*Purpose: To determine the detection rate of IgG subclasses (IgGsc) and C3d-binding HLA-antibodies (abs) in a cohort of highly sensitized kidney transplant patients.

*Methods: We evaluated, in cryopreserved sera from 90 potential renal transplant recipients, HLA-abs Class I and Class II using single antigen beads (SAB). C3d was detected by Luminex (Lifecodes®), IgG 1-4 subclasses were evaluated using monoclonal abs for each IgGsc (Luminex).

*Results: The SAB were able to reveal 1,684 abs, additionally 63 were only detected by C3d assay and 24 by IgGsc (n=1771).The IgGsc assay showed 367 abs with at least one IgGsc, the most common was IgG₁ alone or associated with other subclasses (351, 75.6%); only 7 IgG₂ and 9 IgG₃ were found isolated form IgG₁. IgG₄ was identified in 17 abs, always in combination with IgG₁. A MFI >4000 of the HLA-SAB was directly related with the detection of one or more IgGsc (*p=<0.001). Figure 1.The C3d assay exposed 216 complement-binding abs; 128 of these were identified in the IgGsc as well. Table 1 depict a more detailed analysis of each Class I and Class II category, its IgG detection and C3d-binding capacity.A multivariate analysis using different models showed that the best predictors of IgGsc detection were: finding an HLA-DQ ab and the positivity of the C3d assay (OR, (CI 95%) 1.72 (1.18-2.51), p= 0.004 and 7.41 (5.4-10.1), p= <0.001). Comparatively, finding ≥2 IgGsc was the best predictor for a positive C3d.

*Conclusions: The ability of the IgG monoclonal abs to detect IgGsc depends on the MFI of the HLA-SAB; a high MFI improves the assessment of IgGsc. The HLA-DQ abs that were C3d positive had a higher probability of detection of IgGsc than any other combination. The classification of abs as one or more IgGsc increases the risk of these being C3d positive.

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