A 58-year-old man with bipolar I disorder and stage 3b chronic kidney disease (eGFR 38 mL/min) is evaluated by the PMHNP for ongoing mood stabilization. He has been maintained on lithium 900 mg daily for six years with excellent psychiatric stability, and his most recent lithium level drawn two weeks ago was 0.9 mEq/L. His nephrologist reports a decline in renal function over the past year (eGFR was previously 52 mL/min). The patient denies tremor, gastrointestinal symptoms, or polyuria, but reports mild fatigue and occasional mental sluggishness. The PMHNP evaluates the pharmacokinetic and safety implications of this clinical scenario. Which of the following most accurately reflects the assessment of lithium use in the context of declining renal function?