Supplementary Material for: The Association of Low Hemoglobin Levels with IgA Nephropathy Progression: A Two-Center Cohort Study of 1,828 Cases

<b><i>Aim:</i></b> To investigate the relationship between hemoglobin levels and the progression of IgA nephropathy (IgAN). <b><i>Methods:</i></b> In a two-center cohort of 1,828 cases with biopsy-proven IgAN, we examined the association of hemoglobin levels with the primary outcome of a composite of all-cause mortality or kidney failure defined as a 40% decline in eGFR, or ESKD (defined as eGFR &lt;15 mL/min/1.73 m² or need for kidney replacement therapy including hemodialysis, peritoneal dialysis, or kidney transplantation), or the outcome of kidney failure, assessed using Cox and logistic regression models, respectively, with adjustment for confounders. <b><i>Results:</i></b> At baseline, mean age, eGFR, and hemoglobin levels were 33.75 ± 11.03 years, 99.70 ± 30.40 mL/min/1.73 m², and 123.47 ± 18.36 g/L, respectively. During a median of approximately 7-year follow-up, 183 cases reached the composite outcome. After adjustment for demographic and IgAN-specific covariates and treatments, a lower quartile of hemoglobin was nonlinearly associated with an increased risk of the primary outcome or kidney failure in the Cox proportional hazards models (primary outcome: HR for quartile 3 vs. 4, 1.37; 95% CI, 0.83–2.25; HR for quartile 2 vs. 4, 1.18; 95% CI, 0.68–2.07; HR for quartile 1 vs. 4, 1.91; 95% CI, 1.15–3.17; kidney failure: HR for quartile 3 vs. 4, 1.39; 95% CI, 0.84–2.31; HR for quartile 2 vs. 4, 1.20; 95% CI, 0.68–2.11; HR for quartile 1 vs. 4, 1.83; 95% CI, 1.09–3.07) in the fully adjusted model. Then, hemoglobin levels were transformed to a binary variable for fitting the model according to the criteria for anemia of 110 g/L in the women and 120 g/L in men in China. The participants in the anemia group had an increased risk of developing outcomes compared with the nonanemia group in both genders (primary outcome: male: HR, 1.64; 95% CI, 1.01–2.68; female: HR, 1.68; 95% CI, 1.02–2.76; kidney failure: male: HR, 1.60; 95% CI, 0.97–2.64; female: HR, 1.58; 95% CI, 0.95–2.61) in the fully adjusted model. <b><i>Conclusions:</i></b> A low level of hemoglobin was nonlinearly associated with IgAN progression. The anemic IgAN patients presented a higher risk of developing poor outcomes compared with the nonanemic patients.