CSF Free Light Chains as a Marker of Intrathecal Immunoglobulin Synthesis in Multiple Sclerosis: A Blood-CSF Barrier Related Evaluation in a Large Cohort

Abstract

Objectives: The importance of immunoglobulin G (IgG) oligoclonal bands (OCB) in the diagnosis of multiple sclerosis (MS) was reaffirmed again in the recently revised MS diagnostic criteria. Since OCB testing is based on non-quantitative techniques and demands considerable methodological experience, measurement of CSF immunoglobulin free light chains (FLC) has been suggested as quantitative alternative to OCB. We aimed to establish reference values for FLC measures and evaluate their diagnostic accuracy with regard to the diagnosis of MS. Methods: Immunoglobulin kappa (KFLC) and lambda (LFLC) free light chains were prospectively measured by nephelometry in CSF and serum sample pairs in 1,224 patients. The analyzed cohort included patients with MS, other autoimmune or infectious inflammatory diseases of the nervous system as well as 989 patients without signs for nervous system inflammation. Results: Regarding diagnosis of MS, the diagnostic sensitivity and specificity of intrathecal KFLC ratio were 93.3 and 93.7% using the CSF-serum albumin ratio-dependent reference values, 92.0 and 95.9% for intrathecal KFLC ratio applying the ROC-curve determined cut-off levels, 62.7 and 98.3% for IgG index, 64.0 and 98.8% for intrathecal IgG synthesis according to Reiber diagrams, and 94.7 and 93.3% for OCB. Diagnostic sensitivity and specificity of intrathecal LFLC were clearly lower than KFLC. Conclusions: Intrathecal KFLC and OCB showed the highest diagnostic sensitivities for MS. However, specificity was slightly lower compared to other quantitative IgG parameters. Consequently, CSF FLC may not replace OCB, but it may support diagnosis in MS as a quantitative parameter.

Keywords: IgG index; OCB; cerebrospinal fluid; immunoglobulin free light chains; intrathecal IgG synthesis; multiple sclerosis; oligoclonal bands; serum.

Figure 1

Correlation among cerebrospinal fluid free light chain levels in non-inflammatory neurologic diseases (n = 989) with the respective serum levels (KFLC: p < 0.001, r = 0.744; LFLC: p < 0.001, r = 0.737) and the respective CSF-serum ratio of albumin (Q Alb) (KFLC: p < 0.001, r = 0.638; LFLC: p < 0.001, r = 0.653) are shown.

Figure 2

CSF-serum ratio of kappa (Q KFLC) and lambda (Q LFLC) free light chain is plotted against the respective CSF-serum ratio of albumin (Q Alb) on the log scale. The upper line is the 99% quantile estimated by linear quantile regression and indicates the upper reference value of Q KFLC and Q LFLC dependent on Q Alb and based on a control group of 989 non-inflammatory neurologic diseases and a range of Q Alb from 1.6*10⁻³ to 25.7*10⁻³. Formula for the Q Alb dependent upper reference value (upper line): Q KFLC = 9.50 + 2.08*Q Alb; Q LFLC = 16.37 + 1.36*Q Alb. The lower line is the 1% quantile line and the dashed line in the middle is the 50% quantile (median) line.

Figure 3

CSF- serum ratio of kappa free light chain (Q KFLC) (left) and lambda free light chain (Q LFLC) (right) is plotted against CSF-serum ratio of albumin (Q Alb). Solid line indicates the upper reference value introduced in Figure 2. AI-CNS-D, other autoimmune disease of the central nervous system; CIDP, chronic inflammatory demyelinating polyneuropathy; CIS, clinically isolated syndrome; CNS-I, viral and bacterial CNS infection; GBS, Guillain-Barré syndrome; ME, metabolic encephalopathy; MS, multiple sclerosis; PP-PNP, paraproteinemic neuropathy and/or neuropathy with monoclonal gammopathy of unknown significance; P-CNS-I, post-infectious CSF syndrome.

Figure 4

Immunoglobulin Free light chain levels in neurological diseases. Horizontal solid line indicates median. All groups have been compared to NIND and MS. Significant P-values for pairwise comparisons (Mann-Whitney U-test) are displayed. ^****p < 0.0001, ^***p < 0.001, ^**p < 0.01, ^*p < 0.05. AI-CNS-D, other autoimmune disease of the central nervous system; CIDP, chronic inflammatory demyelinating polyneuropathy; CIS, clinically isolated syndrome; CNS-I, viral and bacterial CNS infection; CSF, cerebrospinal fluid; GBS, Guillain-Barré syndrome; ME, metabolic encephalopathy; MS, multiple sclerosis; PP-PNP, paraproteinemic neuropathy and/or neuropathy with monoclonal gammopathy of unknown significance; NIND, non-inflammatory neurological diseases; P-CNS-I, post-infectious CSF syndrome. CSF (A) and serum (B) levels of KFLC as well as CSF (C) and serum (D) levels of LFLC are displayed.

Figure 5

ROC curves of different FLC measures for discrimination between MS (n = 75) and non-MS (n = 1,149) patients. AUC and 95% confidence interval are given. KFLC, immunoglobulin kappa free light chain; LFLC, immunoglobulin lambda free light chain; Q Alb, CSF-serum ratio of albumin; Q KFLC, CSF-serum ratio of KFLC; Q LFLC, CSF-serum ratio of LFLC; KFLC index (Q KFLC/Q Alb); LFLC index (Q LFLC/Q Alb); ROC, receiver operating characteristics; AUC, area under the curve.