not overlook a meningitis case. They conclude that in children older than 1 month with a traumatic LP, the WBC:RBC and O:P ratios will differentiate the majority of patients without meningitis from those with meningitis. However, they find it prudent to advise reliance on all clinical and laboratory information, which would include bacteriologic results, before discharging a child without treatment after a traumatic LP.