Influenza B with positive strep PCR

[fireguy]

Anyone else seeing a trend where patients present for typical flu symptoms (fever, malaise, chills, cough, congestion) and the nursing staff reflexively does a strep and fly swab with BOTH the strep and flu B coming back positive?  Noticed this last year and have seen multiple cases this year.  My shop uses PCR for both strep and flu testing. Most of the patients have NO strep symptoms (normal pharynx, normal tonsils).  Seems like a lot of carriers with Flu B but not Flu A if it is indeed a carrier testing positive situation.  I suspect there is something in teh strep PCR that gives a false positive with Flu B but have no literature or information to prove and validate so my director is mandating (with pressure from infection control nurse) that we treat ALL positive strep patients regardless of symptoms,   

 

any thoughts?

 

Ben

[LT_Oneal_PAC]

A significant portion of the population are carriers. IIRC, something like a quarter of patients.

[fireguy]

yeah I knew that.  But if it were simply that they are all carriers, why am I only seeing it with flu B and not flu A?  if it were a carrier thing, then the numbers should be comparable

 

[LT_Oneal_PAC]

10 hours ago, fireguy said:

yeah I knew that.  But if it were simply that they are all carriers, why am I only seeing it with flu B and not flu A?  if it were a carrier thing, then the numbers should be comparable

 

Guess I don’t know. Unless you’re seeing a statistical aberration in a small sample size. I don’t see why a PCR detecting bacterial DNR would respond positively to RNA from a virus.

[kargiver]

Its theoretically possible if the intermediate strand of cDNA made in the RT-PCR bears some semblance to the Strep DNA being tested (specifically where the specific primers sit for amplification of the DNA fragment identifying strep or influenza B).  Since they are different reactions, the primers used to amplify fragments of either flu or strep would be limited to their respective tubes, and with RT-PCR quantitative increases are read in the instrument during the cycling phase to show amplification of the respective fragments.  What needs to be clarified is if gels are being done dost RT-qPCR to look at fragment size (and if so, how big are the respective fragments for flu vs. strep).  If not, and there is cross reactivity between primer sites, then yes, it is possible a flawed assay is being used.  

However, when I used to design PCR and RT-qPCR assays, I personally would make sure there was nothing cross-reactive (and certainly nothing cross-reactive for pathogens that are known to infect similar parts of the body) so if a company is selling this product (and assuming it is FDA-approved for testing) I strongly doubt there is a primer issue with cross-reactivity.  

Without knowing the primers being used and the sequences targeted, can't say with 100% certainty why you are seeing some "cross-reactivity" but I suspect that you are catching carriers for Strep as well.

G

[Parashooter79]

If you suspect contamination I would suggest ordering a culture as well. You can have prelim results in about 24 hours on a blood plate. Have the Micro lead do a comparison study to figure out the issue. As with anything Micro related you must understand even the best technique can have contamination.