Kidneys and electrolytes
Kalium
A salt that is essential for the heart and muscles; an abnormal value is often due to the blood draw itself, medicines or the kidneys.
Everything about this testWhat it measures
Potassium is a salt that is mainly found inside the cells; only about 2 percent is in the blood. It is needed for signal conduction in the heart, nerves and muscles. The kidneys regulate the amount precisely, with help from the adrenal hormone aldosterone. Because there is so little potassium in the blood, small shifts already affect the heart rhythm. That is exactly why the measurement is sensitive to how the blood is drawn: damaged blood cells in the tube release a lot of potassium and make the result come out falsely high.
- Too high a potassium (hyperkalemia) with kidney damage, with blood pressure medicines (ACE inhibitors, ARBs, potassium-sparing water pills) or with an adrenal problem
- Too low a potassium (hypokalemia) due to water pills, diarrhea, vomiting or excessive sweating
- Monitoring of medicines that affect potassium, according to the Dutch GP guidelines (NHG) on chronic kidney damage and cardiovascular risk management
High
A slightly raised potassium in someone without symptoms is very often due to the blood draw itself and not to the body. Leaving the tourniquet on for a long time, repeatedly making a fist, a difficult blood draw, a finger prick where the finger is squeezed, or a tube that takes too long or gets too warm on its way to the laboratory: in all these cases blood cells break down (hemolysis) and potassium is released into the sample. The laboratory often reports hemolysis with the result. A truly raised potassium fits reduced kidney function, medicines such as ACE inhibitors, ARBs, spironolactone, potassium-sparing water pills and NSAIDs, potassium supplements or salt substitutes containing potassium, poorly controlled diabetes and a shortage of adrenal hormone (Addison's disease). A slightly raised value causes no symptoms; high values cause muscle weakness, tingling and heart rhythm disorders, and that is an emergency.
Low
A low potassium mainly results from losses: through the kidneys with water pills (thiazides, furosemide) and with a lot of licorice candy or licorice root, through the bowel with diarrhea, vomiting or laxatives, and through the skin with excessive sweating. Overproduction of aldosterone by the adrenal gland (with high blood pressure), insulin, salbutamol and a magnesium deficiency also lower potassium. Symptoms are muscle weakness, cramps, constipation, tiredness and, at low values, heart rhythm disorders. This is why the GP checks potassium regularly in people who take water pills.
If your result is abnormal
With an unexpectedly raised potassium, the GP first arranges a new, careful blood draw, without leaving the tourniquet on for long and with fast transport to the laboratory. If the value remains raised, the GP assesses kidney function and medicines and adjusts them if needed; with a very high value or symptoms, a heart tracing (ECG) and referral follow immediately. With a low potassium, water pills and losses are reviewed, potassium is supplemented if needed, and with an unexplained or persistent drop, the adrenal gland is examined.
Reliability
The potassium measurement is accurate, but the result is particularly sensitive to how the sample is drawn and transported. According to the Dutch Society for Clinical Chemistry (NVKC), an unexpectedly raised potassium should therefore always be confirmed with a new, careful blood draw before any action is taken. According to the Dutch GP guidelines (NHG), potassium is a standard monitoring value with kidney damage and with the use of blood pressure medicines, because both too high and too low a potassium can disrupt the heart.
Limitations
- Hemolysis from leaving the tourniquet on for a long time, making a fist, a difficult blood draw, a finger prick or rough transport gives a falsely raised value; this is the most common cause of a high potassium.
- A very high number of platelets or white blood cells raises potassium in the tube after the blood draw (pseudohyperkalemia).
- Medicines (ACE inhibitors, ARBs, spironolactone, water pills, NSAIDs, insulin, salbutamol), potassium supplements and licorice affect the value.
- Heavy exertion just before the blood draw temporarily raises potassium; the value recovers within about an hour.
How common is this
In healthy adults who take no medicines, a truly abnormal potassium is rare; a falsely raised value due to the blood draw, on the other hand, is fairly common. A raised potassium mainly occurs in people with chronic kidney damage (about 1 in 10 adults) who take ACE inhibitors, ARBs or potassium-sparing medicines. A low potassium is mainly seen with the use of water pills and after diarrhea or vomiting.
Who is this useful for
Useful with reduced kidney function, when taking water pills, ACE inhibitors, ARBs, spironolactone or potassium supplements, with heart rhythm disorders, muscle weakness or cramps, with persistent diarrhea or vomiting, and with high blood pressure that is hard to treat (an adrenal problem may then play a role). According to the Dutch GP guideline (NHG) on cardiovascular risk management, potassium is checked together with kidney function after starting or increasing these medicines. In healthy adults without medicines or symptoms, the result is almost always normal, and the main abnormality is a falsely raised value due to the blood draw.
Downsides and risks
- A falsely raised potassium due to hemolysis is common and, without a repeat measurement, can lead to unnecessary worry or even unnecessary emergency care.
- With a finger prick, the chance of hemolysis is greater than with a blood draw from a vein.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results