Faecal blood loss has been measured using autologous erythrocytes labelled with radioactive chromium for several decades, using generally similar methods. We conducted a systematic review of studies employing this technology to determine the degree of blood loss associated with use of aspirin, nonsteroidal anti-inflammatory drugs (NSAIDs) and cyclo-oxygenase-2 selective inhibitors (coxibs).
A systematic search of PubMed and the Cochrane Library (to December 2006) was conducted to identify randomized trials in which treatment with aspirin, NSAIDs, or coxibs was continued for at least 7 days, and with at least 7 days of washout for crossover trials. Rates of faecal blood loss associated with these agents were determined in the randomized trials identified. Comparators were placebo, active, or no treatment. Outcomes of interest were mean daily faecal blood loss, and the number or proportion of individuals recording faecal blood above 5 ml/day and above 10 ml/day.
Forty-five reports of 47 trials were included, including 1,162 individuals, mostly healthy volunteers and predominantly young men. Only 136 patients (as opposed to healthy volunteers; 12%) were included, and these were mostly older people with an arthritic condition. Most NSAIDs and low-dose (325 mg) aspirin resulted in a small average increase in faecal blood loss of 1 to 2 ml/day from about 0.5 ml/day at baseline. Aspirin at full anti-inflammatory doses resulted in much higher average levels of blood loss of about 5 ml/day. Some individuals lost much more blood than average, at least for some of the time, with 5% of those taking NSAIDs having daily blood loss of 5 ml or more and 1% having daily blood loss of 10 ml or more; rates of daily blood loss of 5 ml/day or 10 ml/day were 31% and 10%, respectively, for aspirin at daily doses of 1,800 mg or greater.
At baseline, or with placebo, faecal blood loss is measured at 1 ml/day or below. With low-dose aspirin and some NSAIDs, average values may be two to four times this, and anti-inflammatory doses of aspirin result in much higher average losses. A small proportion of individuals respond to aspirin or NSAIDs with much higher faecal blood loss of above 5 ml/day or 10 ml/day. There are significant limitations regarding the quality and validity of reporting of these studies, such as limited size and inclusion of inappropriate participants. The potential for blood loss and consequent anaemia requires more study.