Lecture Outlines | Course Schedule | Main Menu | Resources

Pg 90 – 109  Chapter 4

Cover
Blood Urea Nitrogen (BUN)
Creatinine Levels in Serum
BUN to Creatinine Ratio
Serum and Urine Osmolality
Uric Acid (Serum and Urine)

BUN
Major nitrogenous end product of PROTEIN and AMINO ACID catabolism
Produced by LIVER and distributed throughout intracellular and extracellular fluid
Urea nitrogen is excreted from the body primarily by the KIDNEYS and a little by sweat or intestinal bacteria.
KIDNEYS – almost all urea is filtered out of blood by glomerular function. Some urea reabsorbed with water most most is removed in urine.
Amount of urea excreted depends on hydration of patient
     If dehydrated then low tubular flow so more urinary filtrate so more urea absorbed so high serum level.
     If overhydration occurs then high tubular flow rate and less is reabsorbed so low serum level.
Also can rise from renal and non-renal factors
     Increased with high dietary protein intake or increased catabolism (corticosteriods therapy or muscle wasting disease)

Elevated BUN
Can be prerenal, intrarenal or postrenal problem
Prerenal = Poor renal blood flow like in shock or renal stenosis. Impairment of perfusion slows filtration rate.
Intrarenal = Damage to renal parenchyma.
Postrenal = Obstruction in kidney or urinary tract which increase the tubular reabsorption of urea
BUN is used to evaluate renal function. With serum creatinine it is used to monitor patients in renal failure.

Creatinine in Serum
Creatinine is an amino aide and waste product of protein metabolism. Derived from creatine and synthesized in liver, kidneys and pancreas and stored in muscle tissue.
Creatinine is released into the extracellular fluid and excreted through the kidneys
Creatinine is filtered by the glomeruli and NOT reabsorbed
When kidneys are working properly, serum creatinine level is low but with renal function impaired – Creatinine level increases
If half of nephrons are damaged then serum creatinine level rises to about double

Normal creatinine level:
Men = 0.6 –1.5 mg/dL
Women = 0.6-1.1 mg/dL
Pregnancy = reduced siChildren = 0.2 – 1.0 mg/dLnce creatinine clearance is increased

Unlike BUN, it is not affected by protein metabolism and minimally affected by hydration.

Nursing indications:
Red-topped tube – 10 ml venous blood
Venipuncture must be smooth with even blood flow or excessive turbulence can cause abnormal false measurements
Elevated values: acute & chronic renal failure, uremia, renal artery stenosis, CHF, shock, rhabdomyolysis, acromegaly
Decreased values: advanced liver disease, long-term corticosteriod therapy, hyperthyroidism, muscular dystrophy, paralysis.
Instruct patient to fast for 8 hrs before test. Ingestion of meat can cause a false elevated result.
Prolong delay of specimen to laboratory can cause ammonia to form and warming can cause falsely elevated results.
For infants and small children, a heelstick puncture is used to fill a capillary pipette
Creatinine rises and falls more slowly than BUN levels it is often preferred method for long-term assessment of renal function

Creatinine Clearance Test

The total amount of creatinine excreted in urine in a 24 hour period is called creatinine clearance.During renal failure, diminished glomerular filtration occurs thus increasing the secretion of creatinine. In chronic renal failure and uremia becomes very severe, an eventual reduction occurs in the excretion of creatinine by both the glomeruli and the tubules.
Decreases by 10% per decade after age 40 years whereas serum creatinine shows little variation.
**Used to assess renal function and creatinine excretion. Used to monitor the progression of renal disease
Elevated values = muscular dystrophy paralysis, anemia, leukemia hyperthyroidism
Decreased values = glomerulonephritis, CHF, acute tubular necrosis, shock , polycystic kidney disease, dehydration

Nursing indications:
Can be shorten to 4 or 12 hours but best is 24 hrs. Needs to be refrigerated or on ice.Instruct patient to avoid excessive intake of meat before the test, encourage adequate hydration before and urine test and omit coffee and tea
Usually instruct patient to void at 8:00 am then all subsequent urine specimens are collected for 24 hrs.
No vigorous exercise.
Label container with proper time and date.

