Electrolyte Replacement Guide (K, Mg, Phos, Ca)

Adult limits from the labels, for the common replacement products. The rates are maximums, not targets. The serum value, the ECG and the rhythm set the dose.

You'll see K⁺, Mg²⁺, phosphate and Ca²⁺ on the report. Treating high potassium is a different job. Calcium, insulin with dextrose, albuterol and sodium bicarbonate live on the hyperkalemia page. This page is about replacement.

1. Treat the rhythm and ECG first. An arrhythmia, an ECG change or paralysis changes how fast you move. Confirm the value on a sample that is not hemolyzed. Don't hold treatment for the repeat.
2. Check the other electrolytes together. Low magnesium or phosphate makes potassium hard to correct. Check them together and replace them in one plan (protocol-dependent order).
3. Pick the route first. If the gut works and the case allows, oral potassium is the route. The KCl label keeps IV for when oral isn't feasible. Peripheral limits sit below central line limits (potassium phosphates label). Above these rates, go central with continuous ECG monitoring, per protocol.
4. Calculate the bag, not just the dose. Volume (mL) = mEq ÷ mEq per mL of the bag. Confirm the concentration on the label before the pump is set.
5. Recheck after each dose. Recheck the same electrolyte and ECG, and re-examine the patient before any repeat dose (product labels direct this for phosphate).

Potassium Phosphates Dosing

Dose on actual body weight. The data are thin for patients well above ideal body weight, so consider adjusted body weight (label). The single-dose ceiling is 45 mmol of phosphorus, about 66 mEq of potassium. In moderate renal impairment (eGFR 30 to below 60 mL/min/1.73 m²), start at the low end of the range. It is contraindicated when eGFR is below 30 or in ESRD (label).

Diabetic Ketoacidosis

• Potassium below 3.5 mmol/L at presentation: start replacement at 10 mmol/h and postpone insulin until K⁺ is above 3.5 mmol/L.
• K⁺ below 5.0 mmol/L: Replace to hold K⁺ between 4 and 5 mmol/L. For most patients, 20–30 mmol of potassium per litre of IV fluid holds it there. Check K⁺ as you go.
• Phosphate: Routine replacement is not indicated. Consider it only when phosphate is below 1.0 mmol/L (about 3.1 mg/dL) with muscle weakness or respiratory or cardiac compromise. Randomised trials show no benefit from routine replacement, and fast replacement can cause hypocalcemia. Watch the units. 1 mg/dL is about 0.32 mmol/L, so the mg/dL tables above and this mmol/L threshold don't match.
• Magnesium: The consensus report doesn't address it. Follow protocol.

Refeeding Syndrome

High-risk patients need careful refeeding. ASPEN 2020 grades the drop: a fall of 10–20% in phosphorus, potassium or magnesium is mild, 20–30% is moderate, and over 30% is severe. The window is 5 days from restarting calories. NICE CG32 starts at no more than 10 kcal/kg/day, then climbs slowly to full needs over 4 to 7 days (5 kcal/kg/day in extreme cases). Keep the cardiac rhythm on continuous monitoring.

Worked example: a 70 kg adult with serum phosphorus below 1 mg/dL. 70 × 0.44 to 70 × 0.64 gives 31 to 45 mmol. The top of that range hits the 45 mmol single-dose cap.

Worked example: 10 mEq/h of potassium from a 40 mEq/L bag is 0.25 L/h, which is 250 mL/h.

Worked example: 150 mg/min (the label maximum) comes to 1.5 mL/min of 10%, or 0.75 mL/min of 20%. Don't push from the 50% stock (500 mg/mL). The label wants it diluted to 20% or less before any IV use.

