ICU ID Secrets (following up on my post* from last week):
Ten things to remember about methicillin-resistant Staphylococcus aureus (MRSA) polymerase chain reaction (PCR) nasal swabs:
Ten things to remember about methicillin-resistant Staphylococcus aureus (MRSA) polymerase chain reaction (PCR) nasal swabs:
1. The MRSA nasal PCR is mostly helpful in patients with pneumonia or at least high suspicion of it since nasal colonization correlates with MRSA presence in the rest of the respiratory tract
2. The MRSA nasal PCR is mostly useful for its high negative predictive value (NPV) for MRSA pneumonia. The NPV number to remember is 95%. The use of MRSA PCR screening in pneumonia can reduce length of stay and antibiotic costs
3. The MRSA nasal PCR is unhelpful in predicting the likelihood of MRSA pneumonia since its positive predictive value (PPV) is not great. The PPV number to remember is 30%. This happens because the upper airways' colonization with MRSA is quite frequent while true MRSA pneumonia
is quite infrequent. In other words, a patient with pneumonia AND a positive MRSA PCR is still MORE likely to have a different organism causing pneumonia (and not MRSA!)
4. Regarding previous points #2 and #3: Unlike sensitivity and specificity, PPV and NPV depend on disease prevalence and move in opposite directions as prevalence increases or decreases. For example, the higher the prevalence of MRSA pneumonia in the ICU, the higher the PPV and
the lower the NPV. Knowing the microbiology in your ICU will help you use the nasal MRSA PCR result more efficiently
5. MRSA pneumonia is usually aggressive; patient is โsepticโ and the lungs can be destroyed. If the patient improves overnight and has an unimpressive lung infiltrate, a positive MRSA PCR likely represents colonization
6. The sensitivity of the nasal MRSA PCR is reduced if the patient has received anti-MRSA antibiotics for more than 24-48 hours. If you order the test Friday night and on Monday morning you discover that it has still not been sent, it is not going to be very helpful.
The sensitivity of the test also decreases if the nares have been decolonized with mupirocin. Keep this in mind if you work in an ICU with cardio-thoracic patients who frequently undergo pre-operative S aureus screening. Of interest,
antibiotic treatment seems to be less effective in reducing S. aureus colonization in the lower airways, but we donโt have enough data (or I am not aware)
7. Even though there is some correlation between MRSA nasal colonization & colonization elsewhere in the body, itโs probably risky to use a negative MRSA nasal PCR result to rule out the presence of MRSA elsewhere. In addition, the sicker the patient the less trust I would place
on a negative nasal PCR test. Think about a neutropenic, septic patient with a central line: Would you use a negative nasal MRSA PCR test as a justification to avoid MRSA coverage?
8. Use nasal MRSA PCR test as procalcitonin. It may be helpful to โde-escalateโ & simplify the antibiotic regimen, but it should not dictate the decision to initiate MRSA coverage in a crashing septic patient. I would even argue that if a patient with pneumonia is critically ill,
it may be imprudent to de-escalate right after a negative nasal PCR since the implications of a false negative test may be dire
9. The MRSA nasal PCR has equal or higher sensitivity to culture, most notably in patients who receive anti-MRSA antibiotics as the PCR can also detect DNA from non-viable organisms. Also, the PCR also can be performed in a few hours, while the culture result takes days
10. The combination of nasal plus throat culture is more sensitive (up to 20%) than nasal culture for detecting MRSA colonization. Therefore, some authors recommend throat culture among pts with negative nasal MRSA PCR. I have read about it but have not seen it practiced
Thanks for reading!
Please correct my mistakes and share your experience
#foamed #foamcc #meded #medtwitter #medstudent
Please correct my mistakes and share your experience
#foamed #foamcc #meded #medtwitter #medstudent
Useful references:
Dangerfield B, et al. Antimicrobial Agents & Chemotherapy 2014; 58(2): 859-64
Giancola SE, et al. Diagnostic Microbiology & Infectious Disease 2016; 86: 307-10
Baby N, et al. Antimicrobial Agents & Chemotherapy 2017; 61(4): e02432-16
Dangerfield B, et al. Antimicrobial Agents & Chemotherapy 2014; 58(2): 859-64
Giancola SE, et al. Diagnostic Microbiology & Infectious Disease 2016; 86: 307-10
Baby N, et al. Antimicrobial Agents & Chemotherapy 2017; 61(4): e02432-16
* x.com
Loading suggestions...