What labs are used to monitor renal impairment?

Laboratory Monitoring for Renal Impairment

The essential labs to monitor renal impairment are serum creatinine with calculated estimated glomerular filtration rate (eGFR) and urine albumin-to-creatinine ratio (or protein-to-creatinine ratio), measured at intervals determined by the stage of kidney disease and clinical context. 1

Core Laboratory Tests

Serum Markers of Filtration Function

• Serum creatinine alone should never be used as the sole assessment of kidney function because it remains within normal range until GFR declines to approximately half of normal, and is affected by muscle mass, age, and creatinine generation 1

• Estimated GFR (eGFR) calculated from serum creatinine using prediction equations (MDRD or CKD-EPI) is the recommended method for assessing kidney function, incorporating age, sex, race, and body size 1, 2

• Blood urea nitrogen (BUN) provides additional information about nitrogen retention and volume status, though it is less specific than creatinine for renal function 1

Urine Markers of Kidney Damage

• Urine albumin-to-creatinine ratio on untimed (spot) urine samples is the preferred method for detecting and monitoring proteinuria, with a cutoff of >30 mg/g indicating abnormal albumin excretion 1

• Total protein-to-creatinine ratio should be used when albumin-to-creatinine ratio is high (>500-1000 mg/g) to better quantify severe proteinuria 1

• Urinalysis with microscopy is essential for detecting cellular casts (especially tubular epithelial casts suggesting acute tubular necrosis, or red blood cell casts indicating glomerular disease), crystals, and cells 1, 3

• 24-hour urine collections are not necessary and provide less accurate GFR estimates than prediction equations based on serum creatinine 1

Electrolytes and Metabolic Parameters

• Serum electrolytes including sodium, potassium, chloride, calcium, phosphorus, and magnesium should be monitored to detect complications of renal impairment 1, 3

• Serum bicarbonate (total CO2) declines progressively with worsening renal function and indicates metabolic acidosis 1

• Serum potassium requires particular attention as hyperkalemia is a life-threatening complication, especially when using ACE inhibitors or ARBs 2

Monitoring Frequency Based on Disease Stage

Chronic Kidney Disease (Stable Patients)

• Annual monitoring for patients with eGFR >60 mL/min/1.73m² who are at high risk (African American, diabetes, hypertension, hepatitis C, HIV with low CD4 or high viral load) 1, 2

• Twice yearly (every 6 months) monitoring for patients with eGFR 30-60 mL/min/1.73m² (Stage 3 CKD) to detect progression 1, 2

• More frequent monitoring (every 3-4 months) for patients with eGFR <30 mL/min/1.73m² (Stage 4-5 CKD) due to higher risk of complications 1

Acute Kidney Injury or Rapidly Changing Function

• Serial serum creatinine measurements (daily or more frequently) are required to detect rapid deterioration, as eGFR equations are only valid for stable or slowly declining function 1

• Fractional excretion of sodium (FENa) calculated from simultaneous urine and serum sodium and creatinine helps differentiate prerenal (<1%) from intrinsic renal (>1%) causes of acute injury 3

After Medication Changes

• Check renal function and electrolytes within 2-4 weeks after initiating or increasing doses of ACE inhibitors, ARBs, or diuretics to detect acute changes 2

• A transient eGFR reduction up to 25% after starting ACE inhibitors or ARBs is acceptable and does not require discontinuation unless severe 2

Special Circumstances Requiring Direct GFR Measurement

Prediction equations may be inaccurate and clearance methods should be considered in: 1

• Extremes of age (<18 or >70 years)
• Extremes of body size (severe malnutrition, obesity, or unusual body habitus)
• Skeletal muscle diseases, paraplegia, or quadriplegia
• Vegetarian diet
• Rapidly changing kidney function
• Dosing of potentially toxic drugs excreted by kidneys

Common Pitfalls to Avoid

• Do not rely on serum creatinine concentration alone as elderly patients may have significantly reduced GFR despite "normal" creatinine due to decreased muscle mass 1

• Do not measure creatinine clearance from 24-hour urine collections without cimetidine as this overestimates GFR (CCr/GFR ratio up to 1.7) due to tubular creatinine secretion and is less reliable than eGFR from serum creatinine 1, 4

• Ensure laboratory calibration of creatinine assays to international standards as differences between laboratories can cause up to 20% error in eGFR estimates 1

• Use eGFR for chronic monitoring but serum creatinine for acute changes since eGFR equations were developed for stable renal function 1

• In patients with diabetes or at high risk, use albumin-specific testing rather than standard dipstick which may miss microalbuminuria 1