The lithium level is therapeutic and the patient is asymptomatic, so no changes are needed at this time; continue current monitoring schedule and recheck labs at the next routine interval.

A therapeutic lithium level with rising creatinine is not reassuring. The renal function change requires evaluation regardless of the drug level.

Evaluate the rising creatinine as possible lithium-related renal decline by calculating eGFR, obtaining urinalysis, and coordinating with nephrology to assess risk-benefit of continuing lithium.

Lithium nephrotoxicity is a recognized long-term complication, with chronic tubulointerstitial nephritis and nephrogenic diabetes insipidus being the most common renal manifestations. A creatinine increase of 0.5 mg/dL in 6 months in a 70-year-old is concerning and may represent lithium-related renal decline, though other causes (dehydration, NSAIDs, ACE inhibitors, obstruction) should also be evaluated. The lithium level being "therapeutic" means the dose is achieving target blood levels — it says nothing about renal tolerance. eGFR trending, urinalysis (assess for nephrogenic diabetes insipidus — polyuria, dilute urine), and nephrology consultation are indicated.

Reduce the lithium dose to lower the serum level, as the creatinine elevation is directly dose-related and a lower lithium level will allow the kidneys to recover renal function over time.

Reducing the dose may be appropriate if lithium is continued, but the first step is assessing the degree of renal impairment and whether lithium should be continued at all. Lithium nephrotoxicity is not simply dose-related.

Recheck the creatinine in 6 months, as mild creatinine elevations are an expected part of normal aging in a 70-year-old and do not warrant immediate intervention or workup.

This degree of creatinine change in 6 months exceeds normal age-related decline and warrants evaluation, not passive monitoring. Age-related GFR decline is typically 1 mL/min/year, not a 0.5 mg/dL creatinine jump in 6 months.