**4. Discussion**

In this retrospective study, we performed a detailed evaluation of the importance of C3 in kidney biopsy specimens and serum for the outcomes of IgAN in children. We found that the presence of >+1 C3 deposits in kidney biopsy is a predictor of worsening renal function (GFR < 90 mL/min) in this group of children, which is the first such study in a large sample collected throughout a European country. However, we did not show similar importance of a reduced serum C3 level at the onset of the disease.

In the study population, C3 deposits in kidney biopsy, mostly rated at +1 or +2, were identified in 66% of patients, which is a rate similar to that reported in an adult population studied by Wu et al. [18].

In patients with IgAN, immune complexes may activate the alternative and lectin pathways of the complement system and initiate inflammation [13,15,19]. Recent studies in adult patients show that the severity of C3 deposits in kidney biopsy and reduced serum C3 level may affect long-term renal outcomes [3,20–22].

In our study from 2015, we evaluated the usefulness of serum Immunoglobulin A/complement factor 3 (IgA/C3) ratio for predicting the severity of histological lesions in kidney biopsy children with IgA nephropathy. We found positive correlations between the IgA/C3 ratio and proteinuria, serum creatinine and serum IgA level. We also determined that the higher the MEST score the higher the IgA/C3 ratio. We also determined the optimal cutoff values of IgA/C3 serum ratio for specific MEST score [23].

In the study by Caliskan et al. in 111 adult patients with IgAN, C3 deposits > +1 were found to be a prognostic factor for the development of chronic kidney disease (CKD) stage G5 or reduction in GFR by ≥50% compared to the baseline [20].

We also confirmed the importance of C3 standing > +1 as an adverse prognostic factor for renal survival in children but in our study, the endpoint was GFR < 90 mL/min, and thus we demonstrated the prognostic significance of C3 deposits for CKD from stage G2 onwards, which is a novel finding, and these observations were made in children, which also contrasts to the conclusions from the studies in adult patients with IgAN [20].

In the study by Kim and Koo in 66 adult patients with IgAN, a prognostically adverse effect of C3 deposits > +1 for the development of ESRD or doubling of serum creatinine level was also shown, and this study also showed an effect of reduced serum C3 level on the renal outcomes, although its predictive value was lower than that of the urinary protein to creatinine ratio [22]. In our study, we were unable to confirm the effect of reduced serum C3 level on renal survival with normal GFR but this may have been related to a low number of children (*n* = 13, or 8.78%) with reduced serum C3 level at baseline. This finding needs to be replicated in a larger patient sample.

An additional, though already previously known finding of our study is confirmation of an adverse effect of male gender and reduced GFR at baseline on long-term renal outcomes [8–11].

Among children with C3 deposits > +1 and those with a reduced serum C3 level at baseline, we did not find significant differences in the MEST-C score, similarly to the study by Kim et al. who did not find significant differences in the rates of M, E, S and T between groups with C3 deposits > +1 and ≤ +1 [22].

In addition, we found a reduced serum C3 level at the end of follow-up in 10 patients, of whom 3 showed a reduced serum C3 level at baseline. Of these, only one patient had GFR < 90 mL/min at the end of follow-up, which might also confirm no prognostic significance of a reduced serum C3 level at baseline in children, but again, these patient groups were too small to allow any definitive conclusions.
