Potassium blood test: what it measures, normal levels, and what high potassium (hyperkalemia) or low potassium means

About 98 percent of the body's potassium is inside cells; the blood carries the remaining 2 percent, and it is that fraction the test measures. Because the pool in the blood is so small, modest shifts of potassium into or out of cells change the result quickly. Insulin, adrenaline and an alkaline blood pH push potassium into cells and lower the level; acidosis, tissue breakdown and insulin deficiency release it and raise the level. The kidneys, under the hormone aldosterone, do the long-term balancing by excreting what is eaten.

The electrical activity of every nerve and muscle depends on the potassium gradient across cell membranes, which is why the heart is the organ at risk when the level is far off in either direction: high potassium slows conduction and can stop the heart; low potassium provokes extra beats and dangerous rhythms, especially in people with heart disease or on certain medications. That is also why the reference range is narrow and why values just outside it are taken seriously when symptoms or a heart condition are present.

The measurement itself is fragile. Red cells contain about thirty times more potassium than plasma, so any hemolysis in the tube, a prolonged tourniquet, repeated fist clenching during the draw, a delay in separating the sample, or very high platelet or white cell counts release potassium into the sample and produce a falsely high result. Laboratories flag hemolyzed samples, but a surprising high potassium in a well person is repeated before anyone acts on it.

Mild changes in potassium usually cause no symptoms. As potassium falls (hypokalemia), muscle and nerve function suffer first:

High potassium (hyperkalemia) can also be silent until it is dangerous. Symptoms, when present, include muscle weakness or heaviness, fatigue, nausea, tingling, and slow, irregular or skipped heartbeats; very high levels can stop the heart, which is why a markedly high result is treated as urgent.

A high potassium on a self-ordered test is sometimes an artifact: red cells break during a difficult draw or when a sample waits too long before processing (hemolysis) and release potassium into the tube. An unexpected high value in someone who feels well is usually repeated before anything else; a very high value with weakness or palpitations needs same-day medical attention.

Potassium is on every basic and comprehensive metabolic panel and every electrolyte panel. It is monitored deliberately in people with chronic kidney disease; in anyone taking ACE inhibitors, ARBs, spironolactone or eplerenone, potassium-sparing diuretics, or the combination of these with NSAIDs, all of which raise potassium; in anyone on loop or thiazide diuretics, which lower it; in people with heart failure, diabetes with kidney involvement, or adrenal disease; and in anyone with unexplained muscle weakness, cramps or palpitations.

It is also part of the standard evaluation of high blood pressure that resists treatment, because a low potassium in that setting suggests excess aldosterone, and of eating disorders, laxative or diuretic misuse, and prolonged vomiting or diarrhea, in which potassium can fall to dangerous levels.

Potassium comes with the basic metabolic panel, the comprehensive metabolic panel and the electrolyte panel, which your doctor can order or you can order online through TestWell for a draw at a Quest or Labcorp patient service center. A licensed physician reviews the order; results post to your portal within one to three business days with each electrolyte shown against its range. Adding magnesium is sensible when potassium is low, because a low magnesium keeps potassium from correcting.

Fasting is required for the glucose on a metabolic panel. To avoid an artificially high potassium, relax the arm and do not pump your fist during the draw, and mention it if the tourniquet was on for a long time or the draw was difficult; hard exercise in the hours before also raises the value transiently. Take prescribed medicines as usual and list them, along with potassium supplements, salt substitutes (many are potassium chloride) and NSAID use, for the ordering physician.

Most laboratories report a normal adult potassium of 3.5 to 5.0 millimoles per liter, with some ranges extending to 5.1 or 5.2; the range on your report applies, and serum values run slightly higher than plasma values because clotting releases a little potassium. There is no difference by sex or age in adults.

Clinicians grade an abnormal potassium by distance from the range, speed of change and the presence of heart disease or symptoms. A value a tenth or two outside the range in a well person on a stable medication is usually rechecked; values well above or below the range, particularly with weakness, palpitations or a known heart condition, are treated the same day, often with an electrocardiogram first because the heart tracing shows whether the level is affecting conduction.

The first question about a high potassium is whether it is real. Pseudohyperkalemia from the draw itself, described above, is the most common explanation for an isolated high result in a healthy person, and a repeat drawn carefully often reads normal. When a high potassium is real, reduced kidney excretion is the usual mechanism: chronic kidney disease, acute kidney injury, and medications that reduce excretion, including ACE inhibitors, ARBs, spironolactone and eplerenone, potassium-sparing diuretics, NSAIDs, trimethoprim and heparin. Combinations, such as an ACE inhibitor with spironolactone in someone with kidney disease, are the classic setting.

Intake and shifts come next. Potassium supplements and potassium-based salt substitutes, particularly with any of the drugs above, can raise the level; so can a very high intake of potassium-rich foods in someone whose kidneys cannot excrete it, though diet alone rarely does it in a person with normal kidneys. Acidosis, uncontrolled diabetes with insulin deficiency, tissue breakdown from rhabdomyolysis, burns, tumor lysis or hemolysis, and adrenal insufficiency, in which aldosterone is missing, all release or retain potassium.

Symptoms are unreliable until the level is high: weakness, fatigue, nausea and a slow or irregular pulse. The danger is the heart, and the electrocardiogram, not the symptom list, determines urgency. Anyone with a clearly high potassium and kidney disease, heart disease or symptoms should be seen the same day rather than rechecked next week.

Low potassium means the body has lost potassium faster than it was replaced or has shifted it into cells. Loop and thiazide diuretics are the most common cause in outpatients; vomiting, diarrhea, laxative misuse and nasogastric suction are next, because gut fluid is rich in potassium and the vomiting itself triggers the kidney to waste more. Low intake alone, as in eating disorders or alcohol use disorder, contributes but is rarely the sole cause. Magnesium deficiency, often from the same diuretics, alcohol or proton-pump inhibitors, prevents potassium from being retained, so a low potassium that will not correct usually has a low magnesium behind it.

Hormonal causes are primary hyperaldosteronism, the most common curable form of high blood pressure, in which the adrenal glands make too much aldosterone and the kidney wastes potassium, and Cushing's syndrome or high-dose steroids. Shifts into cells from insulin, beta-agonist inhalers, decongestants, refeeding after starvation and rare inherited channel disorders lower the blood level without changing the body total.

Symptoms begin with muscle weakness, cramps, constipation and fatigue and progress to palpitations and dangerous rhythms, especially in people on digoxin or with heart disease. A mildly low value with an obvious cause and no symptoms is usually managed with a review of medications and diet and a recheck; a clearly low value, or any low value with palpitations, muscle weakness or a heart condition, needs same-day attention and an electrocardiogram.

For a high potassium: repeat on a carefully drawn, promptly processed sample; creatinine and eGFR; glucose and bicarbonate for acidosis; a medication and supplement review; and, if kidney function is normal and the value persists, morning cortisol and aldosterone for adrenal causes. For a low potassium: magnesium; bicarbonate and chloride, which separate vomiting and diuretics from other causes; a medication review; and, in anyone with high blood pressure, an aldosterone-to-renin ratio for primary hyperaldosteronism. In both directions an electrocardiogram is the first step when symptoms or heart disease are present, and how quickly the level is corrected is a decision for the treating clinician, not a matter of diet alone.