Serum indices based on creatinine and cystatin C, including creatinine/cystatin C ratio (Cr/CysC), ratio and difference of estimated glomerular filtration rate (eGFR) based on cystatin C and creatinine (eGFRcys/eGFRcre and eGFRDiff), and serum creatinine × eGFRcys, are recently identified serum markers for sarcopenia. The gene discussed is CST3; the disease is sarcopenia.