A 42-year-old male with bipolar I disorder has been maintained on valproic acid 1500 mg daily for two years with good mood stability. He presents for routine monitoring and reports no mood symptoms. His most recent valproic acid trough level is 62 mcg/mL, drawn appropriately at 12 hours post-dose. Three months ago, his level was 85 mcg/mL on the same dose. He denies any medication changes, missed doses, or new medications. He reports he recently started a high-protein, low-carbohydrate ketogenic diet six weeks ago for weight loss and has lost 14 pounds. His AST and ALT are mildly elevated at 48 and 52 U/L respectively, up from baseline values of 28 and 30 U/L. Serum albumin is 4.2 g/dL, ammonia is 42 mcmol/L, and CBC with platelets is within normal limits. The PMHNP is evaluating the significance of the decreased valproic acid level and hepatic enzyme changes. Which of the following best represents the appropriate evaluation?