Serum glucose adjusted cut-off values for normal cerebrospinal fluid/serum glucose ratio: implications for clinical practice.

Abstract

Calculation of the cerebrospinal fluid/serum glucose (CSF/SGlu) ratio is part of the routine CSF work-up, however, different cut-off values ranging from 0.3 to 0.5 have been suggested so far to distinguish physiological from pathological conditions. The objective of this study was to determine cut-off values for normal CSF/SGlu ratio dependent on serum glucose concentrations. We screened our database for paired CSF and serum samples, which have been collected by lumbar puncture, were processed within 1 h after withdrawal, showed cell count <15/3, erythrocyte count <1500/3 and normal CSF total protein resulting in 1036 sample pairs. Glucose concentrations in CSF and serum were measured by enzymatic spectrophotometry. Median glucose concentration in CSF was approximately 60% of that in serum. CSF/SGlu ratios negatively correlated with serum glucose levels (R=-0.586, p<0.001) and cut-off values for normal CSF/SGlu ratio defined as the 5th percentile were 0.5 for patients with serum glucose concentrations <100 mg/dL, 0.4 for those with a glucose level of 100-149 mg/dL and 0.3 for serum glucose concentrations ≥150 mg/dL. CSF/SGlu ratio inversely correlates with serum glucose concentrations in a non-linear manner. These findings suggest that cut-off values for normal CSF/SGlu ratio must be adjusted to serum glucose levels, probably explaining the considerably varying cut-offs that have been reported so far.