Indicators of Renal Impairment in Laboratory Tests
The three laboratory findings that indicate renal impairment are elevated serum creatinine, blood urea nitrogen (BUN) greater than 25 mg/dl, and protein in urine. 1
Serum Creatinine and GFR
- Elevated serum creatinine is a key indicator of reduced renal function, reflecting decreased glomerular filtration 1
- While serum creatinine may remain in the normal range during early renal dysfunction, an elevated level clearly indicates impaired kidney function 1
- A low estimated glomerular filtration rate (eGFR) calculated from serum creatinine indicates renal impairment - values below 60 ml/min/1.73 m² indicate chronic kidney disease stage 3 1
- Increased (not decreased) GFR is not associated with renal impairment; in fact, decreased GFR is the hallmark of kidney dysfunction 1
Blood Urea Nitrogen (BUN)
- BUN greater than 25 mg/dl is an indicator of renal impairment 2, 3
- Elevated BUN reflects reduced kidney function and has been shown to correlate with reduced renal blood flow 1, 2
- BUN elevation is associated with increased mortality in patients with cardiovascular conditions, independent of serum creatinine-based estimates of GFR 2
- In critically ill patients, BUN levels greater than 40 mg/dL are consistently associated with reduced creatinine clearance 3
Proteinuria
- Protein in urine (proteinuria) is a significant marker of kidney damage and glomerular filtration barrier derangement 1
- Proteinuria indicates established renal parenchymatous damage and is associated with increased cardiovascular risk 1
- Microalbuminuria (low-grade albuminuria) predicts the development of overt diabetic nephropathy and cardiovascular events 1
- Urinary protein excretion is measured by protein-to-creatinine or albumin-to-creatinine ratios in spot urine samples 1
Important Clinical Considerations
- Negative ketones in urine is not an indicator of renal impairment 1
- Serum creatinine alone may underestimate renal dysfunction, particularly in elderly patients with reduced muscle mass 4
- Both formulae for estimating GFR (MDRD and Cockcroft-Gault) help detect mild impaired renal function even when serum creatinine values are still in the normal range 1
- Monitoring trends in renal function markers is often more informative than single measurements, particularly when evaluating medication effects 1
- Early detection of renal impairment is critical as it constitutes a potent predictor of future cardiovascular events and mortality 1