Abstract: Background: IgA nephropathy is the most common primary glomerulonephritis in Asian population. Kidney biopsy is the gold standard for diagnosing the disease. The exploration and discovery of non-invasive markers with diagnostic value of the disease is still a hot research topic. Objective: To investigate the diagnostic value of serum IgA and IgA/C3 ratio in children with primary IgA nephropathy and its relationship with pathological grade. Design: Diagnostic test accuracy. Methods: Taking renal biopsy pathology as the gold standard, children undergone kidney biopsy were divided into primary IgA nephropathy (true positive) and non-IgA primary glomerulonephritis (true negative). Taking IgA and IgA/C3 ratio as the test standard, a diagnostic model for predicting primary IgA nephropathy in children was established. Demographic characteristics, serum immunoglobulin and complement test results, renal biopsy pathological examination reports and other laboratory test results were intercepted. According to age, children were divided into 1-4 years old, -7 years old, -11 years old and -18 years old subgroups and according to the 24 h urinary protein level at the time of admission, children were divided into two subgroups of <50 and ≥50 mg·kg-1·d-1. Main outcome measures: The diagnostic power of serum IgA and IgA/C3 ratio for primary IgA nephropathy. Results: A total of 150 cases in the primary IgA nephropathy group and 474 cases in the non-IGA nephropathy group were included into analysis. The serum IgA and IgA/C3 ratio in primary IgA nephropathy group was higher than that in non-IgA nephropathy group, and the proportion of serum IgA elevation in all ages was higher than that in non-IgA nephropathy group. The AUC of serum IgA and IgA/C3 ratio was 0.824 and 0.851, the sensitivity was 80.0% and 74.3%, and the specificity was 73.1% and 82.7%, respectively. The AUC of IgA and IgA/C3 was the largest in 1-4 years old subgroup. The diagnostic cutoff values of IgA and IgA/C3 increased with age. The sensitivity of serum IgA and IgA/C3 ratio in the subgroup with 24 h urinary protein level <50 mg·kg-1·d-1 was 90.3% and 93.5% respectively. The sensitivity ranged from 86.8% to 100% in different age subgroups of children with 24 h urinary protein level <50 mg·kg-1·d-1 , indicating that serum IgA and IgA/C3 ratio had a high recognition degree in the subgroup with 24 h urinary protein level <50 mg·kg-1·d-1, and the diagnostic cutoff values were also increased with age. There were no significant differences in gender, age, serum IgA, C3 and IgA/C3 ratio between children with MESTC score < 4 and ≥4, or between children with LEE grade Ⅰ-Ⅱ and Ⅲ-Ⅴ. Conclusion: Serum IgA/C3 ratio and IgA elevation are important for differential diagnosis of primary IgA nephropathy, especially in patients with 24 h urinary protein level < 50 mg·kg-1·d-1.

Key words: Children, IgA nephropathy, IgA/C3 ratio, Pathological grade, Diagnostic value