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Selection of M- red cells

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We have an antenatal patient with previously detected anti-M.

We referred the booking sample to our RCI lab who did not titre the anti-M as it did not react by IAT.

The report we got back recommended we select M- units for cross-matching by IAT.

This is contrary to the British Society for Haematology guidelines which say M- must be selected only if detected at 37oC.

When I queried the advice I was told this is their policy.

Any thoughts?

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I agree with you that it does sound daft, but I am in the position to tell you why this is the recommendation, despite it apparently being contrary to BSH Guidelines, as I was still working when the decision was made (albeit, I don't agree with it!).

The huge majority of hospital blood transfusion laboratories now use column agglutination technology (CAT) as their "first line of attack", and many of them use the CAT that uses gel in the column, rather than glass beads.  This form of CAT is particularly adept at detecting anti-M in plasma by IAT, even though the anti-M may not actually be reacting at strictly 37oC.  There are several reasons as to why this is so, but a couple of them are that "cold" reacting IgM anti-M can sensitise red cells particularly quickly (in a matter of seconds at room temperature), but can take quite a time, even at 37oC, to dissociate back off.  This means that the anti-M is often still sensitising the red cells at the end of the incubation time and, of course, centrifugation is, in itself, a potentiator of agglutination.  In addition, the gel in the column is at a slightly low pH, and many examples of anti-M "like" a slightly acid environment to sensitise the M antigen.  This all means that this particular kind of CAT is very good at detecting anti-M, but making it appear to react at 37oC, when, actually, it doesn't.

This left the RCI Laboratories with a dilemma.  Cross-matching blood by tube at strictly 37oC takes a long time, and the RCI Laboratories are not, shall we say, over staffed!  This meant that RCI Laboratories were being stretched to breaking point by having to cross-match for samples containing anti-M, as so few hospital laboratories could now perform IAT by tube technique (particularly with the need to demonstrate competence in the technique being used).  The alternative was to test many more units for the M antigen, and to provide M Negative units to the hospitals, whether they use CAT of that type or not, so that they could perform their own cross-matching, and actually end up with compatible units.  In theory, this was great, but it left the RCI Laboratories with fewer phenotyped units with which to select for, say Fy(a-b+) for a patient with anti-Fya, as the M Negative units that were also Fya Negative had been sent out to cover a cross-match in a patient who had an anti-M, who didn't actually need M Negative blood.

Having said all of that (and I KNOW I said it at length), anti-M can, rarely, cause severe haemolytic disease of the foetus and new-born (particularly in people from the Far East, even when the anti-M does NOT react overtly at 37oC), and so I have a certain amount of sympathy with giving M Negative units, even in the situation in which you find yourself with your patient, as M Positive units, albeit found to be compatible by IAT, could well stimulate the anti-M to become stronger during the pregnancy, "turn it" to an IgG production and increase its (at present, non-existent) IAT titre.

I hope that helps and that you haven't fallen asleep reading it!!!!!!!!

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Thanks Malcolm, I was hoping you would reply.

I didn't fall asleep but had to reread a few times!

I left their employ 4 years ago. I don't recall selecting M- units if the anti-M was not detected by IAT. I did wonder if the policy had changed since I left, but from what you say it was already in place.

The unfortunate thing for us is that we produce our own red cells, which are compatible and (probably) safe for these patients, but we have to import M- units from the UK due to the RCI report. This means we have to pay for the units and the cost of air freight for blood which we are unlikely to use for the antenatal patient.(To cover delivery). (Rant over)

Having said (all of) that, we have a not insignificant number of residents of Thai stock who may fall into the category of been at risk of severe HDFN from a non-detectable anti-M! 

So maybe we will have to accept it.



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