Most physicians provide adjunctive antibiotic therapy for large abscesses following incision and drainage – the sorts where you need a bucket and a hose. Less clear has been the small abscess – but, in the age of MRSA, the fear factor has led many to cover these, regardless. Recent evidence suggests there is a small absolute benefit to antibiotic use and clinical cure, with an NNT around 14, along with other apparent benefits regarding re-infection and spread to household contacts. These trials, however, still enrolled patients with abscesses much larger than typically encountered in routine practice.
This trial is specifically designed to break the glass on “smaller skin abscesses” – just like in the title! What does small mean to these authors? It means a suppurative cavity of 5cm in diameter, or, up to the size of a cupcake:
So, before we even start, we can see we may end up with issues regarding generalizability to many of the abscesses we encounter in the Emergency Department.
This trial is comprised of three arms – clindamycin, trimethoprim-sulfamethoxazole, and placebo – and enrolled 786 patients in an attempt to detect a 10% difference between arms while accounting for 20% attrition rate. The primary outcome was test of cure at 10 days after therapy, with a variety of secondary outcomes, including new infections at one month and treatment-related adverse events.
The winner, if one can be crowned, was not placebo. At the test of cure visit in the intention-to-treat population – and likewise, the population that could be evaluated – placebo lagged behind both clindamycin and TMP-SMX by approximately a 12% absolute magnitude of difference. Recurrent infections at the same site, or another site, were lowest in the clindamycin group at 6.8% – and similar between TMP-SMX and placebo, at 13.5% and 12.4%, respectively. However, clindamycin was implicated in the highest rate of adverse events, at 21.9%, compared with TMP-SMX and placebo, at 11.1% and 12.5%, respectively. Most of the difference in adverse events can be attributed to diarrhea illness, although clostridium difficile was not isolated in any cases. There was one case of systemic hypersensitivity reaction thought to be related to TMP-SMX.
There were two main drivers for the difference in test of cure between the placebo cohort and the two antibiotic cohorts, and these were use of rescue antibiotics during the follow-up period and new infections at another site. The use of rescue antibiotics is not necessarily a reliable measure of treatment failure, but it is still reasonable to suggest this difference would not arise by chance alone, despite the small sample. Regarding generalizability to practice, the minority of abscesses were cupcake-sized, but these were still fairly substantial infections. The median size of the abscess was about 2.2cm in diameter, with surrounding erythema of 5.9cm in greatest dimension.
The takeaway, then, hinges on the generalizability of their population to your individual patient. If these are “smaller” skin abscesses, then I wager the bulk of my abscess encounters are for “tinier” abscesses. I doubt this changes much current practice with regard to antibiotics, or antibiotic selection, for those treating abscesses in the 2+cm range, but I expect the differences in cure rates shrink for smaller lesions. It falls within the realm of acceptable practice variation to weigh the harms of antibiotic use with the chance of recurrence or new infection for those lesions.
“A Placebo-Controlled Trial of Antibiotics for Smaller Skin Abscesses”