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Question on guidelines for compatibility testing

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Hi everyone

I would like to ask a question related to the 'Guidelines for compatibility procedures in blood transfusion laboratories' prepared by the BCSH.

In section 8.6, The immediate Spin XM, there is written that this technique cannot be used in patients where ABO grouping reveals very weak anti-A or anti-B (except in group AB patients).

I would like to know if this weak reaction can be quantified; for example: you cannot use the IS if you obtain a 2+ reaction or lower using CAT.

I think that a laboratory should have a standard cut-off reaction to determine which patients can have units allocated by the IS.

I would like to know your opinions on this matter.

Best regards


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Hi Jesmond,

This was the reply. Hope it helps.

Thanks for the opportunity of commenting on this Malcolm!

I think this is common sense really - if the anti-A or -B is difficult to detect in a reverse group it's going to be difficult to detect in an I/S crossmatch. If a laboratory wants to define a cut-off they would probably need to undertaken some validation of their I/S technique compared with their reverse group technique, using some weak antibody examples. You would be looking for a 'clear' positive reaction - I guess a 2+ would be safe.

Sorry, not very helpful!


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Hi Malcolm

Thanks for your response. Don't worry, it will do. At least I know that each lab can define its own cutoff, and that there is no other guidelines and recommendations.

Thank you again for your great help


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So - if the patient has a weak (<2+) reverse type does this mean that an antiglobulin crossmatch becomes necessary? Here in the USofA the FDA has determined that for those of us gel users an ISxm is necessary when we perform an IgGxm in gel. During my validation of the gel xm for ABO compatibility detection I found that pts with no detectable reverse grouping were IgG compatible even though the ABO type between pt/donor was definitely incompatible.

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