Two labs to monitor for those taking lithium are

• A. Lithium level and sodium
• B. Potassium and sodium
• C. Calcium and phosphate
• D. Hemoglobin and hematocrit

Answer: (A)

Monitoring lithium therapy hinges on keeping the drug’s level in a safe range and watching electrolyte status that affects how lithium is cleared by the kidneys. The first key lab is the lithium level itself. Because lithium has a narrow therapeutic window, its blood concentration must be checked regularly to stay within the effective range without causing toxicity. A trough level is typically drawn after steady-state is reached to guide dosing.

The second important lab is the sodium level. Lithium and sodium share similar pathways in the kidneys, so changes in sodium balance influence lithium reabsorption. When sodium is low (hyponatremia or dehydration), the kidneys reabsorb more sodium and also more lithium, raising lithium levels and increasing the risk of toxicity. Adequate or higher sodium intake tends to promote lithium excretion and helps keep levels in balance.

This pairing—lithium level and sodium level—directly addresses safe dosing and the main mechanism that can unpredictably push lithium into toxic territory. Other labs like potassium, calcium, phosphate, or blood counts don’t have the same immediate relevance to the safety and dosing of lithium in routine monitoring.