A male client arrives at the mental health clinic and asks the nurse for more lithium and the antidepressant amitriptyline that he uses to help him sleep. After reviewing the assessment findings with the healthcare provider, a serum creatinine is obtained. Which information supports the reason for this laboratory test?

A. Creatinine primarily reflects kidney function, not the metabolism of lithium in the liver. B. Amitriptyline's effects on lithium toxicity are not directly related to creatinine levels. However, both drugs can impact kidney function. C. The decision to change medication based on creatinine levels involves assessing the impact on kidney function, not the combination of lithium and amitriptyline per se. D. Lithium is excreted by the kidneys, and monitoring creatinine levels helps assess renal function, guiding the appropriate dosage and preventing lithium toxicity.