U and E blood test: Urea and Electrolytes

U and E, often written as U+E, U&E or U/E is a medical abbreviation that stands for Urea and Electrolytes, two commonly prescribed blood tests.

Table of Contents

What is U and E blood test prescribed for?

U and E blood test may be prescribed as part of a routine health check-up or to investigate other conditions: it is mainly used to evaluate kidney function or may be used to diagnose or evaluate other conditions such as dehydration, metabolic acidosis or alkalosis, electrolyte imbalances, lung disease.

Urea or Blood Urea Nitrogen (BUN)

The blood test for urea is also known as Blood Urea Nitrogen (BUN) test. It is mainly used to assess kidney function or to diagnose kidney disease and dehydration. BUN may also be prescribed to evaluate the effectiveness of therapies such as hemodialysis.

Urea is formed in the liver and represents the end product of protein metabolism. After being ingested, proteins are broken down by digestive enzymes and then enter the bloodstream as amino acids; the amino acids are then combined to form new proteins needed by the body. If proteins and amino acids are not needed, they are transformed into energy in the liver through metabolic processes that result in the production of urea, which goes back into the bloodstream and reaches the kidneys to be excreted from the body through urine.

BUN is often tested with creatinine for a better understanding of kidney function.

Electrolytes panel

The electrolyte panel includes four different values

Carbon dioxide (CO₂)
Chloride (Cl^–)
Potassium (K⁺)
Sodium (Na⁺)

Carbon dioxide is mainly used to indirectly measure the bicarbonate ion (HCO₃) and to roughly evaluate the acid-base balance. The levels of HCO₃are regulated by the kidneys.

Chloride is mainly used in conjunction with the other electrolytes to evaluate the acid-base balance and body hydration.

Potassium is the major positively charged ion inside the cells and is involved in nerve signaling and muscle contraction (heart included!). It is excreted by the kidneys without resorption hence it depends upon dietary intake.

Sodium is the major positively charged ion outside the cells and is involved in maintaining the water balance, regulating blood pressure and in nerve signaling and muscle contraction. It is excreted by the kidneys but may be resorbed.

Other electrolytes not included in the standard electrolyte panel but present in the body are:

Calcium (Ca²⁺)
Magnesium (Mg²⁺)
Phosphate (PO₄^3-)

U and E test results normal range

For Urea or BUN, the normal ranges are:

3-12 mg/dL in newborns
5-18 mg/dL in infants and children
10-20 mg/dL or 3.6-7.1 mmol/L (SI units) in adults and slightly higher in the elderlies.

For carbon dioxide (CO₂) the normal ranges are:

13-22 mEq/L in newborns
20-28 mEq/L in infants and children
23-30 mEq/L or 23-30 mmol/L (SI units) in adults and in the elderlies.

For chloride (Cl^–) the normal ranges are:

96-106 mEq/L in newborns
90-110 mEq/L in infants and children
98-106 mEq/L or 98-106 mmol/L (SI units) in adults and in the elderlies.

For potassium (K⁺) the normal ranges are:

3.9-5.9 mEq/L in newborns
4.1-5.3 mEq/L in infants
3.4-4.7 mEq/L in children
3.5-5.0 mEq/L or 3.5-5.0 mmol/L (SI units) in adults and in the elderlies.

For sodium (Na⁺) the normal ranges are:

134-144 mEq/L in newborns
134-150 mEq/L in infants
136-145 mEq/L in children
136-145 mEq/L or 136-145 mmol/L (SI units) in adults and in the elderlies.

The normal or reference ranges may vary slightly as they are determined by each lab according to their equipment, testing method and population used as reference for “normal values”.

Abnormal test results explained

To interpret abnormal test results, you need to look at the bigger picture. A single value in the low or high range without other information can’t be used to diagnose anything (as a general rule, but there are some exceptions).

