A-278 Use of Antibody Index Calculation to Detect Intrathecal VZV IgG Antibody Synthesis to Aid in the Diagnosis of VZV Infections of the CNS

Abstract Background Diagnosis of VZV CNS vasculitis (CNS-VZV) is aided by detecting intrathecally (i.t.) synthesized VZV-specific antibodies (Abs) through testing paired serum (S) and CSF specimens and determining an antibody index (AI). Currently, laboratories in the United States do not provide AI testing for VZV. The currently used methods (e.g., EIAs, IFAs) lack clinical utility as they cannot discriminate if VZV Abs in the CSF are i.t. produced due to CNS infection or whether they are present due to passive diffusion across the blood-CSF-barrier. To optimize diagnostic options for CNS-VZV, we evaluated a quantitative VZV IgG ELISA (Euroimmun AG, Lübeck, Germany) to calculate AI values using Reiber’s formula on paired S/CSF specimens to assess the diagnostic accuracy of this method. Methods The clinical sensitivity cohort consisted of four patients diagnosed with CNS-VZV (meningitis or vasculitis) as determined by symptoms, imaging, response to antiviral therapy, and/or supporting test results (e.g., lymphocytic pleocytosis, elevated protein in CSF). The patient group for clinical specificity included 11 patients with suspected CNS-VZV who ultimately had an unclear (n=4) or non-CNS-VZV (n=7) diagnosis. The reference range cohort included paired clinical S/CSF samples from 25 patients without infectious diseases testing. Paired S/CSF samples were processed on the same batch using a fully automated ELISA analyzer capable of onboard, ad hoc runtime dilution preparation (AP22 ELITE, DAS ITALY). The AI values were calculated using the Reiber formula, and sensitivity (Sn), specificity (Sp), and positive (PPV) and negative (NPV) predictive values were calculated. The AI was calculated using the following equations: If Total IgG CSF (mg/dL):Total IgG serum (mg/dL) < Quotient limit Total IgG, then AI = (VZV IgG Ab CSF [units × dilution]:VZV IgG serum [units × dilution])/(Total IgG CSF [mg/dL]:Total IgG serum [mg/dL]). If Total IgG CSF (mg/dL):Total IgG serum (mg/dL) > Quotient limit Total, then AI = (VZV IgG Ab CSF [units × dilution]:VZV IgG serum [units × dilution])/Quotient limit Total IgG. Quotient limit Total IgG = 0.93 × [(Qalbumin2) + (6 × 10-6)]1/2 - 1.7 ×10-3. Qalbumin = Albumin CSF (mg/dL):Albumin serum (mg/dL). Results The VZV IgG ELISA with Reiber AI calculation method showed Sn, Sp, PPV, and NPV values of 100% (4/4), 91% (10/11), 80% (4/5), and ?0 (10/10), respectively. One of the Sp patients had an equivocal AI result and was classified as negative, whereas the Sp patient with a positive AI result had an unclear diagnosis. VZV AI results for 96% (24/25) of the reference range patients were negative, with one equivocal. The mean AI of the 25 patients was 1.05, and the lower and upper 3 standard deviation limits were 0.6 and 1.5, respectively. These values match the AI qualitative interpretive criteria recommended by Reiber (0.6 to <1.3 = negative, 1.3 to 1.5 = equivocal, >1.5 AI = positive). Conclusion We show that the VZV IgG ELISA, alongside the Reiber formula for AI calculation, is effective for identifying patients with i.t. synthesized VZV IgG Abs. This method can assist clinicians in evaluating patients with suspected CNS-VZV infections who are negative by alternative methods.