What Is Urine – And What Is In It?
May 5, 2026
What Is Urine – And What Is In It? Urine is liquid waste produced by the kidneys to remove excess fluids and toxins from the blood. Function: The primary function of urine is to remove waste and...
Written by Dr. Andrew Stein MD, Consultant Nephrologist (UHCW Coventry). Last updated: July 2026
A Urea and Electrolytes (‘U&E’) test is one of the most common blood screening tests used in modern medicine.
It provides a ‘snapshot’ of your renal (kidney) function, your body’s hydration levels, and the delicate balance of essential minerals.
The test is divided into three primary categories: electrolytes, waste products, and occasionally, bone minerals.
1. Key Electrolytes (‘Es’)
Electrolytes are minerals that carry an electric charge and are vital for muscle and nerve function.
Sodium (Na): Regulates water balance and blood pressure.
Potassium (K): Critical for heart rhythm and muscle contraction.
Chloride (Cl): Helps maintain proper blood volume and pH.
Bicarbonate (HCO3): Acts as a buffer (alkali) to keep your blood from becoming too acidic or alkaline.
2. Metabolic Waste Products
These are substances the kidneys should naturally filter out of the blood.
Creatinine (‘creat’): A waste product from muscle breakdown. It is the most reliable marker of kidney function – the higher the number, the better is the kidney function.
Urea (‘U’): A byproduct of protein breakdown. A marker of kidney function. High levels can also indicate dehydration or heart failure. It is less reliable than creatinine.
3. Bone Minerals & Biochemistry
While often requested as a separate ‘Bone Profile,’ these are frequently tested alongside U&Es:
Calcium (Ca) and Phosphate (PO4): Vital for bone density and cellular energy.
Magnesium and Vitamin D: Often added to check for underlying deficiencies.
Doctors order this test to monitor several critical health factors:
Kidney Health: High creatinine or urea is caused by Acute Kidney Injury (AKI) or Chronic Kidney Disease (CKD).
Heart Safety: Abnormal potassium levels (too high or too low) can lead to dangerous cardiac arrhythmias (irregular heartbeats), or muscle weakness.
Hydration & Fluid Balance: Helps identify fluid overload (oedema) or severe dehydration.
Neurological Function: Imbalances in sodium can lead to confusion, delirium, seizures, or even coma.
Medication Monitoring: Many heart and blood pressure medications (like ACE inhibitors, ARBs, SGLT2is or diuretics) require regular U&E checks to ensure the dosage isn’t harming the kidneys.
It is rare for a U&E test to be ordered alone. You will often see it paired with:
Top Tip: In a clinical setting, you may hear a doctor ask for “FBC, U&Es, Gluc, LFTs and Bone.” This provides a comprehensive overview of your metabolic and haematologic health.
The U&E blood test is a foundational tool for assessing how well your kidneys are filtering waste and whether your body’s chemistry is in balance.
Whether you are monitoring a chronic condition like CKD or investigating a new symptom like fatigue, these results are key to your diagnosis.
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