U&E Interpretation

Question 1

What does a standard urea and electrolyte profile account for?

Answer 1

• Serum creatinine
• Estimated glomerular filtration rate (eGFR)
• Serum urea
• Serum sodium
• Serum potassium

Question 2

Why check U&Es?

Answer 2

• Renal impairment can lead to imbalances in certain electrolytes
• Kidney excretes urea and creatinine - used as markers of renal function
• Assessing urea levels in the case of suspected upper gastrointestinal haemorrhage.
• Suspected electrolyte disturbances due to non-renal causes, medication monitoring (e.g. after starting ACE inhibitors)

Question 3

What are some drugs that may require U&E monitoring?

Answer 3

• ACE inhibitors and ARBs
• Diuretics: spironolactone, thiazide diuretics (e.g. indapamide), loop diuretics (e.g. furosemide)
• Direct-acting oral anticoagulants (DOACs)
• Carbamazepine
• Lithium
• Digoxin

Question 4

Describe creatinine.

Answer 4

• Waste product of muscle metabolism excreted entirely by the kidney
• Levels determined by extent of filtration from bloodstream by kidney - high levels indicate renal dysfunction
• As the serum creatinine rises, the eGFR will decrease, indicating worsening kidney dysfunction.

Question 5

Describe urea.

Answer 5

• Waste product of protein breakdown produced in the liver.
• Kidneys predominantly excrete urea, and it can be used as a surrogate marker of renal function

Question 6

What causes raised serum urea levels?

Answer 6

• Renal dysfunction: decreased excretion of urea into the urine.
• Dehydration: urea rises quickly in dehydration, even in the presence of normally functioning kidneys. This is physiologically mediated by anti-diuretic hormone (ADH), released from the posterior pituitary gland in response to intravascular volume depletion. ADH increases urea and water reabsorption in the collecting ducts.
• Upper gastrointestinal bleeding: blood in the upper GI tract is broken down into proteins, which are transported to the liver via the portal vein and metabolised into urea.

Question 7

What causes reduced serum urea levels?

Answer 7

Non-pathological, associated with pregnancy and those on a low-protein diet.

Question 8

Describe sodium.

Answer 8

• Main determinant of plasma osmolality, and serum levels are closely related to hydration (volume) status and regulated by ADH
• Symptoms of both hypernatremia and hyponatremia are primarily neurological. Mild symptoms include fatigue, weakness and confusion, but can progress to severe symptoms such as seizures and coma

Question 9

Describe hypernatraemia. PART 1

Answer 9

• Serum sodium level >146mmol/L. It is most commonly caused by dehydration (e.g. unreplaced skin or GI losses of hypotonic fluid)

Rarer causes of hypernatraemia include:
- Diabetes insipidus
- Drugs (e.g. loop diuretics)
- Osmotic diuresis (e.g. in hyperglycemic states)
- Extreme levels of salt ingestion

Question 10

Describe hypernatraemia. PART 2

Answer 10

• In most cases, where there is a clear history implicating dehydration (e.g. vomiting/diarrhoea), treatment involves gentle rehydration with intravenous hypotonic fluids (e.g. 5% dextrose).
• Essential to monitor sodium levels during treatment, as correcting the hypernatraemia too rapidly can lead to intracerebral fluid shifts and central pontine myelinolysis

Question 11

Describe hyponatraemia.

Answer 11

• Serum sodium level <135mmol/L. It is often caused by a failure to excrete water normally i.e an excess of water in the body in relation to sodium (e.g. SIADH)
• Treatment can vary according to the underlying cause and the patient’s fluid status. Important to regularly monitor sodium levels to avoid rapid correction.