Hypokalaemia (serum potassium < 3.5 mmol/L)

• No tests in the community: refer to the emergency department for intravenous replacement under continuous ECG monitoring.
• Serum potassium < 2.5 mmol/L, muscle weakness or paralysis, ECG abnormality or arrhythmia, vomiting preventing oral intake.

Laboratory tests

• Serum electrolytes (repeat serum potassium, serum sodium, chloride)
• Bicarbonate
• Serum magnesium
• Serum creatinine with eGFR
• Plasma glucose
• TSH (if tachycardia or episodes of paralysis)if needednon-French guidelineTachycardia, signs of thyrotoxicosis or episodes of weakness or paralysis: thyrotoxicosis can cause hypokalaemia by shift (thyrotoxic periodic paralysis) ^[2].
• Serum digoxin (if on digoxin)if needednon-French guidelinePatient treated with digoxin: hypokalaemia promotes arrhythmias on digitalis, and serum digoxin should be measured ^[2].

Urine tests (spot sample, same day, before any potassium replacement)

• Urinary electrolytes (sodium, potassium)
• Urinary creatinine: urinary potassium:creatinine ratio
• Urinary chloride (if the cause is not found on history and examination)if needednon-French guidelineHypokalaemia whose cause is not explained by the history and examination: in the United States, an American Family Physician review has urinary chloride measured with urinary sodium, potassium and creatinine, as well as renin and aldosterone ^[3]. In France, urinary chloride has no NABM code: it is not reimbursed ^[4].

If hypertension: after correction of serum potassium

• Plasma aldosterone and renin, with aldosterone:renin ratio (conditions: see the note)if neededHypokalaemia, spontaneous or on a diuretic, in a hypertensive patient with normal GFR: screening for primary hyperaldosteronism ^[1,2]. To be done with normal potassium, in the morning, more than 2 hours after getting up, seated for 5 to 15 minutes ^[5].
• Renin-aldosterone: in the morning, more than 2 h after getting up, seated for 5 to 15 min, with normal serum potassium, on a normal-sodium diet; aldosterone antagonists stopped (see the note).