Potassium

Every nerve impulse and every muscle contraction begins with potassium and sodium swapping places across the cell wall. For that your body keeps potassium inside the cells and sodium outside: of all the potassium, ninety-eight percent sits in your cells, mainly in your muscles, and only two percent in your blood. That small part decides whether your nerves fire and your heart muscle contracts in the right rhythm. Too little gives muscle weakness, cramp and a heart that skips; too much can stop the heart. That is why your kidneys hold the level in your blood within a narrow band, and excrete within hours what you take in too much of. That regulation runs through aldosterone, a hormone from your adrenal glands that instructs the kidneys to hold on to sodium and excrete potassium. Anything that drives or slows that axis shifts potassium: diuretics drive excretion, liquorice mimics aldosterone, and blood pressure medicines such as ACE inhibitors slow it. Insulin joins in too: it pushes potassium into the cells, which is why a large meal lets the level fall temporarily and a deranged blood sugar lets it rise. Your report shows potassium in mmol/L. Normal is about 3.5 to 4.9 mmol/L, where the labs overlap. Below 3.0 mmol/L and above 6.0 mmol/L the requester is phoned the same day, because the heart rhythm can then be at risk.

Potassium is the value that guards your heart rhythm. Anyone who takes diuretics, eats a lot of liquorice, has had a spell of diarrhoea or vomiting or sweats heavily in the heat can sink to the low end unnoticed, and you only notice when muscles weaken, cramp sets in or the heart skips. Anyone on blood pressure medicines, or with kidneys that work less well, runs the opposite risk of potassium rising. In both cases a blood value is the only way to know before the heart lets you know. For anyone who is healthy, potassium is mainly interesting as a lever for blood pressure. Potassium from food counteracts sodium: it makes the kidneys excrete salt and relaxes the vessel wall. Anyone who eats plenty of vegetables, fruit, legumes, potatoes and nuts takes in three to four grams of potassium a day and has a lower blood pressure than someone who mainly eats processed food, where the salt is in and the potassium is out. That potassium-rich diet does not lift the blood value with healthy kidneys, because the kidneys excrete the surplus; the value in your blood stays in the band while your blood pressure falls. The pitfall of this value is the draw itself. Red blood cells are full of potassium, and if they break in the tube, through a tight tourniquet, a long clenched fist, a difficult draw or a tube that has been in transit for a long time, the lab measures a high potassium that does not exist in your body. That is the most common reason for a high value, and a repeat with a calm draw solves it.

A low potassium almost always means loss, through the kidneys or through the gut. The most common explanation first: diuretics, especially the thiazide kind for high blood pressure; diarrhoea or vomiting; laxatives or a lot of liquorice, because the liquorice root mimics aldosterone and makes the kidneys excrete potassium; and a low magnesium, which makes the kidneys leak potassium, so that potassium only comes back once magnesium is topped up. Rare is an adrenal gland making too much aldosterone, and that suspicion arises with a low potassium beside a high blood pressure that responds poorly to medicines. Below 3.0 mmol/L it should be topped up under supervision, because the heart rhythm can then get into trouble. A high potassium is first a question about the draw. Broken red cells in the tube give a high value that says nothing about your body, and the lab often reports that as haemolysis; if that is not reported and the value does not fit your symptoms or medicines, ask for a repeat with a loose tourniquet and a relaxed arm. If the value is real, kidneys that work less well are the most common reason, followed by medicines that slow excretion, such as ACE inhibitors, sartans, spironolactone and anti-inflammatory painkillers, and potassium-rich salt substitutes. Rare are tissue damage, in which potassium is released from broken cells, and an adrenal gland making too little aldosterone. Above 6.0 mmol/L an ECG belongs with it the same day. Read potassium beside sodium, because those two swap places and often shift together, beside creatinine and the eGFR, which say whether the kidneys can get rid of it, and beside magnesium, which keeps a low potassium going.