Diagnosis

Dear Mrs. MRSA, 

I have a large welt on my left forearm.  It is very painful, and seems to be getting larger by the day.  It looks to me like a spider bite, but I haven't seen any spiders around my house so far this year.  A friend told me it could be MRSA.  How would I find out?

Sincerely, 
Confused in California

Dear Confused,

• To diagnose MRSA a health care provider will swab of the inside of the nose, or take a sample of the boil that appears to be infected with MRSA.  The swab is then applied to a petri dish and grown with nutrients that facilitate the growth of s. aureus.  If it turns out that s. aureus is growing it is then tested to see if it can be killed with antibiotics.  If the s. aureus is not killed with the antibiotics it is diagnosed as MRSA.
• The FDA has approved a new diagnostic test that can have results within about 5 hours.  Instead of a swab of the infected site or the inside of the nose, a blood sample is collected.  The blood is collected in tubes that have special ingredients that will help bacteria grow if it is in fact in the blood.  The blood is then examined under the microscope to see if s. aureus is growing.  The blood test is about 95% accurate, but does result in false positives.  It is a good initial screening tool for providers and hospitals so that appropriate care can be taken that MRSA is not spread to other patients.  Positive blood test results are then confirmed with a nasal screening to rule out false positives.  
• For more information about the progression of complications from MRSA visit MRSASurviors.org.  

MRSA vs a spider bite

Image source http://www.webmd.com/skin-problems-and-treatments/understanding-mrsa

Colonization vs. Infection 

• If you are colonized with MRSA you will not necessarily have any symptoms of disease.  You can be colonized with MRSA on your whole body, or just a part of your body.  You could have MRSA and not even know it unless you had a culture grown in the laboratory.  An active infection progression is quite varied depending on the strain of MRSA you have and the part of your body that is infected.  Like I mentioned before the most common areas to have MRSA infection are on skin and soft tissue and surgical sites.  The most serious kinds of MRSA infection are bloodstream infections leading to sepsis, pneumonia, and osteomyelitis.    
• It is very important that MRSA infections be treated right away so the likelihood of complications will decrease. 

References 

• Sullivan KV, Turner NN, Roundtree SS, McGowan KL.  Rapid detection of methicillin-resistand Staphylococcus aureus (MRSA) and methicillin-susceptible Staphylococcus aureus (MSSA) using the KeyPath MRSA/MSSA blood culture test and the BacT/ALERT system in a pediatric population. Arch Pathol Lab Med.  2013; 13(8):1103-1105. doi: http://dxdoi.org/19.5858.arpa.2012-0422-OA
• Jefferson E.  FDA clears first test to quickly diagnose and distinguish MRSA and MSSA. www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm254512.htm.  Updated April 10 2014.  Accessed April 20 2014.  
• MRSA Infection.  MedicineNet website.  http://www.medicinenet.com/mrsa_infection/page7.htm#what_is_the_prognosis_outlook__of_a_mrsa_infection.  Updated February 4 2015.  Accessed April 21 2014.