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Cord Blood DAT


MinerJ

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Hi All,

Its been a while since I came back to this forum, but glad I feel like I have gained a lot more insight. I feel like I'm a bottomless cup. 

So I come with a question, for which there is not going to be a definitive answer (but with BB, is there ever one?), but hopefully, I would gain a bit of understanding.

Background: So, our laboratory has started sending samples to reference laboratory for genotyping of the foetus by FDNA, which is great, since we would figure out the Rh(D) status of the baby (on most occasions) before they are born! So our laboratory has set up a flow chart which basically mentions that you do not need to send cord sample of Rh(D) negative mother if the baby is shown to be Rh(D) positive (or D positive, I am quite wary when trying to talk about Rh group), and only send cord if baby of Rh(D) negative mother if the FDNA shows the baby is Rh(D) negative, just to confirm the accuracy of FDNA. It sounds kinda counterintuitive, but we will soon be just not processing any cord sample for the ones we performed FDNA on. That means no cord Blood Group or DAT on a lot of post-delivery patients.

Question:  By missing out DAT, we would possibly be missing out on detecting ABO incompatible HDN. How significant do you think it is in the early stages? Is it OK to wait to see if the patient shows signs of jaundice and for them to send a DCT sample afterwards? 

Bonus Question: What does your Hospital/Laboratory do in the event of positive DAT on cord sample, and why do you do it?

I had a read through one of the articles stating about the significant of DAT, but they called the Rh blood group as Rhesus, so I'm not going to take them too seriously

pdf

Cheers,

Jermin

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Hi Jermin

We took up FDNA testing just over a year ago but still test all cord samples from D negative mothers to confirm the accuracy of the FDNA results.  We perform a retrospective comparison of results on a weekly basis and have found a number of discrepancies during that time.  All of these discrepancies have been carefully investigated by us and IBGRL, where appropriate.  Two of them were false positive FDNA results whereby the phenotype of the neonate didn't match the genotype...this scenario (D negative baby following a D positive genotype test result) will occur, albeit rarely, owing to the presence of a non-expressed RHD gene.  We have also had two false negative FDNA results due to insufficient foetal DNA in the maternal blood sample.  By testing the cord the risk to the Mum was minimized as she was able to receive post-natal anti-D immunoglobulin.  Rather more worrying, two discrepancies have highlighted poor practice on labour ward - a cord that was sampled from the sluice (yes, really) that IBGRL was able to show couldn't have belonged to the mother in question and twins who were initially both grouped as D negative but patient recall showed one of the them to be D positive!  By testing the cord and feeding-back any discrepancies to IBGRL it also helps to inform the accuracy of the test.

Although we group the cord samples, we stopped performing DATs on these samples many years ago and only perform a DAT in the presence of maternal antibodies or for clinical investigation of jaundice.

 

Hope this helps

 

Letty

 

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I would TOTALLY agree with Letty's post.  Indeed, the reports from the IBGRL ALWAYS state that there may be discrepancies from time to time - not least because there may be insufficient cffDNA harvested from the maternal circulation, and because of mutant RHD genes.  They are few and far between, but if you read any paper by Kirstin Finning, who runs that laboratory, she never quotes 100% accuracy.

Most cases of ABO HDFN are sub-clinical (just need a bit of phototherapy) and, quite often, the DAT is negative at birth anyway, and "develops" a couple of days into life.  If the ABO HDFN is clinically significant, the doctors, nurses and midwives should easily be able to see the symptoms and then order a DAT.

In terms of tests on a positive DAT, we would only perform elutions if the baby is affected or we know that the mother has an alloantibody.

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10 hours ago, Letty said:

a cord that was sampled from the sluice (yes, really)

I don't even know how to begin to question that.

Thanks for your replies as it has really helped. Since we are on the topic of FDNA, our laboratory is still getting our head around the accuracy of the test. If the FDNA can give a false negative result, and the accuracy as stated on reports show there is a chance of discrepancy, should we take that risk? Shouldn't we either fully be behind FDNA result, or we are not. If FDNA initially showed D negative, but the baby turns out to be D positive, then wouldn't there have been a chance patient would have had a sensitising incident long before we figured out the true D status?

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The results from both the study in the Netherlands and the UK have shown that the accuracy of the testing of cffDNA is extremely accurate (no test is ever 100% accurate).  In the case of the UK, the testing has been thoroughly studied by NICE (a tough lot of people to convince, believe me!), and if they recommend it, then I think you would be justified in getting fully behind it.  Yes, there will be the odd discrepancy, but these have been demonstrated to be very few and far between (and have also been explained).  I really would suggest you read some of the latest papers by Ellen van der Schoot and/or Kirstin Finning if you need convincing, and even, perhaps, the NICE Guidelines (but be aware that the nomenclature in the NICE Guidelines is appalling!).

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Hi Malcolm,

I agree with what you have to say. I have no problem relying on FDNA result, as accuracy is pretty damn high. The people who we are trying to convince/explain are the Midwives who don't understand why we are double checking something if you are meant to rely on it the initial result- and we kinda understand what they have to say. I will read the latest papers as you mentioned. I'm sure it would be helpful, 

Thanks,

Jermin

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1 hour ago, Jermin said:

Hi Malcolm,

I agree with what you have to say. I have no problem relying on FDNA result, as accuracy is pretty damn high. The people who we are trying to convince/explain are the Midwives who don't understand why we are double checking something if you are meant to rely on it the initial result- and we kinda understand what they have to say. I will read the latest papers as you mentioned. I'm sure it would be helpful, 

Thanks,

Jermin

I know what you mean Jermin.  I have had problems, on occasions, convincing midwives that we do need to test samples from pregnant ladies at 28 weeks of pregnancy and also, more worryingly, convincing them that we need to regularly test samples throughout the pregnancy when a pregnant woman has an atypical antibody known to cause severe HDFN.

I always wonder why some of them (a small minority, thank goodness) think that they know more than the experts (the people who work in transfusion science, and doctors who work in transfusion medicine), when none of us (I hope) would claim to be experts in their particular field.

END OF RANT!  :angered::angered::angered::angered::angered:

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