Blood tests are performed to diagnose levels of potassium that are too high (hyperkalaemia) or too low (hypokalaemia). The most common cause of hyperkalaemia is kidney disease, but many drugs can decrease the amount of potassium excreted from the body and result in this condition. Hypokalaemia can occur if you become dehydrated, for example due to diarrhoea and vomiting, or if you are sweating excessively. Potassium can also be lost through your kidneys in urine; in rare cases, potassium may be low because you are not getting enough in your diet. Drugs can cause your kidneys to lose potassium, particularly diuretics ('water pills'), resulting in hypokalaemia. Once your doctor discovers the reason for the too-high or low potassium levels, it can be treated.

Blood potassium levels are routinely measured as part of the clinical investigation of many diseases. It is measured in those who take diuretics or heart medications, and in the investigation of high blood pressure and kidney disease. It is also used to monitor patients on kidney dialysis or diuretic therapy, and patients receiving intravenous therapy on a drip.

A raised blood potassium level is called hyperkalaemia. This may indicate the following health conditions:

• kidney disease
• Addison's disease
• injury to tissue
• infection
• diabetes
• excessive intravenous potassium intake (in patients on a drip)

Certain drugs can also cause hyperkalaemia in a small percent of patients. Among them are non-steroidal anti-inflammatory drugs (such as ibuprofen); beta-blockers (such as propanolol and atenolol), angiotensin-converting enzyme (ACE) inhibitors (such as captopril, enalapril, and lisinopril), and potassium-sparing diuretics (such as triamterene, amiloride, and spironolactone). A low blood level of potassium is called hypokalaemia. and may occur in a number of conditions, including:

• vomiting,
• diarrhoea, and
• deficient potassium intake (this is rare).

If you have diabetes, your potassium may fall after you take insulin, particularly if your blood glucose levels have been out of control for a while. Low potassium is commonly due to 'water pills' (diuretics); if you are taking these, your doctor will check your potassium level regularly.

The way that your blood is taken and handled may cause the potassium level in the sample to be falsely high. If you clench and relax your fist a lot while your blood is being collected, this can make potassium rise. If blood comes out of your veins too fast or too slow, the blood cells can burst and release potassium into the blood, giving a falsely raised potassium result. Some tubes that are used to collect blood into contain potassium salts as a preservative If your blood is collected into one of these by mistake potassium will be falsely high.

Potassium can also be elevated if the specimen takes a long while to travel from your GP surgery to the laboratory.

If there are any questions as to how your blood was collected, your doctor may request that the test be repeated before starting any treatment.