Simple things like drinking more or less than usual, exercising, sweating, diet, medications, may cause changes in the test results. Pregnancy also causes changes in many test results. It is always advised to have your GP or prescribing physician to evaluate your test results as they know why they were prescribed, they have access to your previous test results, know your anamnesis and has all the other information needed to draw conclusions.

Blood urea nitrogen (BUN) may be increased in renal failure (kidney disease both chronic and acute), urinary tract obstructions, dehydration, gastrointestinal bleeding, burns, shock, high-protein diets and when certain medications are used such as many antibiotics.

BUN may be decreased in hepatic failure (acute and chronic liver disease), low protein diets, overhydration and nephrotic syndrome.

Carbon dioxide (CO₂) may be increased in metabolic alkalosis, severe vomiting or diarrhea, emphysema, aldosteronism or with the use of some medications like aldosterone, hydrocortisone and some diuretics.

CO₂may be decreased in renal failure, diabetic ketoacidosis, metabolic acidosis, shock and with the use of some medications such as nitrofurantoin and thiazide diuretics.

Chloride (Cl^–) may be increased (hyperchloremia) in kidney dysfunctions, dehydration, hyperventilation, respiratory alkalosis, metabolic acidosis, hyperparathyroidism, excessive infusion of normal saline (NaCl 0.9% solution), Cushing syndrome and with the use of some medications such as androgens, estrogens and chlorotiazide.

Cl^– may be decreased (hypochloremia) in salt-losing nephritis, overhydration, respiratory acidosis, metabolic alkalosis, burns, Addison disease, congestive heart failure, vomiting and diarrhea, and with the use of some medications such as corticosteroids and diuretics.

Potassium (K⁺) may be increased (hyperkalemia) due to excessive dietary intake, kidney failure, crush syndrome, infections, acidosis, hypoaldosteronism and with the use of some medications such as antibiotics, potassium-sparing diuretics, lithium, antineoplastic drugs.

K⁺ may be decreased (hypokalemia) due to insufficient dietary intake, burns, hyperaldosteronism, Cushing syndrome, diarrhea and with the use of medications such as insulin, diuretics and laxatives.

Sodium (Na⁺) may be increased (hypernatremia) due to excessive dietary intake, Cushing syndrome, hyperaldosteronism, diabetes insipidus, excessive sweating, burns, vomiting and diarrhea, or due to use of some medications such as anabolic steroids, antibiotics and oral contraceptives.

Na⁺ may be decreased (hyponatremia) due to insufficient dietary intake, Addison disease, excessive water intake, kidney disease, congestive heart failure, vomiting or diarrhea, and with the use of some medications such as diuretics, tricyclic antidepressants, ACE inhibitors.

U and E critical values

Critical values are those representing a medical emergency which requires immediate attention due to life-threatening implications. U and E critical values are:

Urea (BUN) > 100 mg/dL
Carbon dioxide (CO₂) < 6 mEq/L
Chloride (Cl^–)< 80 or > 115 mEq/L
Potassium (K⁺) < 2.5 or > 6.5 mEq/L
Sodium (Na⁺) < 120 or > 160 mEq/L

Sources

Mosby’s Diagnostic and Laboratory Test Reference – 14^th Edition
KD Pagana – Elsevier, 2019
ISBN: 978-0-323-60969-2
https://www.elsevier.com/books/mosbys-diagnostic-and-laboratory-test-reference/pagana/978-0-323-60969-2
Guide to Diagnostic Tests – 7^th Edition
D Nicoll – McGraw-Hill Education, 2017
ISBN: 978-1-25-964090-2
Acute kidney injury.
R Bellomo – The Lancet, Aug 2012
DOI: https://doi.org/10.1016/S0140-6736(11)61454-2
Urea.
HK Wang – Sub-Cellular Biochemistry, Oct 2014
DOI: https://doi.org/10.1007/978-94-017-9343-8_2

Richard Troy

Medical Blogger committed to accuracy, I am passionate about delivering my readers critical insights into the medical field. Former paramedic, instructor and medical school student. You can read more about me in the dedicated section of the website.