Objective: At our facility, we measure creatinine (Cr) and cystatin C (CysC), using JSN-estimated glomerular filtration rates (eGFRCr) and (eGFRCys), to aid in the diagnosis of chronic kidney disease. We compared the two markers to assess whether they match, and to examine the trends in cases where they diverge.

 Subjects: A total of 8,431 individuals who underwent a health check-up between April 2016 and March 2021, with five consecutive simultaneous measurements of Cr and CysC. The cohort consisted of 4,245 men (aged 24–94 years) and 4,186 women (aged 29–87 years).

 Methods: Based on the evidence-based CKD Clinical Practice Guidelines 2023, "Referral Criteria from Primary Care Physicians to Nephrologists or Specialized Medical Institutions," we used eGFR values for Cr and CysC along with urinary protein levels to categorize individuals into normal, continued lifestyle advice/management, and referral groups. The consistency of referral criteria between eGFRCys and eGFRCr the 2020 fiscal year was statistically analyzed. Additionally, we evaluated whether eGFR values changed positively or negatively over the five years and expressed these changes as slopes. The medical histories of discrepant cases were also reviewed.

 Results: The consistency rate of referral criteria for the 2020 fiscal year was 87%. When using eGFRCr for classification, the results were: 99% for normal, 25% for continued lifestyle advice/management, and 67% for referral. When comparing slopes, 0.4% of cases showed discrepancies. Among these, 15 cases had higher eGFRCr and lower eGFRCys (including 4 with thyroid disease and 1 with metastasis due to cancer treatment), and 17 cases had lower eGFRCr and higher eGFRCys (including 7 with thyroid disease).

 Discussion: The low consistency rate of 25% for continued lifestyle advice/management was likely due to the overall higher values of eGFRCys. The correlation between eGFRCr and eGFRCys was good when comparing slopes, and the discrepancies observed were likely influenced by thyroid dysfunction or cancer, as has been reported in previous studies.

 Conclusion: Consistency rates were high for normal and referral categories but low for continued lifestyle advice/management. Discrepant cases were mostly found among individuals with thyroid disorders or other known conditions.

 Objective: The creatinine/cystatin C (Cre/CysC) ratio is attracting attention as a potential readily available biomarker of the muscle mass. We evaluated the correlation between the Cre/CysC ratio and skeletal muscle mass index (SMI) measured using dual-energy X-ray absorptiometry. We also aimed to investigate the associations of the Cre/CysC ratio with sarcopenia and locomotive syndrome (LS) components.

 Design, Setting and Participants: A total of 157 subjects (male, n=96; female, n=67) who underwent the Advanced course and Basic course anti-aging health checks at Tokai University Tokyo Hospital were enrolled in the study.

 Measurements: Cre/CysC levels were compared with or without components of sarcopenia (decreased SMI and/or muscle strength) or the LS risk test (two-step test stand-up test and the 25-question geriatric locomotive function scale).

 Results: Cre/CysC was significantly lower in male subjects with low SMI, low grip strength and sarcopenia. A lower Cre/CysC ratio was also observed in women with low grip strength. With respect to LS, Cre/CysC levels decreased as the results of the two-step test and stand-up test deteriorated. Cre/CysC values were significantly lower in LS stage 2 patients than in non-LS stage 2 patients in both men and women.

 Conclusion: The present study revealed that high Cre/CysC values were associated with sarcopenia in men and with LS in both sexes. The Cre/CysC ratio is expected to be a useful marker for sarcopenia and LS in the context of general health screening, where precise body composition measurements are not easily applied.