Kidney Function Test (KFT) - 10 Markers

1. Serum Urea (Blood Urea Nitrogen - BUN)

• What it measures: End-product of protein catabolism, excreted by glomerular filtration
• Normal range: 7-20 mg/dL (BUN); 15-40 mg/dL (serum urea)
• Interpretation: Rises when GFR falls, but is also affected by non-renal factors - high protein diet, dehydration, GI bleeding, fever, and liver disease (decreases in hepatic failure)
• Limitation: Not as specific as creatinine for renal function; wide reference range due to dietary protein influence
• Brenner & Rector's The Kidney - Normal range of urea nitrogen in blood/serum is 5-20 mg/dL (1.8-7.2 mmol urea/L)

2. Serum Creatinine

• What it measures: End-product of creatine/phosphocreatine metabolism in muscle; freely filtered at the glomerulus
• Normal range: Males: 0.7-1.2 mg/dL; Females: 0.5-1.0 mg/dL
• Interpretation: Inversely related to GFR - halving of GFR roughly doubles serum creatinine. A better and more specific marker than urea, as it is less affected by diet
• Key limitation: Serum creatinine remains within normal range until ~50% of GFR is already lost - poor early sensitivity. Affected by muscle mass, age, sex, and race
• Harper's Illustrated Biochemistry - "Creatinine is a better marker of renal function than urea because its blood concentration is not significantly affected by non-renal factors"

3. Blood Urea Nitrogen : Creatinine Ratio (BUN:Cr)

• Normal ratio: 10:1 to 20:1
• Clinical use:
  • BUN:Cr >20 → Pre-renal azotemia (dehydration, reduced perfusion)
  • BUN:Cr <10 → Low protein intake, liver disease, rhabdomyolysis
  • BUN:Cr 10-20 → Intrinsic renal disease or post-renal cause

4. Glomerular Filtration Rate (GFR) / eGFR

• What it measures: Volume of plasma filtered by the glomerulus per minute; the single best overall index of renal function
• Normal: ~90-120 mL/min/1.73 m² (decreases with age)
• Estimation methods:
  • Creatinine clearance (urine + serum creatinine over 24 h): Formula = (U × V) / P
  • CKD-EPI equation (preferred) - uses age, sex, race, serum creatinine ± cystatin C
  • MDRD equation - older, less accurate
  • Gold standard: Inulin clearance (freely filtered, not secreted/reabsorbed - but impractical clinically)
• CKD staging by GFR: G1 (≥90), G2 (60-89), G3a/b (30-59), G4 (15-29), G5 (<15, kidney failure)
• Tietz Laboratory Medicine - "Serum creatinine can remain within the reference interval until notable kidney function has been lost"

5. Cystatin C

• What it measures: Low-molecular-weight protein produced at a constant rate by all nucleated cells; freely filtered at the glomerulus, not secreted, fully catabolized in tubules
• Advantage over creatinine: Not affected by muscle mass, age, sex, or diet - better endogenous GFR marker
• Use: eGFR-cystatin C (CKD-EPI_cys) or combined CKD-EPI_creat-cys equation improves GFR estimation, especially in patients with abnormal muscle mass
• Normal: ~0.5-1.0 mg/L (varies by assay)
• Tietz Laboratory Medicine - KDIGO recommends CKD-EPI cystatin C equation for standardized results

6. Urine Routine & Microscopy (Urinalysis)

• Harper's Illustrated Biochemistry - "Physical characteristics to be assessed include urine volume, odor, color, appearance, specific gravity, and pH"

7. Urine Protein (Proteinuria)

• Normal: <150 mg protein / 24 hours; <30 mg albumin / 24 hours
• Proteinuria: >150 mg/24 h - sign of renal disease
• Categories:
  • Glomerular proteinuria: Albumin predominates (nephrotic syndrome, diabetic nephropathy)
  • Tubular proteinuria: Low-molecular-weight proteins (Fanconi syndrome)
  • Overflow proteinuria: Bence-Jones (myeloma)
• Microalbuminuria: 30-300 mg albumin/24 h - earliest detectable marker of diabetic nephropathy
• Macroalbuminuria (overt): >300 mg/24 h
• Harper's Illustrated Biochemistry - "Microalbuminuria is an early marker of renal damage in diabetes mellitus"

8. Urine Specific Gravity & Osmolality (Concentrating Ability)

• Specific gravity normal: 1.003-1.030
• Osmolality: Urine can be concentrated to 800-1400 mOsm/kg; plasma ~285-295 mOsm/kg
• Interpretation:
  • Fixed specific gravity (~1.010) in all samples = isosthenuria (severe tubular damage)
  • Inability to concentrate urine = tubular dysfunction (e.g., nephrogenic diabetes insipidus)
• Water deprivation test: Used to distinguish nephrogenic from central DI; failure to concentrate urine reflects tubular disease

9. Serum Electrolytes (Na⁺, K⁺, HCO₃⁻, Cl⁻)

• Why included in KFT:
  • Sodium: Deranged in renal failure (hyponatremia/hypernatremia)
  • Potassium: Hyperkalemia is a life-threatening complication of renal failure (reduced excretion)
  • Bicarbonate: Low in CKD due to metabolic acidosis (reduced ammoniagenesis + H⁺ secretion)
  • Chloride: Reflects acid-base status
• Anion gap: Elevated in uremic acidosis (accumulation of phosphate, sulfate, organic acids)
• Normal K⁺: 3.5-5.0 mEq/L; Na⁺: 135-145 mEq/L; HCO₃⁻: 22-28 mEq/L

10. Serum Uric Acid

• Normal range: Males: 3.5-7.2 mg/dL; Females: 2.6-6.0 mg/dL
• Relevance to kidney function:
  • 70% of uric acid is excreted by the kidneys; rises in renal failure (hyperuricemia)
  • Uric acid nephropathy can itself cause renal failure (tumor lysis syndrome, gout)
  • Hyperuricemia is an independent risk factor for CKD progression
• Also used as: A marker of pre-eclampsia severity and acute kidney injury

Summary Table

• Harper's Illustrated Biochemistry 32nd Ed, Tests of Kidney Function
• Tietz Textbook of Laboratory Medicine 7th Ed, Creatinine & GFR markers
• Brenner & Rector's The Kidney, Blood Urea Nitrogen & Evaluation of Kidney Disease