Reduce the lithium dose and increase monitoring frequency, as the declining GFR indicates progressive renal impairment that has reduced lithium clearance, resulting in the elevated level

Lithium is almost exclusively renally eliminated, so any decrease in GFR directly impacts clearance. The PMHNP should also consult nephrology to evaluate for lithium-induced nephropathy, which can present as chronic tubulointerstitial nephritis after prolonged use. Lithium is >95% renally eliminated, making renal function the primary determinant of serum levels. The rising creatinine and declining GFR, combined with the elevated lithium level despite stable dosing, indicate decreased renal clearance. Long-term lithium use can cause chronic tubulointerstitial nephritis and nephrogenic diabetes insipidus. Dose reduction, increased monitoring, and nephrology consultation are all appropriate. The risk-benefit of continued lithium use must be reassessed.

Maintain the current lithium dose

Since the patient is asymptomatic, and a level of 1.2 mEq/L remains within the acceptable therapeutic range of 0.6-1.2 mEq/L. The creatinine increase is likely age-related and does not necessarily indicate lithium-induced renal damage. Renal function naturally declines with aging, and this patient's GFR is still adequate for lithium clearance. Schedule routine follow-up in 3 months with repeat labs. A lithium level of 1.2 mEq/L represents the upper limit of the therapeutic range, and in an older adult with declining renal function, this warrants concern rather than reassurance. The significant creatinine rise from 1.0 to 1.6 with GFR dropping from 75 to 42 represents a major change that may indicate lithium nephrotoxicity.

Immediately discontinue lithium and start valproic acid as a replacement mood stabilizer

Because continued lithium use in the setting of declining renal function is contraindicated and will inevitably lead to end-stage renal disease. Lithium-induced nephropathy is irreversible and progressive as long as the drug is continued, and the only way to halt renal decline is complete cessation. Valproic acid has no renal elimination and is the safest alternative. Abrupt lithium discontinuation in a patient stable for 15 years carries significant risk of manic relapse and is not warranted based on these findings alone. While lithium dose reduction and possible transition may be needed, the situation calls for careful evaluation, not emergency discontinuation. Lithium nephropathy is not inevitably progressive to ESRD in all cases.

Add a thiazide diuretic to reduce the lithium dose requirement, as thiazides enhance lithium reabsorption in the proximal tubule

Allowing therapeutic levels to be maintained at lower doses. This strategy protects renal function by reducing total lithium exposure while maintaining mood stability. The combination is commonly used in geriatric psychiatry for this purpose and would allow the GFR to stabilize at its current level. While thiazide diuretics do increase lithium levels (by 20-40% through enhanced proximal tubular reabsorption), adding a thiazide to an already elevated lithium level of 1.2 mEq/L would be dangerous. The goal is to lower, not maintain, the lithium level in this patient with declining renal function. Adding a thiazide could push the level into the toxic range. The appropriate first step is to reduce the lithium dose, not add medications that increase lithium retention.