B2M and mucositis: Preexisting–CKD had the best accuracy (90.3%, 95% CI 85%‐94.1%), followed by serum β2M level ≥ 3.7 mg/L (85.4%, 95% CI: 79.5%‐90.1%) and mucositis grade 3/4 (71.9%, 95% CI: 64.8%‐78.2%) (Table 2).