CST3 and Parkinson disease: Although this PD-focused study did not include measurements of other freely filtered biomarkers such as Cystatin C, which is unaffected by muscle mass, diet, or sex, prior comparative studies have shown that in individuals with eGFR values between 60 and 89 mL/min/1.73 m2, the same range observed in our study population (see Table 1), SCr alone estimates GFR as accurately as Cystatin C or the combined SCr and Cystatin C Eqs.