Can abnormal cystatin C or low cystatin C-based eGFR explain a low GlycoMark?

Low GlycoMark reflects low circulating 1,5-anhydroglucitol (1,5-AG), a marker shaped by renal handling as well as glycemic exposure. In a 70-year-old man with abnormal cystatin C or reduced cystatin C–based eGFR, renal impairment is therefore a credible competing contributor to a low result—especially if kidney disease is advanced.

Clinical interpretation

• 1,5-AG showed a weak inverse association with measured GFR in type 2 diabetes cohorts, even after HbA1c adjustment. Its relationship with glycemic measures became weaker when GFR was <30 mL/min/1.73 m².
• Laboratory/assay materials caution that stage 4–5 CKD, terminal renal failure, and dialysis can produce low or otherwise unreliable GlycoMark results.
• Thus, reduced kidney function should decrease confidence that a low GlycoMark exclusively represents recent hyperglycemic excursions. This concern is greatest in severe CKD; mild-to-moderate impairment does not necessarily eliminate glycemic interpretability.

Renal and drug mechanisms

• 1,5-AG is freely filtered at the glomerulus and ordinarily almost completely reabsorbed in the proximal tubule. Reduced tubular reclamation increases urinary 1,5-AG loss and lowers serum GlycoMark.
• Glucosuria competitively impairs 1,5-AG reabsorption. This is the principal biologic route by which hyperglycemia lowers GlycoMark.
• SGLT2 inhibitors induce glucosuria and can therefore lower GlycoMark despite improved glycemia. SGLT5 is a major proximal-tubular transporter involved in 1,5-AG reabsorption.
• In chronic renal failure, higher fractional 1,5-AG excretion has been associated with lower serum 1,5-AG even with normal glycemia and negligible urinary glucose, supporting impaired tubular handling as an independent influence.

Bottom line

• Abnormal cystatin C or low cystatin C–based eGFR plausibly complicates interpretation of low GlycoMark, but the inference is indirect because available data address renal function generally rather than cystatin C specifically. Interpret GlycoMark alongside eGFR severity, glucose/HbA1c data, glucosuria, and SGLT2-inhibitor exposure.