Normal = Male: 1-2 g/day, female: 0.8-1.8 g/day

BUN to Creatinine Ratio

Creatinine is changed ONLY by renal dysfunction thus a comparison of BUN with serum creatinine is helpful.
Normal ratio is 10-15:1 but will vary based on protein intake and muscle mass.
Increase ratio if patient is dehydrated and creatinine does NOT change (25:1)
Decrease ratio in low protein diet, overhydration or severe liver disease (8:1)
Both measures are also useful for monitoring nephrotoxic drugs (i.e. gentamicin, tobramycin)

Serum Osmolality

Osmolality is a measure of the number of particles dissolved in a solution.In blood osmolality is created by protein, glucose, chloride, sodium, bicarbonate and urea dissolved in the plasma. Osmolality is affected by increases or decreases in fluid volume or by an increase or decrease in blood particles.
** Used to assess the patient’s FLUID status and Identify any ADH abnormalities.

Normal: Adults: 285-298 mOsm/kg

Increased values = alcoholism, aldosteronism, diabetes insipidus, high protein diet, dehydration, hypercalcemia, hyperglycemia, hypernatremia & hyperkalemia
Decreased values = fluid overload, hypernatremia, liver failure with ascites, Addison's disease
4 interfering factors
     1.  Medications
     2.  Diuretics
     3.  Hemolysis of specimen
     4.  Mineralocorticoids

Urine Osmolality

Collected from a 24-hour urine specimen or 10 ml sample,  Normal: 500-800 mOsm/kg H₂O. Urine osmolality varies based on the patient’s fluid status and metabolic waste products being excreted.
     Overhydrated = Urinary osmolality decreases as output increases
     Dehydrated = Urine osmolality increases as the output decreases.
Urine osmolality is based on concentration ability of kidneys and serum levels of protein, urea, glucose, sodium, bicarbonate and chloride.
Purpose = Assess the ability of kidneys to dilute or concentrate urine and identify ADH abnormalities
Increased values = dehydration, Addison's disease, diabetes mellitus, diarrhea, hyperglycemia, hypernatremia cirrhosis,
Decreased values = overhydration, hyponatremia, hypocalcemia, aldosteronism, diabetes insipidus

Serum Uric Acid

Uric acid is end product of protein metabolism and excreted by kidneys and bowels. Normally 2/3 of uric acid is excreted by the kidneys and the other 1/3 by in bile and intestinal secretions.Temporary increase in serum uric acid from ingestion of food high in purine (meat, fish), strenuous exercise, or heavy alcohol ingestion will usually return to normal within 1 day.

Normal value: Male  3.6-8.5 mg/dL, female = 2.3 – 6.6 mg/dL

Purpose of test is to confirm the diagnosis of gout and helps detect renal impairment that causes prerenal azotemia and renal failure
4 interfering factors: starvation, caffeine, vitamin C ingestion, high purine diet
Elevated values = gout, shock, polycystic kidney disease, renal failure, diabetic ketoacidosis, leukemia, lead poisoning, polycythemia vera, acute alcohol ingestion, psoriasis, pernicious anemia, toxemia of pregnancy
Decreased values = Hodgkin’s disease, multiple myeloma

Urine Uric Acid

End product of protein metabolism, uric acid and urate crystals excreted by kidneys.
Purpose of test: Urinary excretion of uric acid in patients with renal calculi or those at risk for development of calculus. Also used to assess the effect of enzyme deficiency or metabolic abnormality that results in the overproduction of uric acid.

Normal:  250-750 mg/24-hr specimen

Review foods high in purines – Table 4-5 on pg. 105

Elevated values = gout, viral hepatitis, leukemia, Crohn’s disease, polycythemia vera
Decreased values = chronic glomerulonephritis, collagen disease, lead toxicity
Interfering factors: high or low purine diet, many medications (aspirin, antiinflammatory drugs, diuretics, vitamin C), failure to collect all urine or failure to store properly
Specimen must be refrigerated or on ice, a list of all medications taken  by the patient should be on the requisition slip.