Low-dose aspirin therapy is used routinely for the prevention of arterial thrombosis. Elderly patients tend to have adverse reactions to aspirin, including renal effects. In high dosages, aspirin tends to increase uric acid excretion, while lower dosages may cause its retention. Aspirin therapy at the anti-inflammatory dosage level has been shown to have a negative effect on renal function in patients with renal insufficiency, cirrhosis, or heart disease. However, no studies had looked at low-dose aspirin therapy and its impact on the renal system. Segal and associates evaluated the effects of low-dose aspirin therapy on renal function and uric acid in elderly patients.

Patients enrolled in the trial had been admitted to a long-term geriatric center. To participate, they had to have been stable for at least two weeks before the study began and throughout the study period. Patients were excluded if they had any medical condition or history that would put them at risk for adverse effects of aspirin therapy. They also were excluded if their creatinine level was higher than 1.6 mg per dL (140 mmol per L) or higher.

Participants in the study were placed on a controlled-protein diet one week before and during the study. A control group that was similar to the intervention group also was assessed. Patients who participated in the study were given 100 mg of enteric-coated aspirin daily for two weeks. Blood and 24-hour urine samples were collected before the first dose of aspirin and then weekly during the treatment and follow-up phase of the study. Blood tests were performed, including measurement of blood urea nitrogen (BUN), serum salicylate, creatinine, uric acid, and albumin levels. The 24-hour urine test included creatinine and uric acid levels. A 24-hour creatinine clearance was calculated from the results.

There were 83 patients in the active treatment group and 40 in the control group. The average age was 81 years (range, 56 to 98). After two weeks of low-dose aspirin therapy, 72 percent of the patients had decreased urinary excretion of creatinine, and 45 percent had decreased uric acid excretion. In addition, mean creatinine levels and uric acid clearances decreased, while serum BUN, creatinine, and uric acid levels increased significantly. When these patients were compared with the group of patients who did not receive aspirin therapy, a significant deterioration in all levels was noted. At the end of the three-week follow-up period, 48 percent of patients who had received low-dose aspirin therapy had a persisted decline in creatinine clearance from baseline. This decline also was significant when this group was compared with the control group.

The authors conclude that short-term, low-dose aspirin therapy may have a significant adverse effect on renal function in elderly patients. They note that this negative effect persisted for at least three weeks after aspirin therapy was discontinued. The authors add that even though drug information sources and textbooks do not recommend monitoring renal function in elderly patients taking low-dose aspirin therapy, it appears that this therapy may have a negative effect on renal function in this group.