Potassium: How to Read Your Blood Potassium Level

Only about 2% of the body's potassium lives in the bloodstream. The other 98% sits inside cells, mostly in muscle. Insulin, aldosterone, and catecholamines shuttle it across the membrane all day long.

Two consequences worth holding onto:

• Total body potassium and serum potassium are different things. Someone on chronic diuretics can have a normal serum value while being substantially depleted overall.
• Acute shifts move potassium without changing total stores. Insulin, beta-agonists, and alkalosis push it into cells. Acidosis and cell injury push it back out.

So the same number means different things in different contexts. Read potassium with acid-base status, kidney function, and the medication list in view.

• Normal: 3.5–5.0 mEq/L (or mmol/L, same number).
• Mild hypokalemia: 3.0–3.4 mEq/L. Often asymptomatic; common with diuretics or GI losses.
• Moderate hypokalemia: 2.5–2.9 mEq/L. Cramps, weakness, palpitations, ECG changes.
• Severe hypokalemia: below 2.5 mEq/L. Arrhythmia risk; usually warrants hospital management.
• Mild hyperkalemia: 5.1–5.9 mEq/L. Often pseudohyperkalemia or medication effect.
• Moderate hyperkalemia: 6.0–6.4 mEq/L. ECG monitoring; treatment usually indicated.
• Severe hyperkalemia: above 6.5 mEq/L. Cardiac arrhythmia risk; emergency.

Speed of change and cause matter as much as the value itself. A potassium of 6.2 in a stable dialysis patient is routine. The same 6.2 in someone with new kidney injury and chest pain is an emergency. Same number, different problems.

First question, every time: was the sample hemolyzed? If the redraw comes back normal, you can stop reading. Mild hyperkalemia (5.1 to 5.9) on a routine outpatient draw is hemolysis far more often than it's anything else.

If the redraw is still elevated, real causes worth thinking about:

• Kidney disease. The kidneys are the primary route of potassium excretion. Acute injury and CKD stage 4 to 5 are common drivers.
• Medications. ACE inhibitors, ARBs, spironolactone and other potassium-sparing diuretics, NSAIDs, trimethoprim, heparin, tacrolimus. Older adults on three of these at once with mild CKD are the textbook setup.
• Adrenal insufficiency. Low aldosterone, less potassium excretion.
• Tissue breakdown. Rhabdomyolysis, tumor lysis, hemolysis, severe burns. Cells spilling their contents.
• Acidosis. Drives potassium out of cells. DKA can read as hyperkalemic even when the patient is profoundly potassium-depleted overall.
• Dietary load (when excretion is already impaired). Salt substitutes (often pure KCl), coconut water, dried fruit, tomatoes, potatoes. Bananas get the headlines but aren't the worst offender. Anyone on RAAS blockade with CKD should know what's in salt-substitute shakers.

The ECG progression to memorize: peaked T waves, then loss of P waves, then widened QRS, then sine wave, then asystole. Treatment in order: calcium gluconate to stabilize the heart, insulin and glucose plus beta-agonists to shift potassium into cells, then dialysis or binders (patiromer, sodium zirconium) for actual removal.

Low potassium is almost always a loss problem, not a low-intake problem. The settings that come up:

• Diuretics. Loop diuretics (furosemide, bumetanide) and thiazides (HCTZ, chlorthalidone) are the leading outpatient cause. Potassium-sparing diuretics (spironolactone, eplerenone, amiloride) blunt this effect.
• GI losses. Vomiting, diarrhea, laxative abuse, NG suction.
• Hyperaldosteronism. Primary (Conn syndrome) or secondary. Suspect in resistant hypertension with hypokalemia.
• Magnesium deficiency. See "When to worry" below. Refractory hypokalemia is often a magnesium problem in disguise.
• Insulin and beta-agonists. Shift potassium into cells. Common iatrogenic cause in DKA treatment and asthma exacerbations.
• Renal tubular acidosis, Bartter syndrome, Gitelman syndrome. Uncommon but classic.
• Alkalosis. Vomiting causes both alkalosis and direct GI loss; the two amplify each other.

Symptoms include muscle weakness, cramps, fatigue, constipation, and palpitations. ECG changes: flattened T waves, U waves, ST depression, and at severe levels, prolonged QT and torsades de pointes.

Potassium monitoring is a core part of safe prescribing. Standard monitoring intervals:

• Starting or up-titrating ACE inhibitors, ARBs, or spironolactone. Recheck within 1–2 weeks, then with each dose change.
• Starting thiazide or loop diuretics. Recheck within 2–4 weeks.
• Stable CKD on RAAS blockade. Every 3–6 months at minimum.
• Heart failure on multiple potassium-affecting drugs. Usually every 2–3 months.

The trend tells you more than any single number. A potassium drifting from 4.5 to 5.3 over six months, with no medication change, almost always means kidney function is slipping or a new drug interaction has snuck in. Worth catching early.

• Potassium above 6.0 mEq/L. Same-day evaluation, ECG. Above 6.5, emergency department.
• Potassium below 3.0 mEq/L. Same-day evaluation, ECG, replacement therapy.
• Refractory hypokalemia. If potassium won't budge despite heavy replacement, check magnesium. Low magnesium causes the kidneys to leak potassium, and you cannot fix one without the other. Memorizing this single principle prevents a lot of dangerous back-and-forth.
• New hyperkalemia in a patient on RAAS blockade. Consider holding the medication, check kidney function and aldosterone status.
• Mild hyperkalemia (5.1–5.5) on a routine draw. First redraw. If hemolysis is ruled out, recheck with attention to medications, kidney function, and salt substitutes.
• Hypokalemia with hypertension. Screen for primary aldosteronism (aldosterone-to-renin ratio).

• Magnesium. Refractory hypokalemia is often a magnesium problem. Always check magnesium when potassium will not correct.
• BUN and creatinine. Kidney function is the dominant variable in potassium handling.
• Bicarbonate (CO₂). Acid-base status affects where potassium sits between cells and blood.
• Aldosterone and renin. For unexplained hypokalemia with hypertension or unexplained hyperkalemia.
• ECG. For any moderate-to-severe potassium abnormality.
• Comprehensive metabolic panel. Sets context; sodium, glucose, and creatinine all interact with potassium handling.