This blog post explains why blood neurofilament level monitoring is not ready for primetime and may never quite replace CSF NFL monitoring in pwMS.
Although blood neurofilament light chain (NFL) levels are a promising biomarker for monitoring MS inflammatory disease activity there are issues that need to be resolved.
We and others have shown that blood levels correlate with spinal fluid levels with a correlation coefficient of ~0.7. This translates to an R-squared value of ~0.5 and tells us that only 50% of the blood levels can be explained by what is detected in the lumbar CSF. We have many examples of patients low NFL levels in the CSF with high blood levels and the converse, i.e. high CSF levels and low blood levels. I think the discrepancy in these individual cases may indicate real biology.
CSF is an admixture of fluids derived from the (1) choroid plexus, where most of the CSF is made, (2) the so-called interstitium or substance of the brain and spinal cord, (3) the meninges and (4) the blood vessels that traverse the subarachnoid or CSF space.
The typical volume of CSF in adults is 125–150 ml. This turns over approximately four times a day, i.e. the daily volume of CSF produced is approximately 600 ml. CSF production and flow are subject to diurnal variation; flow rates of ventricular fluid can vary by as much as a factor of 3.5, with minimum CSF production occurring around 1800h (12±7 ml/h) and a maximum at approximately 0200h (42±2 ml/h).
The integrity of these barriers and the flow of CSF determine the content of CSF protein. The CSF constituents are not uniform and depend on the site from which the CSF is sampled. There is a head-to-tail concentration gradient for total protein along the axis of the CNS, with the lowest concentrations in ventricular fluid and the highest concentrations in the fluid from the lumbar-sacral sac from where the CSF is sampled during a lumbar puncture.
Physical activity influences the CSF protein concentration; subjects who have been lying down for prolonged periods have higher CSF protein concentrations than active subjects. It is important to consider this when interpreting the results of CSF analyses from subjects who have been on their backs for more than 24 hours.
The lumbar subarachnoid space is a cul-de-sac. CSF obtained from lumbar punctures does not necessarily provide accurate information on inflammatory or pathological events in the brain that occur beyond the CSF outflow pathways of the fourth ventricle, i.e. events in relation to the surface of the brainstem, cranial nerves and cerebral hemispheres. In addition, the extracellular space of lesions deep in the brain may not necessarily communicate with all parts of the free CSF space.
For these reasons, spinal NFL levels are more likely to represent spinal cord disease and not brain pathology. In comparison, blood levels are more likely to integrate what is being produced from the whole CNS and the peripheral nervous system. Yes, NFL is not specific to the CNS. Therefore NFL from peripheral nerve disease will affect blood levels.
Another issue is that some pwMS mount an autoantibody response against NFL proteins. These antibodies may reduce the circulating NFL levels and hence may affect peripheral blood levels by artificially lowering NFL levels. The anti-NFL antibodies tend to occur in people with more advanced or progressive MS and hence may explain why peripheral blood NFL levels are so low in people with progressive MS.
Another factor is the circulating blood volume; the study below shows the larger your blood volume the lower your peripheral blood NFL levels will be. Height and body size will need to be taken into account when interpreting blood levels.
A critical factor that needs to be determined is how long does NFL circulate in the periphery and what is the mechanism of its clearance and breakdown. Without knowing the latter we will continue to have difficulty interpreting what peripheral blood NFL levels mean.
Could peripheral blood levels increase as part of normal biology? I was recently told at a meeting that peripheral blood NFL levels increase as a result of running a marathon (unpublished data). Why? Could this be due to microtrauma to peripheral nerves or the neuromuscular junction? Or due to increased release from the brain? We know that minor head injuries raise NFL levels. Could the excessive bobbing around of the brain in the skull during marathon running result in a small NFL leak? Marathon running also affects peripheral fluid balance; the increased NFL levels could be related to an alteration in blood volume or the breakdown and clearance of the NFL in the periphery.
So switching from CSF to peripheral blood monitoring of NFL levels is not going to happen until we have answered some of these questions. In addition, it is unlikely that we are going to make treatment decisions based on isolated peripheral blood NFL measurements. NFL levels will be one of many factors that will need to be taken into account when making clinical decisions. Serial or longitudinal NFL measurements with an area under the curve analysis will be more valuable than a single peripheral blood measurement.
These are some of the reasons why peripheral blood NFL levels are not quite ready for primetime and clinical practice. Do you agree? This is why we at Barts-MS will continue to do lumbar punctures and CSF measurements of NFL in our patients for the foreseeable future. So if you come to see us in our clinic we may offer you a monitoring lumbar puncture.
In addition, to NFL levels there are many other biomarkers that can be measured in CSF that will provide us with information on smouldering MS disease activity. This is why Barts-MS is ideally positioned to tackle the challenge of defining and studying smouldering MS.
Manouchehrinia et al. Confounding effect of blood volume and body mass index on blood neurofilament light chain levels. Ann Clin Transl Neurol. 2020 Jan 1. doi: 10.1002/acn3.50972.
Blood Neurofilament light chain (NfL) has been suggested as a promising biomarker in several neurological conditions. Since blood NfL is the consequence of leaked NfL from the cerebrospinal fluid, differences in individuals’ Body Mass Index (BMI) or blood volume (BV) might affect its correlation to other biomarkers and disease outcomes. Here, we investigated the correlation between plasma NfL, BMI, and BV in 662 controls and 2,586 multiple sclerosis cases. We found a significant negative correlation between plasma NfL, BMI/BV in both groups. Our results highlight the potential confounding effect of BMI/BV on associations between blood NfL and disease outcomes.