Method Description

Signal Recognition Protein Indirect Immunofluorescence Assay:

The patient's sample is tested by a standardized indirect immunofluorescence assay (IFA) that uses composite frozen sections of mouse cerebellum, kidney, and gut tissues. After incubation with patient sample and washing, fluorescein-conjugated goat antihuman IgG is applied. Signal recognition protein (SRP)-specific autoantibodies are identified by their characteristic fluorescence staining patterns. Samples that are scored positive are titrated to an endpoint. Interference by coexisting non-neuron-specific autoantibodies is eliminated or lessened by serologic absorption. This method does not distinguish between antibodies against different SRP proteins.(Package insert: EUROLINE Autoimmune Inflammatory Myopathies 16 Ag (IgG) test instruction. EUROIMMUN Medizinische Labordiagnostika AG; Version: 03/2018)

3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase:

IgG antibodies to 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) are detected by a chemiluminescent assay using the Inova BIO-FLASH instrument. HMGCR antigen is coated on to paramagnetic beads, which are stored in the reagent cartridge lyophilized. When the assay cartridge is ready to be used for the first time, a buffer solution is added to the tube containing the beads, and the beads are resuspended with the buffer. The reagent cartridge is then loaded onto the BIO-FLASH instrument. A patient serum sample is diluted 1:17 by the instrument in a disposable plastic cuvette. An aliquot of the diluted patient serum, HMGCR-coupled beads, and assay buffer are combined into a second cuvette and mixed. This cuvette is incubated at 37° C. The beads are then magnetized and washed several times. Isoluminol conjugated anti-human IgG antibody is then added to the cuvette, and incubated at 37° C. Again, the beads are magnetized and washed repeatedly. The isoluminol conjugate produces a luminescent reaction when "trigger" reagents are added to the cuvette. The light produced from this reaction is measured as relative light units (RLU) by the BIO-FLASH optical system. RLU values are proportional to the amount of bound isoluminol conjugate, which in turn is proportional to the amount of anti-HMGCR antibodies bound to the antigen on the beads. The QUANTA Flash HMGCR assay utilizes a predefined lot specific master curve that is uploaded into the instrument through the reagent cartridge barcode. Based on the results obtained by running two calibrators, an instrument specific working curve is created, which is used by the software to calculate chemiluminescent units from the RLU value obtained for each sample.(Package insert: QUANTA Flash HMGCR 701333. Inova Diagnostics, Inc; v04, 09/2018)

SRP Immunoblot:

The assay is performed using the EUROBlotOne instrument. All reagents required are supplied in the kit. Samples are diluted 1:100 (15 mcL in 1.5 mL sample buffer) and added to the strips placed in incubation trays. The sample and test strips are incubated for 30 minutes at room temperature. Unbound antibodies are removed from trays by washing steps using wash buffer. Bound patient IgG antibodies are detected by adding antihuman-IgG antibodies coupled to horse radish peroxidase followed by incubation at room temperature for 30 minutes. The strips are washed again to remove excess antihuman-IgG antibodies. The substrate is added for 10 minutes (room temperature) and the reaction is subsequently stopped. The strip is scanned, and band intensities are digitized. The digital image is converted to band signal intensities by the EUROLineScan software, which are normalized to an internal standard. Bands corresponding to SRP with signal intensities of 15 U (arbitrary) or greater are reported as positive. The SRP antigen used is recombinant SRP 54. Positive immunoblot results confirm that a patient's serum contains antibodies directed against the SRP 54 subunit. Negative immunoblot results do exclude the presence of SRP antibodies.(Package insert: EUROLINE Autoimmune Inflammatory Myopathies 16 Ag (IgG) test instruction. EUROIMMUN Medizinische Labordiagnostika AG; Version: 03/2018)