CSF Analysis
Opening Pressure
A normal OP is less than 20 cm H2O. Elevations in OP represents an imbalance in the CSF secretion vs reabsorption. OP is measured in the lateral decubitus position with the legs extended.
Causes of increased OP: infection, bleeding (SAH), rapidly growing tumour. Essentially, any cause of Elevated ICP.
Cellular Profile
Normal cell counts in adults are <5 WBC/uL and <5 RBC/uL. More than 3 PMNs/uL are also abnormal.
CSF pleocytosis can be present in infectious and noninfectious states. Requires correlation with the clinical state. PMNs generally predominate in bacterial meningitis but can also do so in viral meningitis (particularly early on within 12-24 hours). Eosinophilia can indicate a fungal or parasitic infection but is nonspecific.
Adjust for traumatic tap: Subtract 1 WBC/uL for every 1000 RBCs/uL
Chemistry
Protein
The typical CSF profile has very little protein due to the blood-CSF barrier. CSF protein can be mildly elevated in diabetes, SAH, traumatic LP.
Oligoclonal bands are elevations in oligoclonally expanded Ig concentrations which can occur in any disorder that disrupts the BBB, or also by intrathecal production of IgG. Can be indicative of MS if greater than the serum. However, oligoclonal bands can also be found in infections (Lyme), autoimmune disease, CNS malignancy, LPDs.
Glucose
Hypoglycorrachia is loosely defined as CSF glucose less than 0.6 (0.5 to 0.8) of serum glucose. However it lags behind serum changes, and often maxes out at 15-17 mmol/L despite severe hyperglycemia.
Low CSF glucose: less than 1.0 mmol/L is strongly predictive of bacterial meningitis. Other causes of low CSF glucose: Mycobacteria, Mycoplasma, treponemal, fungal CNS infection, mumps, enterovirus, LMCV, HSV, VZV. However, tends to be more normal in viral CNS infections. Can also happen in noninfectious diseases such as leukemia or CNS lymphoma, leptomeningeal carcinomatosis, severe SAH, neurosarcoidosis, salicylate poisoning, hypoglycemia.
High CSF glucose: only occurs in the setting of hyperglycemia.
Lactate
Determination of the CSF lactate concentration has been suggested as a useful test to differentiate bacterial from viral meningitis.
Meta-analyses show that the diagnostic accuracy of CSF lactate is superior to CSF glucose, WBC count, protein concentration in differentiating bacterial from aseptic meningitis.