Urea, Electrolytes (U&E’s) and Renal Function
A urea, electrolytes and renal function test (U&E) is one of the most commonly requested blood tests in healthcare. It provides valuable information about kidney function, fluid balance, electrolyte regulation and acid-base balance. U&E results can help identify dehydration, acute kidney injury, chronic kidney disease, electrolyte disturbances and many other conditions affecting the body's ability to maintain normal physiological function.
What You Need to Know
The kidneys perform several essential functions that help maintain homeostasis. They continuously filter the blood, removing waste products while regulating water, electrolytes and acid-base balance. Because these processes are closely linked, abnormalities in one component of a U&E are often accompanied by changes in others.
For example, a patient with dehydration may have elevated urea and creatinine, while also developing abnormalities with sodium and potassium levels. Likewise, kidney injury may impair the body's ability to regulate multiple electrolytes simultaneously. Looking at the overall pattern of results, rather than focusing on a single abnormal value, provides a much clearer understanding of what is occurring clinically.
Many factors can influence U&E results, including hydration status, medications, vomiting, diarrhoea, heart failure, liver disease, endocrine disorders and chronic kidney disease. Equally important is determining whether an abnormal result is new or represents a chronic condition that has remained stable over time.
Key points
U&E assesses kidney function, fluid balance, electrolyte balance and acid-base balance.
It can help identify dehydration, acute kidney injury, chronic kidney disease and electrolyte disturbances.
Electrolytes are essential for normal nerve conduction, muscle contraction and cardiac function.
Abnormal results do not always indicate kidney disease.
U&E should always be interpreted alongside the patient's clinical presentation and other investigations.
Understanding how each component relates to the others improves clinical interpretation.
Components of a U&E and Renal Function Test:
Sodium (Na⁺)
Potassium (K⁺)
Chloride (Cl⁻)
Bicarbonate (HCO₃⁻)
Urea
Creatinine
Estimated Glomerular Filtration Rate (eGFR)
Beyond the Basics
Sodium (Na⁺)
Reference range
135–145 mmol/L
Sodium is the body's primary extracellular electrolyte (it is more abundant in the blood than inside the cells) and plays a vital role in maintaining fluid balance, blood pressure, nerve impulse transmission and muscle contraction. Although sodium is often thought of as a measure of the body's sodium stores, it is more accurately an indicator of the body's water balance. Changes in sodium concentration usually occur because of changes in the amount of water relative to sodium, rather than changes in total body sodium.
Decreased sodium (hyponatraemia): A reduced sodium level may occur with excessive fluid intake, heart failure, liver disease, kidney disease, syndrome of inappropriate antidiuretic hormone (SIADH), vomiting, diarrhoea, diuretic therapy or adrenal insufficiency. Symptoms vary according to both the severity and how rapidly the sodium falls, ranging from nausea and confusion to seizures, coma and death in severe cases.
Elevated sodium (hypernatraemia): An increased sodium level most commonly occurs due to dehydration, inadequate water intake, excessive water loss from vomiting, diarrhoea or sweating, diabetes insipidus or osmotic diuresis. As sodium rises, water moves out of cells, causing cellular dehydration that may lead to thirst, confusion, irritability and altered conscious state.
Sodium abnormalities should always be interpreted alongside the patient's fluid status and clinical presentation. A low sodium level does not necessarily mean the body lacks sodium, just as a high sodium level does not necessarily indicate excess sodium. In many cases, the abnormality reflects an imbalance between sodium and water rather than a true sodium deficit or excess.
Potassium (K⁺)
Reference range
3.5–5.0 mmol/L
Potassium is the body's primary intracellular electrolyte (it is more abundant inside the cell than in the blood), with approximately 98% found inside cells. It plays a critical role in nerve impulse transmission, muscle contraction and maintaining normal cardiac electrical activity. Even small changes in potassium concentration can significantly affect cardiac conduction and increase the risk of life-threatening arrhythmias.
Decreased potassium (hypokalaemia): A reduced potassium level may occur due to vomiting, diarrhoea, diuretic therapy, excessive sweating, inadequate dietary intake or excessive insulin administration. Symptoms may include muscle weakness, fatigue, constipation and cardiac arrhythmias. Severe hypokalaemia may result in paralysis or respiratory muscle weakness.
Elevated potassium (hyperkalaemia): An increased potassium level commonly occurs with acute or chronic kidney disease, medications that reduce potassium excretion (such as ACE inhibitors or potassium-sparing diuretics), metabolic acidosis or extensive tissue breakdown. Severe hyperkalaemia is a medical emergency because it may cause life-threatening cardiac arrhythmias and cardiac arrest.
