Comment on the study ‘Cerebrospinal fluid lactate: measurement of an adult reference interval’ by Sally D Slack,Paul Turley,Victoria Allgar and Ian B Holbrook

Measurement of lactate concentration in the cerebrospinal fluid (CSF) is useful for diagnosing an inflammatory response in the central nervous system (CNS); thus, we read the study by Slack et al. ‘Cerebrospinal fluid lactate: measurement of an adult reference interval’ ¹ with great interest.

Unfortunately, the ability of CSF lactate to predict CNS inflammation is limited. The concentration of lactate in the CSF is not influenced exclusively by the extent of anaerobic metabolism in the CSF, but also by the supply of the energy substrate (glucose). We therefore derived an equation, the so-called coefficient of energy balance (KEB), and have successfully applied it in our practice for several years^2–5

$\text{KEB}=38-18\frac{[\text{lactate}]}{[\text{glucose}]}$

The KEB can be defined as the theoretical average number of molecules of adenosine triphosphate (ATP) produced from one molecule of glucose under given conditions in the CSF compartment.

By investigating the molar concentrations of glucose and lactate in the CSF and calculating the KEB, we can distinguish three ‘energy levels’ in the CSF: (1) KEB values >28.0 indicate normal energy conditions in the CSF, but cannot rule out a possible pathological process in the CNS; (2) KEB values ≤28.0 indicate increased anaerobic metabolism in the CSF, which may be associated with CNS inflammation; (3) KEB values <10.0, including values below 0.0, indicate a very high level of anaerobic metabolism in the CSF, usually related to severe inflammation with an oxidative burst of phagocytes in the CNS.^2,4,5

Cytological investigations of the CSF are also of key importance in the diagnosis of CNS impairment. The combination of KEB and CSF cytology enables differentiation between less intense serous inflammations, and severe inflammations caused by an oxidative burst of neutrophil granulocytes or macrophages in the CNS. The finding of a significant number of CSF neutrophil granulocytes and a high degree of CSF anaerobic metabolism (with KEB values <10.0) is typical of purulent inflammation in the CNS, usually involving extracellular bacteria.^2,4,5 Similarly, the finding of a significant number of CSF lymphocytes or monocytes–macrophages and a high degree of anaerobic metabolism in the CSF compartment (KEB values <10.0) may signify an intense inflammatory response in the CNS with an oxidative burst of macrophages, involving intracellular bacteria, mycotic agents or cancer.^2,5

Thus, our solution is not only to investigate the lactate concentrations in the CSF, but in particular to also determine the extent of anaerobic metabolism in the CSF compartment via the KEB level.