124 Cystatin C (CysC). mgl

Cystatin C: kidney filtration marker, eGFR equations, KDIGO GFR categories and non-renal influences.

Cystatin C is a small protein produced at a constant rate by all nucleated cells. It is freely filtered by the kidney glomeruli and then reabsorbed and broken down in the tubules, so it does not return to the blood. Its blood level therefore depends mainly on the glomerular filtration rate (GFR). Unlike creatinine, cystatin C depends little on muscle mass and diet, so it is useful when creatinine is unreliable: in older or frail people, amputees, very muscular people, or in malnutrition and liver disease.

KidneyGFRCKD

Typical adult range≈ 0.6–1.0 mg/L (lab-dependent)

UseeGFRcys and eGFRcr-cys

Confirms CKDwhen eGFRcr is 45–59

Standardised tointernational reference material

Figure 1. GFR categories (KDIGO). Estimated GFR from cystatin C, or from creatinine and cystatin C combined, places the patient in one of these categories.

Use in practice

Cystatin C is used to calculate eGFRcys, or combined with creatinine in the race-free CKD-EPI 2021 equation (eGFRcr-cys), which is more accurate than either marker alone. KDIGO recommends measuring cystatin C when creatinine-based eGFR may be inaccurate, and to confirm chronic kidney disease in adults with eGFRcr of 45–59 mL/min/1.73 m² without other markers of kidney damage. A higher cystatin C means lower kidney function.

Factors that affect cystatin C

Higher independently of GFR: hyperthyroidism, high-dose glucocorticoids, inflammation, obesity, smoking, some cancers
Lower independently of GFR: hypothyroidism

Raised cystatin C is also associated with higher cardiovascular risk and mortality, partly independently of GFR.

What to do

A raised cystatin C or low eGFR should be repeated and assessed with urine albumin, blood pressure and blood glucose. Kidney function also determines the safe dose of many medicines.

References

Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int. 2024;105(4S):S117–S314. doi:10.1016/j.kint.2023.10.018
Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C–based equations to estimate GFR without race. N Engl J Med. 2021;385(19):1737–1749. doi:10.1056/NEJMoa2102953

This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

Published on 25 April 2024