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Diagnosis and investigation in primary care

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Diagnosis and investigation in primary care:

note that if potassium is <= 2.5 mmol/L urgent treatment (in a secondary care setting is required)
- look for warning signs or symptoms
- weakness or palpitations
- changes on electrocardiography (ECG)
- severe hypokalemia (less than 2.5 mEq per L [2.5 mmol per L])
- rapid-onset hypokalaemia
- underlying heart disease or cirrhosis (1)

In the absence of any evidence which warrants immediate treatment, a careful history and physical examination should be carried out:

history
- always check for diuretics in the drug history - commonest single cause of hypokalaemia
- if not on diuretics then hypokalaemia suggests potassium loss, consider
  - faecal loss: chronic diarrhoea e.g. - colitis, laxative abuse
  - vomit - due to any cause
  - skin e.g. burns, excessive sweating
- history of excessive alcohol intake
- any process which may stimulates uptake of potassium from the extracellular fluid into cells
  - intravenous insulin for treatment of hyperglycaemia (in particular, diabetic ketoacidosis)
  - stimulation of sympathetic β2 receptors (for example, with high dose salbutamol)
  - verapamil overdose
  - after vitamin B12 or folate replacement in megaloblastic anaemia
- a history of muscle weakness (typically after strenuous exercise or a large carbohydrate meal) and severe hypokalaemia may indicate:
  - hypokalaemic periodic paralysis or
  - thyrotoxic periodic paralysis (if symptoms of thyrotoxicosis are also present, particularly in Asian men)
physical examination
- there may be flaccid muscle weakness and signs of arrhythmia
- check for high blood pressure - hypokalaemia associated with raised blood pressure may indicate primary hyperaldosteronism or Cushing’s syndrome
- presence of Kussmaul breathing, hypotension and signs of dehydration in a diabetic patient may suggest diabetic ketoacidosis (1,2,3)

Laboratory investigations

in mild hypokalaemia with an obvious cause
- monitor serum potassium concentration
in moderate and severe hypokalaemia and when the cause is unclear
- initial basic laboratory investigations should include -
- serum electrolytes
- serum magnesium
  - hypomagnesaemia often coexists with hypokalaemia and needs to be corrected for successful treatment of hypokalaemia
- serum bicarbonate
  - to check for acid-base disturbances
- serum glucose
- urine potassium
  - to identify whether hypokalaemia is due to renal potassium loss
  - can be assessed by
    - a spot urine potassium
    - 24 hour urine potassium
    - transtubular potassium gradient
    - urine potassium:creatinine ratio (KCR)
if spurious hypokalaemia is suspected
- send a fresh blood sample to the laboratory (rapidly following venepuncture) for reanalysis of potassium (1)
persistent hypokalaemia of unknown cause will require further investigation and referral to secondary care. Some investigations may be requested from primary care such as:
- Urine studies
- hypokalaemia
- in Conn's syndrome, sodium may be mildly elevated or normal metabolic alkalosis
- plasma renin and aldosterone - low renin and high aldosterone (raised aldosterone: renin ratio) suggests primary hyperaldosteronism - note that a normal or high renin may occur secondary to compensatory mechanisms
  - assess the effect of posture on renin, aldosterone and cortisol (measure at 9am lying and at noon standing) - this provides further information as to the cause of primary hyperaldosteronism
    - if reduced aldosterone and reduced cortisol on standing then ACTH dependent cause e.g. adrenocortical adenoma (Conn's syndrome)
    - if increased aldosterone and reduced cortisol then angiotensin-II dependent cause e.g. bilateral adrenocortical hyperplasia
- cortisol - a morning plasma cortisol may be raised suggesting Cushing's syndrome
- thyroid function tests (if thyrotoxic periodic paralysis is suspected) - serum thyroid stimulating hormone and free thyroxine)
ECG (1,2)
imaging of the adrenal glands (CT or MRI) if there is suspicion of mineralocorticoid, glucocorticoid or catecholamine excess (4)
MRI of the pituitary gland (to exclude Cushing's disease) (4)

Reference:

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Diagnosis and investigation in primary care

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