AF Association
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Unfortunately ended up in my local hospital due to tachycardia. Everything was going well with medics attention, just one problem.  They asked to take blood for testing etc.  Four phials were successfully taken, then off to lab.  Quite a while later medic asked if he could take another phial - one lot had clotted before the test had been dealt with! So off went the second phial, later on the poor same medic came along and apologised - 2 of the phials had clotted!! Please, please anyone have answer to this?!  I am on Pradaxa, was wondering if the blood once it has been released from the vein/artery does clot a lot quicker due to it being thinned out so much in the system?

8 Replies

Don't understand that 


Hi Migmog

I don't understand it either, firstly the blood test is venal not aterial there are very few blood tests carried out on arteries.

You did say "quite a while later" it's possible for example that it was stored badly or that the test was particularly sensitive to clotting, and need to be done on a very fresh sample of blood, or many other things I suppose.

I'd be tempted to ring an anti-coagulation clinic however and run it past them, I am told there is a very complicated test not usually carried out in hospitals but labs, which can check the effectiveness of the NOACs such as Pradaxa, not sure if they have access to such a thing. But they should be able to point you in the right direction and advise you further.

Be well


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I'm not medically trained either but I do know that normally there is a preservative in the phials to prevent this happening. I think you should blame somebody else frankly and not worry about it since this seems to be a problem in phlebotomy.


I totally agree with Bob! This should not happen. My Dad worked as a biomedical scientist testing samples his whole career, and enjoyed telling us all the, not always interesting, details. 

I also used to work in a vets and was involved in taking samples. 

Firstly, they should be using a cannula and what's called a vacutainer (sample tube) , which ensures the blood doesn't come into contact with the air. The vacutainer contains an anticoagulant , which varies depending on the test being run. Lithium heparin is one, another is EDTA. 

Some old school doctors will use a syringe instead of a cannula (this is also how it is done in vets), and clotting can occur if the blood is not in the blood tube ASAP. This may occur if blood pressure is low for example. 

It is relativey common in the vet world for samples to be clotted. Unfortunately, due to the size of the veins, and the fact it could be a sample from the jugular vein (in the neck), cannulas are not used in small animals in most practices. I suspect cost is a factor too.

Sorry for the ramble, but hope you get my point that your blood should not have clotted in the sample tube.

Sounds like an error has been made somewhere. 


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I agree with Bex. 

The person taking the blood has not mixed the anticoagulant in the tube with the blood properly. You often see this when a few blood samples are required. The first tube filled is just put down while the others are filled.

Don't worry.


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Thanks to all for reassurance.   Once again this morning (1.30) back in A&E. Tachycardia once again let's me know it's alive and kicking!! Thankfully no problems with blood congealing this time. So see how the next few days go.


I am a nurse and have seen blood samples clot in the sample bottle for no apparent people who are not on anticoagulants and who are very healthy. Labs have never raised any concerns about this phenomenon and neither have doctors but we have been told to just repeat the test. I wouldn't think too much into it to be honest and believe me, I am very prudent so if I though t it were of concern, I would not say so. I will ask in work tomorrow if you like?? 


Thanks ever so much Vony for your reassurance. Am prepared for anything going wrong now!! 


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