Potassium should always be interpreted promptly, particularly when significantly elevated or reduced. If a potassium result is unexpectedly high without clinical explanation, haemolysis of the blood sample should also be considered, as this can falsely elevate the result.
Chloride (Cl⁻)
Reference range
98–107 mmol/L
Chloride is the body's major extracellular negatively charged electrolyte (anion) and works closely with sodium to maintain fluid balance, osmotic pressure and acid-base balance. Although chloride abnormalities are often overlooked, they frequently provide valuable clues when interpreted alongside sodium and bicarbonate.
Decreased chloride (hypochloraemia): A reduced chloride level may occur with prolonged vomiting, gastric suction, diuretic therapy, metabolic alkalosis or excessive fluid administration. Symptoms are usually related to the underlying cause rather than the chloride abnormality itself.
Elevated chloride (hyperchloraemia): An increased chloride level commonly occurs with dehydration, excessive administration of normal saline, metabolic acidosis, diarrhoea or impaired kidney function.
Chloride should rarely be interpreted in isolation. It is most useful when assessed alongside sodium, bicarbonate and the patient's acid-base status.
Bicarbonate (HCO₃⁻)
Reference range
22–32 mmol/L
Bicarbonate is an important buffer that helps maintain the body's acid-base balance by neutralising excess acids. The kidneys regulate bicarbonate levels by reabsorbing or excreting bicarbonate as required, making it an important indicator of metabolic acid-base disorders.
Decreased bicarbonate: A reduced bicarbonate level commonly indicates metabolic acidosis and may occur with diabetic ketoacidosis, lactic acidosis, severe diarrhoea, kidney failure or sepsis. As bicarbonate falls, the body attempts to compensate by increasing respiratory rate to remove carbon dioxide.
Elevated bicarbonate: An increased bicarbonate level usually indicates metabolic alkalosis and may occur following prolonged vomiting, excessive diuretic use or chronic respiratory disease where the kidneys compensate for elevated carbon dioxide levels.
Bicarbonate should always be interpreted alongside the patient's respiratory status, blood gas results (if available) and other electrolytes, particularly chloride and potassium, to determine the underlying acid-base disturbance.
Urea
Reference range
2.5–8.0 mmol/L
Urea is a waste product produced in the liver during the breakdown of proteins and is excreted by the kidneys. While it is commonly used as a marker of kidney function, urea is also influenced by hydration status, dietary protein intake and liver function.
Decreased urea: A reduced urea level may occur with severe liver disease, low protein intake, malnutrition or overhydration. Low urea levels are generally less clinically significant than elevated levels.
Elevated urea: An increased urea level commonly occurs with dehydration, acute kidney injury, chronic kidney disease, gastrointestinal bleeding, high protein intake or increased protein breakdown. Urea often rises earlier than creatinine in dehydration because the kidneys reabsorb more urea when conserving water.
Urea should always be interpreted alongside creatinine and eGFR. An isolated rise in urea with relatively normal creatinine often suggests dehydration rather than intrinsic kidney disease.
Creatinine
Reference range
Male: 60–110 µmol/L
Female: 45–90 µmol/L
Creatinine is a waste product produced during normal muscle metabolism and is removed almost entirely by the kidneys. Because it is filtered with minimal reabsorption, creatinine is one of the most commonly used indicators of kidney function.
Decreased creatinine: A reduced creatinine level may occur in people with low muscle mass, older adults, pregnancy or severe malnutrition. Low creatinine is generally not clinically significant.
Elevated creatinine: An increased creatinine level commonly indicates reduced kidney filtration and may occur with acute kidney injury, chronic kidney disease, dehydration, urinary tract obstruction or medications that impair kidney function.
Creatinine should never be interpreted in isolation. Muscle mass, age and hydration status all influence creatinine levels, so results should always be considered alongside eGFR, urea and the patient's clinical presentation.
Estimated Glomerular Filtration Rate (eGFR)
Reference range
≥90 mL/min/1.73 m² (normal kidney function)
The estimated glomerular filtration rate (eGFR) estimates how effectively the kidneys filter waste products from the blood. It is calculated using serum creatinine together with age and sex and provides a more accurate assessment of kidney function than creatinine alone.
Decreased eGFR: A reduced eGFR indicates impaired kidney filtration and may occur with acute kidney injury or chronic kidney disease. As kidney function declines, waste products and electrolytes become increasingly difficult to regulate.
