Infrequent patterns in cerebrospinal fluid isofocusing test: Clinical significance and contribution of IgG index and Reiber diagram to their interpretation.

Cerebrospinal fluid (CSF) isoelectrofocusing (IEF) is considered as the gold standard for detecting an intrathecal synthesis of IgG, which is a hallmark of multiple sclerosis (MS). This corresponds to the presence of CSF-restricted IgG oligoclonal bands (OCB) (typically type 2 pattern). Moreover, this technique can also detect a systemic immune reaction with passive transfer of IgG (type 3 and 4 patterns) for which the clinical relevance is less understood. The aim of our study was to determine the frequency and disease associations of IEF type 3 and 4 patterns and to investigate the potential usefulness of including quantitative data (IgG index and Reiber Diagram) in interpreting such IEF profiles. Among 544 patients who underwent CSF IEF (Hydragel CSF isofocusing kit, Sebia®, France) in our Laboratory during a six-year-period, those who presented type 3 or 4 patterns were selected. Clinical data and results of other immunological tests were analyzed for 27 patients followed in the Neurological Department. Frequencies of type 3 and type 4 patterns were relatively low (2.3 % and 3.4 % respectively). Among patients with type 3 pattern included in our study (n = 10), 5 were diagnosed with MS. For the 5 other patients, the diagnosis was a clinically isolated syndrome (CIS) (2 cases), a probable auto-immune encephalitis (2 cases) and a possible genetic neurodegenerative disease (1 case). MS patients had an IgG index >0.7 and fell into area 4 of Reiber diagram while non-MS patients had an IgG index <0.7 and fell into area 1, except the last case. Regarding type 4 pattern (n = 17), the diagnoses were as follows: MS (3), CIS (4), Neuromyelitis optica spectrum disorders with positive anti-AQP4 antibodies (3) and anti-NMDAR autoimmune encephalitis (1). The remaining cases had central nervous system impairment related to vascular, metabolic or tumoral etiologies (3) or peripheral nervous system impairment (3). In this group (type 4 pattern), IgG index was <0.7 in 15/17 cases. Interpretation using Reiber diagram showed an abnormal blood-brain barrier for 8/17 patients. Type 3 and 4 IEF patterns are infrequently observed in routine neurology practice. It is important for the diagnostic laboratory professional as well as for the neurologist to understand their clinical relevance. Our findings highlight the contribution of quantitative evaluation of CSF (IgG index, Reiber diagram) for the interpretation of such situations. Despite the small size of our study population, our results emphasize the importance of reporting the exact type of IEF pattern and not only the positivity or not of OCB.