RESUMO
The advent of immunotherapy has improved outcomes of patients in a number of cancers.1 While immunotherapy helps the immune system recognize malignant cells, it also can lead to adverse effects that mimic autoimmune diseases including, but not limited to, rash, colitis, pneumonitis, and nephritis. Diagnosis of immunotherapy related nephritis is based on serum creatinine trends, which can be falsely elevated in the setting of high muscle mass. Cystatin C is an adjunctive kidney biomarker that can estimate glomerular filtration rate independent of muscle mass. We present a case where the use of cystatin C avoided unnecessary therapy interruption in a young, athletic man being treated with nivolumab for melanoma. Further research is needed to define the role cystatin C in monitoring kidney function during immunotherapy.