The role of aspirin has changed from high-dose treatment for inflammation to low-dose (1,000 to 2,000 mg per day) or mini-dose (less than 500 mg per day) for thromboembolic risk prevention. As a result, over-the-counter preparations account for most aspirin that is taken. While studies have demonstrated that high-dose aspirin therapy can adversely affect renal function and uric acid secretion, the impact of a mini-dose regimen on the kidneys has not been examined. Elderly patients are more likely to use mini-dose aspirin therapy but may also be more susceptible to the adverse effects of aspirin on the kidneys. Caspi and associates studied the effect of mini-dose aspirin (75 to 325 mg per day) on renal function and uric acid secretion in elderly patients.

Elderly patients with a history of renal failure, hyperuricemia, gout or bleeding were excluded from the study, as were patients currently taking anticoagulants, aspirin or non-steroidal anti-inflammatory drugs. Patients were given aspirin in a dosage of 75 mg per day in the first week, 150 mg per day in the second week, 325 mg per day in the third week and zero mg during the fourth week. Baseline and weekly samples of blood and urine were evaluated for uric acid, creatinine and blood urea nitrogen levels, creatinine clearance, uric acid secretion, and urine and plasma levels of aspirin.

At the lowest dosage of aspirin, the excretion rate of uric acid dropped, and the serum levels rose. Creatinine and uric acid secretion rates were parallel during aspirin therapy. One week after aspirin was discontinued, the uric acid clearance rate returned to baseline, while the creatinine clearance rate remained lower. Plasma aspirin levels varied among the participants, but those whose levels were above the median had significantly more changes in renal parameters. The negative effect of mini-dose aspirin therapy was greater in patients with low serum albumin levels or concomitant diuretic therapy.

The authors conclude that mini-dose aspirin therapy, even at a dosage of 75 mg per day, may have a significant negative effect on renal function and secretion of uric acid in elderly patients. This was particularly true in elderly patients who had low albumin levels or were taking diuretics. Physicians should be aware of the possible negative effect of aspirin on renal function, particularly because most aspirin is taken on an over-the-counter basis.

editor's note: Aspirin has become a common preventive against thromboembolic events. Advertisements in the lay media have emphasized this property. Because aspirin is an over-the-counter medication, physicians may be unaware that their patients are taking it on a regular basis. This study shows that, even at very low dosages, aspirin has a negative effect on renal function and uric acid secretion in elderly patients. When recommending aspirin therapy to elderly patients, physicians should weigh the risks and benefits of this therapy. This is particularly true in patients who have low albumin levels or are already taking diuretics.—k.e.m.