Kidney Function Tests (KFT)

1. Urine-Based Tests

Urine Specific Gravity

• Normal: 1.010-1.025
• With diminishing renal function, concentrating ability is progressively lost until specific gravity fixes at 1.006-1.010 (isosthenuria)
• Diluting ability is maintained longer; even in uremia, dilution to 1.002-1.004 may persist
• Urine osmolality is a more precise measure but specific gravity is convenient for office use

Urine Protein / Albumin

• Normal proteinuria: < 150 mg/day (mainly Tamm-Horsfall protein)
• Urine dipstick is most sensitive to albumin; less sensitive to other proteins
• Microalbumin assay detects as little as 0.3 mg/dL - a sensitive early marker of glomerular dysfunction
• Urine β2-microglobulin and lysozyme detect tubular dysfunction
• Significant proteinuria is quantified by 24-hour urine collection

2. Blood/Serum Tests

Serum Creatinine

• Creatinine is the end product of creatine metabolism in skeletal muscle, excreted by the kidneys
• Normal: 0.8-1.2 mg/dL (adults); 0.4-0.8 mg/dL (young children)
• Serum creatinine stays within normal range until approximately 50% of renal function is lost
• Not significantly influenced by dietary intake (unlike BUN)
• Measured classically by the Jaffe reaction (alkaline picrate forms colored complex)
  • Falsely elevated by: cephalosporins, ketones, glucose, fructose, protein, urea, ascorbic acid

Blood Urea Nitrogen (BUN)

• Urea is the primary metabolite of protein catabolism, excreted entirely by the kidneys
• BUN is influenced by: dietary protein intake, hydration status, GI bleeding
• Approximately two-thirds of renal function must be lost before BUN significantly rises
• An increase in BUN = azotemia; high BUN with toxic effects = uremia
• Normal: approximately 7-20 mg/dL

BUN:Creatinine Ratio

• Normal: ~10:1
• Elevated (20:1 to 40:1): prerenal azotemia (dehydration, bilateral obstruction, urinary extravasation)
• Low or normal: intrarenal disease, hepatic insufficiency, overhydration

3. Glomerular Filtration Rate (GFR)

Endogenous Creatinine Clearance

• Most accurate standard measure without exogenous infusion
• Uses 24-hour urine collection + concurrent serum specimen
• Normal: 90-110 mL/min (70-140 mL/min after correction for body surface area)
• Formula: ClCr = (UCr × V) / (PCr × time)

Estimated GFR (eGFR) Equations

4. Cystatin C

• Produced by nearly all nucleated cells; freely filtered by glomerulus and completely reabsorbed by the proximal tubule
• Superior to creatinine as a predictor of cardiovascular mortality
• An early indicator of evolving chronic kidney disease
• Increasingly used to calculate eGFR (independent of muscle mass)

5. Differentiating Acute Renal Failure (ARF) Types

6. CKD Screening Recommendations

• eGFR
• Microalbuminuria test

• GFR < 60 mL/min/1.73 m² or
• Albuminuria persisting for 3 or more consecutive months