Michael Fernandez¹, Vraj Evans¹, Zoe Vayanos¹, Peter Graham^1
1The Royal College of Pathologists of Australasia Quality Assurance Programs (RCPAQAP), St Leonards, NSW, Australia

Introduction: Immunoglobulin G (IgG) subclasses (IgG1, IgG2, IgG3 and IgG4) provide a valuable measurement tool for the clinical diagnosis of various autoimmune conditions and immunodeficiencies¹. Variation in measuring IgG subclasses due to the absence of a single international reference material has been noted^2,3. We sought to review whether method variation in our 2023 IgG subclasses EQA program impacted result interpretations. 

Methods: Survey material from consenting donors was distributed to thirty participating laboratories. Quantitative and qualitative interpretations (low/normal/elevated) based on method-related reference intervals were analysed using RCPAQAP in-house software. Qualitative targets for each IgG subclass were determined by ≥80% consensus.

Results: The Binding Site demonstrated a higher bias for IgG3 but lower for IgG1, IgG2 and IgG4 compared to Siemens across seven out of eight survey rounds. Both IgG1 and IgG3 showed the greatest variation out of the four IgG subclasses. Eleven out of thirty-two possible qualitative interpretations failed to achieve consensus.

Discussion: We largely attributed the differences in qualitative results between the manufacturers to their respective biases and reference intervals, supporting previous publications^2,3. However, other factors, such as internal cut-off ranges, cannot be excluded. Our review highlights the importance of developing a single standardised reference material for IgG subclasses methods.

Statement of originality: This review is original work undertaken by the RCPAQAP.

References
¹ McLean-Tooke A, O’Sullivan M, Easter T, et al. Differences between total IgG and sum of the IgG subclasses in clinical samples. Pathol J RCPA 2013; 45: 675-7.
² Wilson C, Ebling R, Henig C, et al. Significant, quantifiable differences exist between IgG subclass standards WHO 67/97 and ERM-DA470k and can result in different interpretation of results. Clin Biochem 2013; 46: 1751-5.
³ Sarnago A, Pascual RM, Moreno MJ et al. IgG subclasses quantitation: Analytical performance of The Binding Site SPA_PLUS® human assay and comparison with Siemens BNII® assay. Clin Biochem 2018; 51: 85-9.