Renal function monitoring in patients receiving lithium carbonate

As a screening test for renal function, urine concentration was measured following a 12-hour overnight fast in 54 outpatients taking lithium carbonate and 19 patients receiving antidepressant drugs. A significantly greater percentage of lithium patients failed to achieve a maximum urine concentration of 600 mOsm/kg (63% versus 33% in the antidepressant group, p less than .001). This level, a compromise between the sensitivity and specificity of the test, is viewed as a cutoff point for further testing. It is concluded that urine concentration testing is a feasible first-line screen for renal function among lithium-treated patients. Other preliminary studies include routine urinalysis, serum creatinine determination, and estimated creatinine clearance. Second-line testing includes a repeated dehydration test and administration of DDAVP.