Determination of kFLC and K Index in cerebrospinal fluid: A valid alternative to assessintrathecal immunoglobulin synthesis

https://doi.org/10.1016/j.jneuroim.2013.07.006Get rights and content

Highlights

  • Kappa free light chains (kFLCs) were increased in early multiple sclerosis (MS).

  • CSF/serum ratio of kFLC was increased in MS.

  • kFLC Index seems to be a good tool in MS diagnoses.

Abstract

Intrathecal immunoglobulin synthesis is observed in several disorders of the central nervous system, but its detection by current laboratory tests is relatively insensitive and operator depending. We assessed the diagnostic accuracy of a nephelometric assay for k free light chain determination in cerebrospinal fluid and serum. The patients were grouped according to clinical and laboratory criteria. ROC curves for all methods were performed to find the best cut-off value. kFLC Index seems to be more accurate than other parameters. Our data indicate that nephelometric assay for kFLCs in CSF reliably detect intrathecal immunoglobulin synthesis and discriminate multiple sclerosis patients.

Introduction

Intrathecal immunoglobulin synthesis is commonly observed in inflammatory disorders of the central nervous system (CNS) of either infectious or autoimmune origin (DeCarli et al., 1987), and has been shown to be of high diagnostic value. Small amounts of immunoglobulins typically pass into the cerebrospinal fluid (CSF) by passive transfer through the blood–CSF barrier, so it is necessary to differentiate the origin of Ig in the CSF before intrathecal immunoglobulin synthesis can be diagnosed (Reiber and Felgenhauer, 1987). This can be achieved either by calculation of the CSF/serum ratios of immunoglobulins compared with the CSF/serum ratio of albumin (Qalb), which is not synthesized intrathecally, or by the detection of so-called oligoclonal immunoglobulin bands (OCBs) in CSF (Mattson et al., 1982, Walker et al., 1983, Luxton et al., 1990). In general, CSF/serum ratios and quotient diagrams are not sufficiently sensitive to reliably differentiate intrathecal immunoglobulin synthesis from passive transfer across the blood–CSF barrier, and the analysis of OCBs is time-consuming, not quantitative, and subject to investigator bias (Luxton, McLean, 1990).

In theory, the determination of free light chains (FLCs) might be a sensitive alternative to the above-mentioned approaches. Immunoglobulin light chains are typically secreted together with intact Ig by plasma cells. Although the half-life of kFLCs in serum is very short (2–4 h) because of rapid renal elimination, this clearance pathway is not available from CSF; therefore, is possible that the half-life of kFLCs in CSF is comparable to that of other proteins. Thus, even small amounts of intrathecal immunoglobulin synthesis with concomitant kFLC secretion will disproportionately increase the CSF concentration of kFLCs, making them a potentially sensitive marker of intrathecal immunoglobulin synthesis.

Attempts to determine FLCs in CSF have been made previously (Eickhoff and Heipertz, 1978, Lamers et al., 1995, Krakauer et al., 1998). We perform an automated nephelometric assay for the detection of FLCs based on specific monoclonal antibodies against epitopes that are hidden in intact immunoglobulins (Bradwell et al., 2001). We applied this test to CSF/serum pairs and evaluated its diagnostic accuracy in the detection of intrathecal immunoglobulin synthesis in comparison with the determination of OCBs, which is considered to be the most sensitive procedure and was taken as the reference standard for our study (Caudie et al., 2000, Richard et al., 2002).

Section snippets

Patients

CSF/serum pairs from 80 patients with different clinically well-documented neurological disorders were collected; all patients had undergone an elective spinal puncture at the Neuroscience Department of the Tor Vergata University Hospital between September 2012 and January 2013. The study protocol complied with the declaration of Helsinki (1964).

Samples were excluded if artificial blood contamination of CSF or monoclonal bands in both CSF and serum were present (n = 7). The patients were

Methods

Immunoglobulin and albumin concentrations were measured by nephelometry (BN Prospec, Siemens Healthcare Diagnostics Products GmbH, Marburg, Germany) in fresh CSF and serum samples.

OCBs were determined by immunofixation (Hydragel9 CSF Isofocusing; Sebia) on a semi-automated agarose electrophoresis system (Sebia Hydrasys) (Caudie et al., 2000, Richard et al., 2002). The system can detect the OCBs presence in a concentration range of 30–125 μg/L. Analysis was performed by laboratory technicians,

Patients

CSF/serum pairs of 80 patients of the neurological clinic were examined. According to their diagnoses the patients were divided into three groups: group 1 (33 patients, 21 female and 12 male) patients having diseases without inflammation, patients with inflammatory diseases other than MS group 2 (24 patients, 15 female and 9 male) and patients with definitive MS group 3 (23 patients, 14 female and 9 male).

Patient characteristics and patient groups are shown in Table 1.

Detection of kFLCs

The median CSF

Discussion

The use of kFLCs to detect intrathecal immunoglobulin synthesis was reported previously by other groups (Fagnart et al., 1988, Lamers et al., 1995, Jenkins et al., 2001, Fischer et al., 2004, Desplat-Jego et al., 2005, Arneth and Birklein, 2009), but the test is not actually employed into diagnostic use. The determination of FLCs was technically difficult in the past and not feasible in clinical routine. In this study we show that a novel, automated assay for kFLCs is suitable for routine

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