My surgeon has S. aureus!

Don't panic! Just based on probability a surgeon would have a 30% chance of being colonized with MSSA and perhaps a 1% chance of being colonized with MRSA. Researchers at NYU Hospital for Joint Diseases recently screened a total of 135 orthopedic surgeons for MRSA and MSSA nasal colonization. In the 74 attendings, 2.7% were MRSA+ and 23.3% were MSSA+. The story was a bit more interesting in the 61 residents with 59% MSSA+ and none MRSA colonized. Overall, 36% were MSSA+ and 1.5% were MRSA+, which is about what we would have guessed before the study.

These results are similar to Cecilia Johnston's report of healthcare worker colonization at Johns Hopkins a few years ago. She reported 28% S. aureus colonization (95% confidence interval [CI], 22%‐34%) and 2% MRSA colonization (95% CI, 0.04%‐4.0%). I pasted in Cecilia's results to highlight the fact that she calculated confidence intervals for each proportion. The NYU researchers were surprised by the high proportion of residents with MSSA colonization. Sure, the level was high, but if they would have calculated the 95% CI, which was 46%-71%, they might have been less excited.

It's possible that the long hours spent in direct patient care might be risk factors for MSSA colonization, as would frequently performing dressing changes, but these would not be unique to ortho residents. Perhaps they should repeat the study in the same residents at a later point to determine what proportion is transient vs persistent colonization. Of course, it would be nice to repeat the study in other cohorts of ortho residents and ortho nurses.

They also reported that patients screened prior to THR, TKR and major spine surgery at their hospital had 2% MRSA and only 18% MSSA colonization. It would have been nice to read more information about these patients, such as recent antibiotic exposure.

Schwarzkopf et al. in Journal of Bone and Joint Surgery (America): PubMed or JBJS
Johnston et al. in December 2007 ICHE
Easy confidence interval for proportion calculator: link

Rhinoceroses and Total Hip Arthroplasty

Distinctions are very important. I was just visiting Ohio last week and had the chance to visit the Columbus Zoo. It's a pretty cool place if you like zoos. I enjoyed reading about various animals and learned that the Black Rhino is endangered, while the White Rhino is not. Thus, it would make sense to spend your conservation money, if you have some, on Black Rhinos first, since time is running out. In infection control, we ought to do the same thing, but in reverse; spending our limited resources on preventing more common infections first. Also, with the rise of public reporting and other methods of interhospital comparison, efforts must be made to place hospitals on a level playing field. There is a nice study that highlights these two issues in the May ICHE by Surbhi Leekha and colleagues at the Mayo Clinic in Rochester, MN.

They examined at a 5-year cohort (2002-2006) of all total hip arthroplasties (primary and revision) and looked to see who developed SSI, using CDC definitions. After controlling for age, gender and NNIS index, patients who had a revision total hip arthroplasty had twice the odds of SSI compared to primary surgery (OR=2.2, 95% CI 1.3-3.7). The difference was even more stark when outcomes were restricted to deep or organ space SSI with revisional surgery associated with four times the odds of SSI (OR 3.9, 95% CI, 2-7.9). One note, they didn't appear to control for duration of surgery as a confounder, even though it was associated with both revisions and SSI. I think this is correct. They were not completing a risk-factor study, but were interested in outcomes.

The usual caveats apply to these types of studies including a single center study and a relatively unique single center at that. However, this is an important study and if these findings hold up at other institutions, which they most certainly will, this suggests that the case-mix of revision and primary hip arthroplasty must be taken into account when SSI rates are reported and hospitals compared. Perhaps an easier solution, as the authors suggest, is to treat them as two different animals, if you will, and report them separately. Also, if one wanted to target specific infections or high-risk procedures, these results suggest targeting revision surgeries over primary ones.

Note: Surbhi is joining the group at my old Maryland stomping grounds and I know everyone is excited for her to arrive.