Which lab value should be monitored closely during magnesium sulfate therapy?

• A. Serum magnesium level
• B. Platelet count
• C. Hematocrit
• D. WBC count

Answer: (A)

Magnesium sulfate is used to prevent seizures in preeclampsia, and keeping the drug within the right range is essential for safety and effectiveness. The best way to gauge both that the dose is therapeutic and that toxicity isn’t developing is to monitor the serum magnesium level. This value directly reflects how much magnesium is in the bloodstream and guides dosing to maintain anticonvulsant protection without crossing into toxic levels.

Why this matters: as magnesium levels rise toward the upper end of the therapeutic range, the risk of toxicity increases. Signs of toxicity include loss of deep tendon reflexes, slowed or shallow breathing, and decreased urine output. If toxicity is suspected, the infusion is typically stopped and calcium gluconate is available as an antidote, with further clinician guidance.

Other labs, like platelet count, hematocrit, or WBC count, provide information about different pregnancy complications (such as HELLP/DIC, volume status, or infection) but they do not monitor magnesium toxicity or the drug’s anticonvulsant effect. Therefore, monitoring serum magnesium level is the priority during magnesium sulfate therapy. Baseline levels are checked before starting, with follow-up measurements according to protocol (often every few hours during infusion).