CST3 and rhabdomyolysis: Even though it seems that low-GFRcys/GFRcr is calculated on behalf of increase in cystatin C, there are more factors that affect creatinine than cystatin C. Creatinine is a by-product of muscle metabolism, and its release into the circulation vary greatly with age (production decreases with older age, largely because of the reduction in muscle mass), gender (female sex tends to have less muscle mass than male), individual muscle mass, certain diseases and conditions leading to reduction of muscle mass (rhabdomyolysis, movement impairment) and diet [27].