A patient on lithium presents with dark brown urine. What is the next best action?

• A. Check renal function
• B. Increase lithium dose
• C. Check liver function tests
• D. Assess for dehydration only

Answer: (A)

Lithium is cleared by the kidneys and has a narrow therapeutic index, so any sign that suggests dehydration or renal issues can raise the lithium level and risk toxicity. Dark brown urine in this context most importantly prompts checking renal function and the serum lithium concentration to assess for possible accumulation and to guide management. Measuring creatinine and BUN, along with electrolytes, helps determine if renal function is compromised, and obtaining the lithium level shows whether dose adjustment or hydration is needed. Increasing the dose would worsen toxicity risk, and liver tests aren’t the primary concern since lithium isn’t primarily hepatically metabolized. Assessing dehydration alone won’t reveal how well the kidneys are clearing lithium.