• Pediatrics: Dose by weight, and see the weight-based dosing page. The label says little about children. The KCl concentrate gives no pediatric dosing, so pediatric potassium runs on local protocol and pharmacy. Under 12 years, the potassium phosphates label points to the smaller peripheral concentration (0.27 mmol phosphorus per 10 mL). For potassium given as phosphate, the label recommends continuous ECG monitoring and a central venous catheter above 10 mEq/h (20 kg or greater) or above 0.5 mEq/kg/h (under 20 kg). The severe magnesium IM dose of up to 250 mg/kg over 4 h sits in the adult section of the magnesium label. In children, dilute the 50% solution to 20% or less before any IM injection. Calcium gluconate bolus tops out at 100 mg/min (premix label).
• Pregnancy: Continuous magnesium sulfate beyond 5 to 7 days can cause fetal abnormalities. The newborn may show magnesium toxicity (label warning). Obstetric use follows the obstetric protocol. No specific adjustment is established for the other products on this page.
• Renal Impairment: Potassium chloride is contraindicated in renal failure. With renal insufficiency it can cause life-threatening hyperkalemia. Check urine output and renal function before and during replacement. Potassium phosphates are contraindicated with eGFR below 30 or ESRD. For eGFR 30 to below 60 mL/min/1.73 m², start at the low end of the range. Only the kidney clears magnesium. In severe renal insufficiency the label maximum is 20 g per 48 h. Keep urine output at least 100 mL in the 4 h before each dose, and monitor serum Mg, reflexes and respiratory rate. Potassium given with ACE inhibitors, ARBs, potassium-sparing drugs or digoxin raises hyperkalemia risk (potassium phosphates label, section 7).
• Older Adults: Start potassium at the low end of the dose range. Older patients carry more risk of electrolyte imbalance (label).
• Hepatic Impairment: These labels state no hepatic dose adjustment. Magnesium leaves the body through the kidney (see Renal Impairment). No specific adjustment is established for the other products.

• Error: undiluted, pushed or mis-timed potassium. The concentrate has to be diluted before use, and it never goes in as a push. Your pump runs in mL/h, but the limits are in mEq/h. Recalculate from the bag concentration. Then check the dose in mEq, the volume, the final concentration and the rate.
• Error: magnesium 50% confused with grams. 4 g is 8 mL of 50%. The premixed magnesium sulfate in water label (Hospira) covers only pre-eclampsia and eclampsia, not hypomagnesemia. Use a product labeled for the indication.
• Error: magnesium with heart block or kidney impairment. Parenteral magnesium is contraindicated in heart block or myocardial damage. With renal insufficiency, check urine output (at least 100 mL in the 4 h before each dose) and serum Mg before more doses. Keep IV calcium on hand for toxicity (label).
• Error: potassium phosphate in the wrong patient. Contraindicated with hyperkalemia, eGFR below 30 or ESRD, hyperphosphatemia, or either hypercalcemia or significant hypocalcemia. Check K⁺ (give only if below 4 mEq/L) and Ca²⁺ first.
• Error: phosphate, calcium or ceftriaxone in one line. Potassium phosphates don't go in with calcium-containing fluids. Calcium gluconate isn't physically compatible with phosphate or bicarbonate, and it never mixes with minocycline. Never give it Y-site with ceftriaxone, which is contraindicated together in neonates.
• Error: replacing potassium alone. Low magnesium or phosphate blocks the correction. Check the other levels before you call the potassium refractory.
• Error: full feeding at once in refeeding risk. In high-risk patients, NICE CG32 limits starting intake to 10 kcal/kg/day and requires thiamine before the first feed.
• Error: insulin before potassium is known. In DKA, check K⁺ before the insulin. Insulin waits while K⁺ is below the consensus threshold (see the DKA section).

Carrying or giving IV potassium, phosphate or calcium in the field depends on state and agency protocol. In the field, magnesium is set by protocol too.

• Transport: Put the product, concentration, rate and total dose on the handoff. Potassium running above label thresholds needs continuous ECG monitoring.
• Transport: Keep calcium and phosphate on separate lines, or run them in the order pharmacy gives.