Two elderly patients with normal creatinine and elevated cystatin C - a case report

Serum creatinine concentration (Scr) and creatinine based GFR estimating equations (eGFR_cr) are commonly used as an estimate of GFR. However, serum creatinine concentration is also influenced by non-GFR determinants. This case report presents two elderly patients with normal Scr but elevated serum cystatin C concentration (Scys) where the exclusive assessment of Scr would have lead to an overestimation of GFR and would have misclassified the patients as having a normal kidney function.

Patient 1, a 102-year-old woman, presented with a Scr of 0.45 mg/dl, while her Scys was elevated (1.55 mg/l). Depending on which of the five GFR estimating equations was used, the patient could be classified into four different CKD-Stages (2, 3a, 3b and 4). The largest difference between the eGFR-results was 94 ml/min/1.73 m² (Δ-eGFR). Patient 2, an 88-year-old man, also had normal Scr (0.93 mg/dl) but elevated Scys (1.55 mg/l). An iohexol clearance measurement yielded a measured GFR (mGFR) of 44 ml/min/1.73 m². Four out of five GFR equations would have overestimated the patient's kidney function.

The presented cases highlight the influence of non-GFR determinants on Scr and demonstrate the variability of eGFR results depending on the filtration marker and GFR equation used. Especially for older adults, it shows the great clinical importance of understanding the limitations of each filtration marker and of identifying situations in which relying on eGFR_cr alone can lead to false estimation of kidney function. In these situations, cysC based GFR equations may provide improved accuracy of GFR assessment and may protect patients from drug overdosing and the abundant use of contrast agents.