“Associated” or “Secondary” IgA nephropathy? An outcome analysis

Background
Whether differences in outcome between primary (pIgAN) and secondary IgA nephropathy (sIgAN) exist is uncertain.
Methods
We conducted a retrospective, observational study that included all histologically diagnosed IgAN patients between 2010–2017 (N = 306), 248 with pIgAN and 58 with sIgAN. To obtain samples with similar risk of progression, sIgAN patients were grouped as liver disease and autoimmune/viral disease and propensity score matched to corresponding pIgAN samples. Univariate (Kaplan Meier) and multivariate time-dependent (Cox modelling) analyses were performed to identify predictors of the composite end-point (doubling of serum creatinine, end-stage kidney disease or death).
Results
Of the whole cohort, 20% had sIgAN (6% alcoholic cirrhosis, 6% autoimmune disease and 8% viral infections). sIgAN patients were older, had more comorbidities, lower proteinuria and higher haematuria, but similar distribution in MESTC lesions and eGFR as those with pIgAN. They reached the end-point in similar proportions with those with pIgAN (43 vs. 30%; p = 0.09) but their mortality was higher (19 vs. 3%; p<0.0001). Both in unmatched (HR 0.80, 95%CI 0.42–1.52; p = 0.5) and matched samples (log-rank test: liver disease-IgAN vs. pIgAN, p = 0.1; autoimmune/viral-IgAN vs. pIgAN, p = 0.3), sIgAN was not predictive for end-point. In analyses restricted only to sIgAN, those with viral infections (HR, 10.98; 95% CI, 1.12–107.41; p = 0.03) and lower eGFR (HR, 0.94; 95%CI, 0.89–0.98; p = 0.007) had a worse prognosis. Immunosuppression did not influence outcome.
Conclusions
The differences in MESTC score and outcome between pIgAN and sIgAN seems to be minimal, suggesting that “associated” describes better than “secondary” the relationship among the two. Immunosuppression did not to influence outcome of sIgAN.