Kidney Function: Blood Urea

• Kidney function tests include assessments of blood urea and creatinine levels.

Blood Urea

• Urea is the primary waste product of protein breakdown.
• The liver synthesizes urea, which is then transported through the blood to the kidneys for filtration via the glomerulus.
• Plasma urea measurement as a renal function test is less effective compared to plasma creatinine measurement.
• Several factors, such as hydration status and protein intake affect urea levels while plasma creatinine levels are not.
• Urea is hydrolyzed by the urease enzyme to produce ammonia (NH3) and carbon dioxide (CO2).
• Free ammonia in an alkaline medium with an indicator forms a green-colored complex that is proportional to the urea concentration, and is measured at 578 nm.
• To convert urea to blood urea nitrogen (BUN), multiply the result by 0.46.
• For urine urea estimation, predilute the urine specimen 1:50 with NH3-free water.

Serum Urea Reference Values

• Adults less than 65 years: 15 – 50 mg/dL
• Adults more than 65 years: > 70 mg/dL

Serum BUN Reference Values

• Adults less than 65 years: 7 – 23.5 mg/dL
• Adults more than 65 years 7 – 33 mg/dL
• Children: 5 – 18 mg/dL

Urine (24 hours) Reference Values

• Urea: 20 – 35 g/24hr
• BUN: 9.3 – 16.5 g/24hr

Clinical Significance of Increased Plasma Urea

• Impaired kidney function.
• Decreased blood flow to the kidneys, caused by congestive heart failure, shock, stress, severe burns, obstruction of urine flow, or dehydration.
• Excessive protein breakdown or high protein diet.

Clinical Significance of Decreased Plasma Urea

• Severe liver disease.
• Malnutrition.
• Overhydration.

Creatinine

• Creatinine is a non-protein nitrogenous waste product from creatine metabolism in skeletal muscle.
• It diffuses freely throughout body water, is filtered from extracellular fluid by the kidneys, and excreted in urine.
• Creatinine excretion is mainly renal and relatively constant in the absence of disease.
• Measurement of serum or plasma creatinine is a more accurate marker of kidney function than urea.
• Creatinine reacts with picric acid in an alkaline medium to form a greenish-yellow color, measured at 520 nm.

Creatinine Assay Formula

• Creatinine concentration (mg/dL) = A(test) / A(standard) x Conc.(standard)

Creatinine Reference Ranges

• Male: 0.7 – 1.4 mg/dL
• Female: 0.6 – 1.2 mg/dL
• Children: 0.3 – 0.7 mg/dL

Clinical Significance of Increased Serum Creatinine

• Renal failure and renal dysfunction.
• Obstruction of urine outflow.
• Acute renal failure due to sudden reduced blood flow to the kidney in cases of hemorrhage or surgical emergencies.

Clinical Significance of Decreased Serum Creatinine

• Diseases associated with muscle wasting reduce creatinine levels in the blood.

Creatinine Clearance

• Creatinine clearance measures the glomerular filtration rate (GFR).
• Creatinine clearance is detected by estimating plasma and urinary creatinine in a 24-hour urine specimen.
• Proper collection of the urine specimen is essential for the accuracy of the creatinine clearance test.

Creatinine Clearance Formula

• Creatinine clearance = (Urine creatinine x Urine volume (mL)/24hr) / (Serum creatinine x 1440)

Creatinine Clearance Reference Ranges

• Male: 95 – 130 mL/min
• Female: 80 – 120 mL/min (100 ± 20 mL/min)