['To determine whether short-term use of oral cimetidine improves the precision of creatinine clearance (CCr) and reduces the overestimation of glomerular filtration rate (GFR) that occurs with this test in patients with lupus nephritis (because creatinine is secreted by injured renal tubular cells).', 'Double-blind, placebo-controlled, crossover clinical trial.', 'Thirteen patients with lupus nephritis with mild renal insufficiency (mean serum creatinine, 230 mumol/L [2.6 mg/dL]; median, 106 mumol/L [1.2 mg/dL]).', 'Patients were given placebo or cimetidine tablets, 400 mg four times daily for 2 days, with ambulatory 24-hour urine collection during the second 24 hours ("outpatient study"). Simultaneous 4-hour technetium-99-diethylenetriamine penta-acetic acid (Tc99-DTPA) and CCrs were measured immediately after each 24-hour collection ("simultaneous study").', 'Use of cimetidine improved the accuracy of CCr, as measured by the CDTPA-to-CCr ratio (1.07 [cimetidine] compared with 1.33 [placebo]; P less than 0.05). Cimetidine use also improved the precision of CCr (P less than 0.05). In addition, when compared with standard clinical estimators of GFR, creatinine clearance with cimetidine rendered the most precise estimates of GFR and explained more of the variation in GFR estimation than did any other method (R2 = 0.78 compared with R2 = 0.52 to 0.63). These effects were shown under both simultaneous and outpatient conditions. No side effects due to cimetidine occurred.'] Does oral cimetidine improve the accuracy and precision of creatinine clearance in lupus nephritis?