Intrathecal immunoglobulin synthesis: The potential value of an adjunct test
Graphical abstract
Introduction
The diagnosis of multiple sclerosis (MS) is mainly based on MRI findings, but cerebrospinal fluid (CSF) analysis remains an important diagnostic tool to confirm inflammatory nature of the disease and to exclude alternative conditions [1]. Currently, the most reliable evidence of intrathecal immune response are CSF specific oligoclonal bands (OCB) detected in approximately 90% of MS patients [[2], [3], [4]]. OCB positivity independently increases the risk of conversion to MS in patients with clinically isolated syndrome (CIS) and thus has an added value to the MRI features [[5], [6], [7], [8], [9]]. With the role of CSF examination re-emphasized in the new revisions to the McDonald criteria optimization and standardization of OCB (intrathecal inflammation) detection methods is again to be addressed [10].
The recommended method for OCB determination consists of isoelectric focusing with subsequent immunodetection (either blotting or fixation) [2]. Although good reproducibility of different techniques was reported (semi-automated commercially available and in-house high sensitivity assays), the procedure is time consuming and gives only qualitative information. Furthermore, it is still largely dependent on technical skills and enables subjective interpretation, which is an important source of inter-laboratory variability [2,11,12].
Quantification of immunoglobulin free light chains (FLC) rather than intact antibodies was proposed to overcome these issues [13,14]. As FLC are secreted in excess by plasma cells during the immunoglobulin (Ig) production they also accumulate in the CSF in the presence of intrathecal B cell activity. Several studies demonstrated comparable diagnostic accuracy of kappa FLC (KFLC) and OCB in MS, however different methodologies and KFLC parameters were employed [[13], [14], [15], [16]].
In this study we aimed to compare the diagnostic accuracy of various KFLC parameters and OCB positivity in our MS cohort and to further estimate the potential value of KFLC as an adjunct test for detection of intrathecal inflammation.
Section snippets
Patient selection
Written informed consent was obtained from the patients and the study protocol was approved by the National Medical Ethics Committee of Slovenia. Patients hospitalized at the Department of Neurology, University Medical Centre, Ljubljana, who underwent diagnostic lumbar puncture between 2014 and 2016 were included in the study. As inclusion criteria, at least 0.5 mL of patient's CSF and results of routine CSF analysis (leukocyte count, total protein concentration, OCB status) had to be
Calculation of KFLC parameters
Different quantitative measures of intrathecal KFLC production were evaluated. KFLC index was calculated as a quotient of the CSF/serum ratios of KFLC and albumin concentrations. To obtain local secretion of KFLC in CSF compartment each corresponding QAlb-dependent upper normal limit of KLFC was subtracted from CSF/serum KFLC ratio and corrected for absolute KFLC serum concentration, as defined in previous studies [[23], [24], [25]]. Intrathecal KFLC fraction (%) was assessed as a relative
Demographic and basic CSF characteristics
Patients from both groups did not differ significantly in gender distribution (Yates' corrected χ2 = 1.288, p = 0.256), however mean age at the time of lumbar puncture was significantly higher in controls. Among basic CSF parameters, only differences in white blood cell (WBC) counts were significant (Table 1). According to the age related reference value 15% of MS and 26% of control group patients had elevated CSF/serum albumin quotients (QAlb; Yates' corrected χ2 = 1.741, p = 0.187),
Discussion
Similar diagnostic performance of OCB and KFLC was found in our study. Among different KFLC measures, linear function of KFLC index provided the best sensitivity and specificity (both 96%), though gain in sensitivity was not statistically significant compared to the gold standard method. Since statistical analysis depends only on the extent of disagreement between the two methods, likelihood ratio with its inherent trade-off between sensitivity and specificity was rather used for direct
Conclusions
Due to fast, relatively inexpensive, automated and rater-independent analysis KFLC quantification has a reasonable potential for implementation in the diagnostic work-up of MS. Even with the use of monoclonal nephelometric assay, different KFLC parameters are as good as OCB in terms of sensitivity and specificity for MS. If neither of the tests proves clearly superior sequential analysis could be cost-effective whereas gain in diagnostic performance might be achieved by simultaneous testing.
Author contributions
MK and UR designed the experiment and contributed materials/analysis tools. AE and VA performed the experiment and analyzed the data. AE, VA and UR wrote the paper. MK and UR read and approved the final manuscript.
Funding
This work was in part supported by University Medical Centre Ljubljana research grant (PN 20150129). Siemens Healthcare Slovenia contributed N Latex FLC kappa assay.
Conflicts of interest
U. Rot received grants/research support from Biogen Idec and consultation fees/travel grants from Bayer, Biogen Idec, Genzyme, Merck-Serono, Novartis and Teva. A. Emersic, V. Anadolli and M. Krsnik have nothing to disclose.
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