An Asian patient is scheduled to receive a first dose of carbamazepine (tegretol). Prior to initiation you should order

A. A serum potassium level: Routine potassium monitoring is not indicated prior to starting carbamazepine, as the drug does not typically affect serum electrolytes. Electrolyte assessment is unnecessary unless other comorbidities exist. B. 24 hour urine for creatinine clearance: Carbamazepine is primarily metabolized hepatically, not renally, so baseline renal function via creatinine clearance is not required for first-dose safety. Standard renal labs may be monitored for comorbid conditions but are not essential for initiation. C. HLA-B1502 genetic screening: Patients of Asian ancestry have a significantly increased risk of developing Stevens-Johnson syndrome or toxic epidermal necrolysis if they carry the HLA-B1502 allele. Screening prior to carbamazepine initiation allows identification of at-risk individuals and prevents life-threatening cutaneous reactions. D. Serum carbamazepine levels: Therapeutic drug monitoring is performed after steady-state is reached, typically 1–2 weeks after initiation. Measuring levels prior to the first dose does not provide useful information for safety or efficacy.