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False Positive DAT CAP Survey


Logan51

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Hello,

I recently received notification of a false-positive IgG DAT result on a survey.  The notification stated that this was caused by IgG contamination in the survey sample, and that because of this, we weren't graded on our response.   In reviewing the other participant responses, most gel users reported the same result as us; compared to almost none for users of other methods.

Our QC protocols for the IgG DAT call for using A1 cells as a negative control, which has always resulted as expected.  So I had hoped to prove for certain that this is an issue with the survey sample, and not with our assay, by running our own Complement Control Cells as a negative control in an IgG Card.  To my surprise however, the Complement Control Cells that we use for our IgG/C3 cards also yield a positive result in our IgG cards.  The manufacturer of the Complement Control Cells has simply stated that their cells are designed to be run as a control for poly-specific or complement-specific assays, and are not meant to be used as QC for an IgG-specific assay.

My first thought is that our Proficiency Testing Program purchased their cells from the same manufacturer as our Complement Control Cells; and that the written statement from the PT Program, along with statements from our gel manufacturer should sufficiently prove that our assay is working as it should.  That being said, it is a little unsettling that at this moment, I am unable to prove that our IgG DAT assay truly won't react against Complement Coated Cells (if those cells aren't also coated in IgG).  I am tempted to purchase a vial of Anti-C3, which we don't currently use in-house, so that I may manufacture my own C3-Coated Cells and demonstrate a negative result in an IgG Specific DAT.  I realize now though that I have never tried doing this before, and before placing the order I was wondering if anyone can confirm that adding Anti-C3 to an RBC sample would successfully create cells that could be used for this purpose?

Thanks for any help

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This issue has been going on for years.  The complement coated cells are produced artificially.  The method used is designed to be used to show positive reactivity with anti-C3 in a tube technique.  It is not designed to show absence of reactivity in anti-IgG, neither in a tube, or even less in gel.  Indeed the method used will ALWAYS show an unwanted positive reaction in anti-IgG if you use gel.  CAP are aware of this - enough people have complained about this over the years.  Why they don't do something about this is a mystery to me

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Logan 51:  I don't understand why you are thinking about changing anything.  You have the notice from your survey organization that the sample was contaminated, and I am assuming that your testing showed reactivity with Anti-IgG?  I do understand you being troubled by the C3d control cells reacting with anti-IgG, but it seems that you got the result that you should have gotten.

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As Galvani states, this issue returns every year. 

CAP continues to send out the cells complement coated using a method that is KNOWN to cause cross reactivity with anti-IgG, particularly in CAT methods, hence their notification.

It seems that not all Cap subscribers outside USA get this notification, and are therefore in a difficult position: fail.

Interestingly, the CAP website states "for use with manual methods".  What do they mean:not for CAT methods? Tube only? ...

I hope some CAP representatives are reading this blog, and will finally talk to the IH community and do something about this situation. 

 

 

CAPStatement.png

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