Blood urea nitrogen

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Overview

The blood urea nitrogen (BUN) test is a measure of the amount of nitrogen in the blood that comes from urea. Urea is a substance secreted by the liver, and removed from the blood by the kidneys.

Physiology

The liver produces urea in the urea cycle as a waste product of the digestion of protein. Normal human adult blood should contain between 7 and 25 mg of urea nitrogen per 100 ml (7-25 mg/dL) of blood. Individual laboratories may have different reference ranges, and this is because the procedure may vary.

Interpretation

The most common cause of an elevated BUN, azotemia, is poor kidney function, although a serum creatinine level is a somewhat more specific measure of renal function.

A greatly elevated BUN (>60 mg/dl) generally indicates a moderate-to-severe degree of renal failure. Impaired renal excretion of urea may be due to temporary conditions such as dehydration or shock, or may be due to either acute or chronic disease of the kidneys themselves.

Elevated BUN in the setting of a relatively normal creatinine may reflect a physiological response to a relative decrease of blood flow to the kidney (as seen in heart failure or dehydration) without indicating any true injury to the kidney. However, an isolated elevation of BUN may also reflect excessive formation of urea without any compromise to the kidneys.

Increased production of urea is seen in cases of moderate or heavy bleeding in the upper gastrointestinal tract (e.g. from ulcers). The nitrogenous compounds from the blood are resorbed as they pass through the rest of the GI tract and then broken down to urea by the liver. Enhanced metabolism of proteins will also increase urea production, as may be seen with high protein diets, steroid use, burns, or fevers.

A low BUN usually has little significance, but its causes include liver problems, malnutrition (insufficient dietary protein), or excessive alcohol consumption. Overhydration from intravenous fluids can result in a low BUN. Normal changes in renal bloodflow during pregnancy will also lower BUN.

Urea itself is not toxic. This was demonstrated by Johnson et al. by adding large amounts of urea to the dialysate of hemodialysis patients for several months and finding no ill effects.[2]. However, BUN is a marker for other nitrogenous waste. Thus, when renal failure leads to a buildup of urea and other nitrogenous wastes (uremia), an individual may suffer neurological disturbances such as altered cognitive function (encephalopathy), impaired taste (dysgeusia) or loss of appetite (anorexia). The individual may also suffer from nausea and vomiting, or bleeding from dysfunctional platelets. Prolonged periods of severe uremia may result in the skin taking on a grey discolouration or even forming frank urea crystals ("uremic frost") on the skin.

Because multiple variables can interfere with the interpretation of a BUN value, GFR and creatinine clearance are more accurate markers of kidney function. Age, sex, and weight will alter the "normal" range for each individual, including race. In renal failure or chronic kidney disease (CKD), BUN will only be elevated outside "normal" when more than 60% of kidney cells are no longer functioning. Hence, more accurate measures of renal function are generally preferred to assess the clearance for purposes of medication dosing.

Units

BUN is reported as mg/dL in the United States. Elsewhere, the concentration of urea is reported as mmol/L. To convert from mg/dL of blood urea nitrogen to mmol/L of urea, one multiplies by 0.357

(10 dL/1 L)/(28 mg of nitrogen/mmol of urea) = 0.357

Methodology

The test as originally carried out was by flame photometry; now chemical colorimetric tests are more widely used. Three methods are shown below: Diacetyl Monoxime, Urograph and Modified Berthelot Enzymatic methods:

  1. DIACETYL MONOXIME Method
    • This method utilizes the Fearon Reaction, wherein urea in hot solution of diacetyl monoxime condenses to form diazine derivative.
      Reagents to use
      • BUN Std (usually 15 mg/dl);
      • Serum or Plasma (Patient's Sample);
      • Distilled Water;
      • Acid Reagent (Diacetyl monoxime);
      • Color Reagent (Ferrithiocyanate).
      Equipment needed
      • Test tubes;
      • Pipettes;
      • Boiling water bath;
      • Iced cold water bath;
      • Cuvet;
      • Spectrophotometer.
      Procedure
      1. Prepare 3 to 10 mL test tubes
        • Blank - 0.05 mL Distilled water
        • Standard - 0.05 mL Std Solution
        • Test - 0.05 mL Serum or Plasma
      2. To all tubes add 3.0 mL of Acid Reagent and mix by lateral tapping;
      3. Then add Color reagent and mix well again;
      4. Place all tubes into an already boiling water bath for 15 minutes;
      5. Cool in iced water bath for 2 minutes;
      6. Let stand at room temperature for 1 minute;
      7. Read against Blank at 520 nm in Spectrophotometer
      Interpolation Formula
      • 0.357 is the Conversion Factor for BUN to mmol/L
      Normal Value
      2.86 to 7.14 mmol/L (or 8 to 20 mg/dL)
  2. UROGRAPH method
    • This method utilizes Physical and Chemical means via Paper Chromatography and Conway Microdiffusion respectively. Conway Microdiffusion involves the hydrolysis of Urea with buffered Urease, the liberation of Ammonia gas and reaction with indicator system.
      Reagents to use
      • Urastrat strip;
      • Serum or Plasma.
      Equipment needed
      • Test tubes w/ Test Tube Rack;
      • Pipet;
      • Ruler or Caliper.
      Procedure
      1. Place 0.2 mL of Serum or Plasma at the bottom of a 5.0 mL test tube;
      2. Place Urastrat strip in the test tube with the lower portion touching the serum;
      3. Place the tube in a rack in straight position and let stand for 30 minutes;
      4. Read the blue colored band produced using the millimeter graduation of a ruler or a caliper.
      Compute as
      • 0.357 is the Conversion Factor for BUN to mmol/L
  3. Modified Berthelot enzymatic method
    • This method utilizes the principle of Urea Hydrolysis. Urea is hydrolyzed in the presence of Urease to Carbon Dioxide and Ammonium Ions.
      Reagents to use
      • Patient's Serum;
      • Urea N Standard (25 mg/dL);
      • Urea N-Zyme reagent (Buffered Urease);
      • Urea N Color reagent (Sodium Salicylates and Sodium Nitroferricyanide);
      • Urea N Base reagent (NaOH and Sodium Hypochlorite).
      Equipment needed
      • Test tubes w/ Test Tube Rack;
      • Pipettes;
      • Cuvet;
      • Spectrophotometer;
      • 37°C water bath.
      Procedure
      1. Prepare 3 to 10 mL test tubes labeled as Blank, Test and Standard;
      2. Place 0.5 mL Urea N Color reagent to all tubes and add sample as follows:
        • Blank - 0.5 mL Distilled water
        • Standard - 0.5 mL Std Solution
        • Test - 0.5 mL Serum or Plasma
      3. Mix by gentle swirling and pre-warm at 37°C water bath for 5 minutes;
      4. Add 2.0 mL Urea N Base Reagent to all tubes, mix and return to 37°C water bath for 5 minutes;
      5. Read at 630 nm against Reagent Blank in Spectrophotometer.
      Compute as
      • 0.357 is the Conversion Factor for BUN to mmol/L
      Normal value
      8 to 26 mg/dL

See also

Reference

  1. ^ Johnson WJ, Hagge WW, Wagoner RD, Dinapoli RP, Rosevear JW. Effects of urea loading in patients with far-advanced renal failure. Mayo Clin Proc. 1972 Jan;47(1):21-9. PMID 5008253


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