Interpreting CSF lactic acidosis: effect of erythrocytes and air exposure

Elevated cerebrospinal fluid (CSF) lactate concentrations in neurotrauma and sub-arachnoid haemorrhage are associated with a poor prognosis. However, in blood-stained CSF, elevated lactate levels may arise from red cell metabolism, even without ischaemia, potentially reducing specificity. This study was undertaken to quantify the erythrocyte contribution to CSF lactate measurements, with and without, exposure to room air.

Blood was added to CSF to achieve three different red cell concentrations. The CSF was then exposed at 37 degrees C to either room air or 5% CO2 and 95% oxygen. Vancomycin and gentamycin were added to inhibit bacterial growth. Lactate concentrations and red cell concentrations were measured prior to the addition of blood and 10 minutes, 6 hours and 24 hours later. CSF without the addition of blood was used as a control.

In the control specimens there were no increases in CSF lactate concentrations over time, either in air or CO2, whereas all specimens with blood added demonstrated significant increases in lactate at 6 and 24 hours (P < 0.01). The lactate increases in both air and CO2 were correlated directly with red cell counts (R2 = 0.62 to 0.87). At all red cell concentrations, the mean lactate increase was greater in air.

Red cells in CSF cause significant increases in lactate concentrations, more so when exposed to air. This should be considered when interpreting lactate in blood stained CSF. Blood-stained CSF specimens for lactate assay should be collected directly from an external ventricular drain rather than a reservoir bag.