Renal Function Assessment
Renal Function Assessment - Practical interpretation of creatinine, eGFR, measured GFR and renal function during AKI or dialysis.
Practical orientation: Creatinine-based eGFR assumes steady state. In AKI, use trend, urine output and clinical context.
Purposes of renal function assessment
| Purpose | Why it matters |
|---|---|
| CKD staging | Guides risk stratification and stage-specific management. |
| Drug dosing | Many medications require kidney-based adjustment. |
| Prognosis | eGFR and albuminuria predict kidney failure, CV events and death. |
| Transplant evaluation | Donor and recipient assessment may require accurate GFR. |
Measured GFR
Measured GFR uses exogenous filtration markers such as inulin, iohexol, iothalamate or EDTA. It is most accurate but impractical for routine use and does not assess tubular function.
Creatinine
| Creatinine lower than expected | Creatinine higher than expected |
|---|---|
| Low muscle mass, limb loss, cachexia, cirrhosis, low height extremes. | Low GFR, high muscle mass, high meat/creatine intake, drugs inhibiting secretion, assay interference, uroperitoneum. |
Creatinine is secreted in the proximal tubule, especially in advanced CKD, so creatinine clearance can overestimate true GFR.
eGFR in CKD
| Marker | Strength | Limitation |
|---|---|---|
| Creatinine-based eGFR | Widely available and standardized. | Less precise at high GFR and in unusual muscle mass/diet. |
| Cystatin C eGFR | Less dependent on muscle mass. | Affected by thyroid disease, corticosteroids and smoking; assays less standardized. |
| Combined creatinine-cystatin C | Often best performance. | Requires cystatin C availability. |
| Cockcroft-Gault CrCl | Used in many drug labels. | Less accurate for CKD staging. |
Renal function in AKI
Do not use routine eGFR equations in AKI because creatinine is not at steady state. Urine output is useful but can remain preserved despite major GFR loss. Kinetic GFR can be considered when creatinine is changing rapidly.
Renal function on dialysis
Residual kidney function in HD/PD is usually assessed by timed urine collections for creatinine and urea clearance. It is clinically important because residual function improves volume, solute and outcome management.