Lumbar puncture on a patient with suspected meningitis

subarachnoid hemorrhage

Normal CSF has no RBCs. RBCs found in CSF suggest either a traumatic tap or a subarachnoid hemorrhage. If the first tube of CSF has many RBCs but they clear by the last tube, traumatic tap is suggested.

A sample revealing xanthochromia, which is yellow hue resulting from the degradation of hemoglobin, is suggestive of subarachnoid hemorrhage (see above image).

By contrast, someone with bacterial meningitis would be expected to have no or nearly no RBCs (or, if RBCs are present, they would clear by the final sample tube), decreased glucose, elevated protein, more than 1000 leukocytes/mm3 with 85%-90% neutrophils, and a positive Gram stain and culture.

References:

Garcia CG, McCracken GH Jr. Acute bacterial meningitis beyond the neonatal period. In: Long SS, et al. Principles and Practice of Pediatric Infectious Disease. 4th ed., 2012:272-278.

Lehman R, Schor N. Neurologic evaluation. In: Kliegman RM, et al. Nelson Textbook of Pediatrics. 19th ed., 2011.