Elevated eGFR: An elevated eGFR is generally not clinically significant and may occur in pregnancy or during periods of increased kidney filtration. Laboratories commonly report values above normal simply as >90 mL/min/1.73 m² rather than providing an exact number.
eGFR should always be interpreted together with creatinine and the clinical context. A single reduced eGFR does not necessarily indicate chronic kidney disease, particularly if the patient has acute illness or dehydration. Persistent reductions over at least three months are generally required before chronic kidney disease is diagnosed.
In Practice
Understanding what each component of a U&E measures is only the first step. In clinical practice, blood results are interpreted alongside the patient's history, symptoms, observations and other investigations. The following scenarios demonstrate how abnormalities in a U&E contribute to clinical reasoning and help guide further assessment and management.
Scenario 1: Acute kidney injury secondary to dehydration
A 79-year-old woman is admitted from a residential aged care facility with vomiting and diarrhoea for three days. She appears lethargic, has dry mucous membranes and is hypotensive.
U&E
Sodium: 148 mmol/L
Potassium: 4.8 mmol/L
Chloride: 111 mmol/L
Bicarbonate: 23 mmol/L
Urea: 18.6 mmol/L
Creatinine: 186 µmol/L
eGFR: 24 mL/min/1.73 m²
Interpretation
The elevated sodium suggests dehydration, while the markedly elevated urea and creatinine with a reduced eGFR indicate acute kidney injury. In the context of significant fluid losses, these findings are most consistent with pre-renal acute kidney injury secondary to dehydration. Prompt fluid resuscitation and repeat blood tests are required to assess recovery.
Scenario 2: Hyperkalaemia in chronic kidney disease
A 68-year-old man with chronic kidney disease presents with increasing weakness and palpitations. He takes an ACE inhibitor for hypertension.
U&E
Sodium: 138 mmol/L
Potassium: 6.5 mmol/L
Chloride: 103 mmol/L
Bicarbonate: 18 mmol/L
Urea: 21.8 mmol/L
Creatinine: 318 µmol/L
eGFR: 16 mL/min/1.73 m²
Interpretation
This patient has severe hyperkalaemia with impaired renal function and metabolic acidosis. Reduced kidney function limits potassium excretion, allowing potassium to accumulate to potentially life-threatening levels. Hyperkalaemia is a medical emergency requiring immediate assessment, cardiac monitoring and treatment to reduce the risk of fatal arrhythmias.
Scenario 3: Hyponatraemia due to SIADH
A 74-year-old woman is admitted with increasing confusion and lethargy. She was recently diagnosed with small cell lung cancer.
U&E
Sodium: 119 mmol/L
Potassium: 4.2 mmol/L
Chloride: 88 mmol/L
Bicarbonate: 26 mmol/L
Urea: 2.9 mmol/L
Creatinine: 68 µmol/L
eGFR: >90 mL/min/1.73 m²
Interpretation
The profound hyponatraemia with otherwise preserved kidney function is suggestive of syndrome of inappropriate antidiuretic hormone (SIADH), a recognised complication of small cell lung cancer. The patient's neurological symptoms are likely due to cerebral oedema caused by the low sodium concentration. Management depends on symptom severity and the underlying cause.
Scenario 4: Diabetic ketoacidosis (DKA)
A 24-year-old woman with type 1 diabetes presents with abdominal pain, vomiting and rapid, deep respirations. She reports missing several insulin doses.
U&E
Sodium: 131 mmol/L
Potassium: 5.8 mmol/L
Chloride: 97 mmol/L
Bicarbonate: 10 mmol/L
Urea: 12.4 mmol/L
Creatinine: 148 µmol/L
eGFR: 48 mL/min/1.73 m²
Interpretation
The markedly reduced bicarbonate indicates a significant metabolic acidosis. Although the potassium is elevated, total body potassium is usually depleted due to osmotic diuresis. Mild elevations in urea and creatinine are common due to dehydration and reduced kidney perfusion. These results, together with the clinical presentation, are highly suggestive of diabetic ketoacidosis.
Scenario 5: Diuretic-induced hypokalaemia
A 76-year-old man with heart failure presents with increasing muscle weakness and intermittent muscle cramps. He has recently had his loop diuretic dose increased.
U&E
Sodium: 140 mmol/L
Potassium: 2.7 mmol/L
Chloride: 94 mmol/L
Bicarbonate: 34 mmol/L
Urea: 10.2 mmol/L
Creatinine: 102 µmol/L
eGFR: 72 mL/min/1.73 m²
Interpretation
This patient has significant hypokalaemia with an elevated bicarbonate, consistent with metabolic alkalosis secondary to diuretic therapy. Loop diuretics increase urinary potassium losses and may also contribute to volume depletion. Significant hypokalaemia increases the risk of cardiac arrhythmias and requires prompt correction while addressing the underlying cause.