5th April 2024

Investigations

Blood tests, including urea and electrolytes, serum bicarbonate, glucose and magnesium, provide useful information in hypokalemia investigations. If the patient is on digoxin, then levels should be taken at the appropriate time and a review of medications for other causative agents, such as loop/thiazide diuretics. Concurrent hyponatremia with hypokalemia is suggestive of thiazide diuretic use or marked volume depletion. Low magnesium should be corrected to allow recovery of serum potassium. 

Patients with moderate to severe hypokalemia should have an ECG performed to determine whether cardiac function is affected and to detect digoxin toxicity. Ventricular arrhythmias, including torsades de pointes, ventricular tachycardia and ventricular fibrillation, can occur. ECG findings with potassium levels < 3.0mmol/L include:

• Flat T waves
• ST segment depression
• Presence of U waves 
• Pseudo prolongation of QT interval due to flattened T waves merging into the u waves.  

Other investigations include urine potassium, sodium and urine osmolality, which is necessary to interpret urinary potassium levels. Low urine potassium is suggestive of poor intake, gastrointestinal loss or a shift into the intracellular space. High urinary sodium indicates renal loss.

A low urinary sodium in combination with high urinary potassium indicates secondary hypoaldosteronism. 

Additional tests may be indicated and directed toward finding the underlying cause of hypokalemia:

• Bone profile (calcium and phosphate)
• Arterial blood gas (to establish metabolic acidosis/alkalosis)
• Urinary calcium (in suspected Bartter’s syndrome)
• Serum renin, aldosterone and cortisol
• Low dose dexamethasone suppression test in suspected Cushing’s syndrome
• Pituitary MRI to further evaluate Cushing’s syndrome 
• CT imaging of the adrenal glands
• Renal angiogram id renal artery stenosis is suspected
• Sweat chloride test in suspected cystic fibrosis