What electrolyte abnormalities does foscarnet cause?

Foscarnet is associated with hypocalcemia, hypomagnesemia, hypokalemia, hypophosphatemia, and hyperphosphatemia. Ionized calcium can fall even when total calcium is less dramatic because foscarnet chelates divalent ions.

Why can foscarnet cause seizures?

Seizures occur when foscarnet-driven mineral and electrolyte changes increase neuronal excitability. Low ionized calcium and low magnesium are the core board clues, especially when renal function worsens during therapy.

Why does foscarnet work when acyclovir or ganciclovir fails?

Foscarnet directly binds the pyrophosphate-binding site of viral DNA polymerase and does not require viral kinase activation. That lets it bypass thymidine kinase deficient HSV and many kinase activation failures in CMV therapy.

How do you reduce foscarnet nephrotoxicity?

Use adequate IV hydration, monitor creatinine closely, and adjust dosing to renal function. The kidney problem drives many downstream electrolyte issues, so renal monitoring and electrolyte monitoring travel together.

Pharmacology · antiviral toxicity

Foscarnet Electrolytes

The testable pattern is not just "nephrotoxic." Foscarnet is a pyrophosphate analog that skips viral kinase activation, hits viral polymerase directly, and then punishes the patient with kidney injury plus divalent-ion chelation.

From the Attending

If the stem says ganciclovir or acyclovir stopped working, ask one question first: did the virus break the activation step? Foscarnet matters because it does not need that step. Then immediately ask what the drug is doing to calcium, magnesium, phosphate, potassium, and creatinine.

Opening challenge

A 36-year-old man with AIDS has CMV retinitis that progresses despite therapeutic ganciclovir levels. Genotyping shows a viral kinase mutation that prevents drug activation. The team switches to foscarnet. What toxicity should be watched most closely in the first week?

Right. Foscarnet bypasses kinase activation, but it chelates divalent ions and injures renal tubules. Watch ionized calcium, magnesium, phosphate, potassium, and creatinine together.

Mechanism theater

The phosphate decoy jams the viral machine

Press the stages. The animation shows why foscarnet does not care about thymidine kinase or CMV kinase resistance, then why the same phosphate chemistry drags calcium and magnesium into the toxicity story.

Growing DNA strand

Divalent ions Ca / Mg

Pyrophosphate pocket

No kinase needed

Ionized Ca 1.30 mmol/L
Magnesium 2.0 mg/dL

Tap a stage

No kinase needed

Acyclovir and ganciclovir are prodrugs: a viral kinase has to switch them on. If that kinase is mutated, they stall at a dead gate. Foscarnet arrives already active and walks straight to the polymerase.

RouteSkips viral kinase activation

PatternBeats TK-deficient HSV and UL97-mutant CMV

PearlNo kinase step means kinase resistance cannot lock it out

Toxicity triage

Do not memorize one electrolyte

Foscarnet is a cluster problem: calcium, magnesium, phosphate, potassium, and creatinine all move together. The board trick is deciding which abnormality is driving the symptom in front of you.

Ionized calcium

Tetany, QT prolongation, seizures

Foscarnet can lower ionized calcium through divalent-ion chelation. Total calcium may not tell the whole story. If the patient has perioral tingling, carpopedal spasm, or seizures, think ionized calcium first.

Magnesium

The correction blocker

Low magnesium lowers seizure threshold and makes potassium and calcium harder to correct. If potassium refuses to rise, replete magnesium instead of just pushing more potassium.

Kidney

The amplifier

Renal tubular injury raises foscarnet exposure and destabilizes the electrolyte panel. Hydration and renal adjustment are not boring details. They are the thing preventing the toxicity cascade.

From the Attending

Do not answer "hypocalcemia" reflexively. Ask what the stem is showing. Tetany or seizure with low ionized calcium points to calcium. Refractory hypokalemia points to magnesium. Rising creatinine points to the kidney as the reason the whole panel is getting worse.

Patient on foscarnet develops perioral tingling and carpopedal spasm. What panel do you check first?

Ionized calcium is the active fraction. Foscarnet chelation can make that fraction dangerous even when the total calcium looks less dramatic.

Next step unlocked: after calcium, ask whether magnesium is preventing correction.

Clinical routing

When to reach for it

Foscarnet is not a gentle default. It is the direct polymerase tool when kinase-activated antivirals fail or when marrow toxicity makes ganciclovir dangerous.

Ganciclovir: great for CMV until the activation step breaks

Needs viral CMV kinase activation, then host kinases finish the job. Main toxicity is marrow suppression. If the ANC is already low, this drug can turn a viral problem into a neutropenic bacterial problem.

Foscarnet: no kinase gate, direct polymerase block

Does not require phosphorylation. That is the rescue logic for kinase-resistant HSV or CMV activation failure. The tradeoff is kidney and electrolyte surveillance.

Cidofovir: another polymerase option, different protection rule

Uses host phosphorylation and is strongly nephrotoxic. The protectant association is probenecid plus hydration. Do not import that probenecid rule into foscarnet questions.

FOS = phosphate feel

It mimics pyrophosphate, so it slots into the polymerase exit pocket and blocks the next base.

Tap to reveal

No kinase, no permission slip

Foscarnet walks past the activation checkpoint already active, so TK-deficient HSV and UL97-mutant CMV cannot lock it out.

Tap to reveal

Two monitors travel together

Creatinine tells you exposure risk; ionized calcium and magnesium tell you seizure and rhythm risk.

Tap to reveal

Image strip

Tap any image to expand. The photos anchor the actual disease targets and the kidney/electrolyte context without putting fake labels on real images.

Kidney anatomy illustration — Kidney anatomy · toxicity target

CMV retinitis fundus image — CMV retinitis · retinal necrosis and hemorrhage

Transmission electron micrograph of herpes simplex virus — HSV particle · resistant mucocutaneous target

Kidney nephron anatomy diagram — Nephron anatomy · renal toxicity context

Board walkthrough

One patient at a time

Use the exam tools before you answer: right-click or long-press to cross out, double-click or double-tap to highlight. The highlighted clues glow only after you commit.

Medically reviewed by Fatima Ali, DO and Kaitlyn Cocuzzo, MD · Last reviewed June 2026

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