Creatinine-Cystatin C Discordance in Patients With Neurotoxicity After Hematopoietic Stem Cell Transplantation or Chimeric Antigen Receptor T-Cell Therapy: A Case Series

Accurate assessment of kidney function is essential in recipients of hematopoietic stem cell transplantation (HSCT) and chimeric antigen receptor T-cell (CAR-T) therapy, who are frequently exposed to renally-cleared medications with narrow therapeutic indices. Serum creatinine-based estimated glomerular filtration rate (eGFR Cr ) may overestimate kidney function compared to cystatin C-based estimates (eGFR cys ) in patients with sarcopenia, potentially leading to medication accumulation and toxicity. Here, we describe five patients with discordance between eGFR Cr and eGFR cys who developed neurologic complications following HSCT or CAR-T therapy. We reviewed clinical and radiographic data from patients who underwent HSCT and CAR-T therapy between January 2024 and March 2026 at two major academic centers. We identified five patients with baseline eGFR discordance, defined as eGFR cys at least 30% lower than eGFR Cr , who subsequently developed suspected medication-related neurotoxicity. Among three HSCT recipients, one developed fludarabine-associated toxic leukoencephalopathy, and two developed cefepime neurotoxicity. In each case, eGFR Cr suggested preserved kidney function, whereas combined creatinine-cystatin C estimates were lower and supported consideration of medication dose modification. Two CAR-T recipients developed severe neurologic syndromes initially attributed to immune effector cell-associated neurotoxicity syndrome (ICANS). However, delayed onset, prolonged encephalopathy, limited response to ICANS-directed therapies, and concomitant exposure to cefepime and fludarabine raised concern for contributing medication-related neurotoxicity. These cases highlight the potential clinical relevance of creatinine-cystatin C discordance in patients receiving renally-cleared neurotoxic medications. Prospective studies are needed to determine whether cystatin C–based kidney function estimates can improve medication dosing and reduce neurotoxicity risk within this population.