Why would you wash red cells for patients with PNH?

Q. I’m currently doing my rotations at Children’s Memorial Hospital’s blood bank and I was reading the standard operating procedure for washing red cells. One of the conditions in which they need to be washed is paroxysmal nocturnal hemoglobinuria. Do you know why this is?

A. The reason for washing red cells for people with PNH is to get rid of any ABO incompatible plasma.

In a person without PNH, ABO incompatible plasma doesn’t cause any perceptible hemolysis. The antibodies in the donor unit probably just get diluted out enough that they don’t have much of an effect. Or perhaps they get sopped up by other ABO antigens on other cells (did you know that you have A and B antigens on cells besides red cells?! Weird.).

But patients with PNH are super susceptible to complement-induced red cell destruction. They lack the ability to anchor certain proteins (including proteins that protect the red cell against complement) to the red cell membrane. Patients with PNH have a hard time down-regulating even a small amount of complement activation – so theoretically, a transfusion of even a small amount of non-ABO compatible plasma could lead to hemolysis. There have been a few cases of hemolytic transfusion reactions in patients with PNH that have been attributed to this phenomenon…so in 1948, blood banks began washing red cells before giving them to patients with PNH.

However, this practice has been called into question. The Mayo Clinic reviewed 38 years of experience with transfusing patients with PNH, and only found one documented episode of post-transfusion hemolysis). Their conclusion was that the important thing is to use group-specific blood products for patients with PNH; washing seems to be an unnecessary precaution.

Coag tests in DIC

Q. I had some confusion on why the PT, PTT, and TT are prolonged in disseminated intravascular coagulation. Intuitively I thought they might be shorter because everything is already present and turned on due to the constant state of coagulation, but the only way I can think it might be prolonged would be that are the factors being used up which then shows up as a long PT, PTT and TT?  If you could just clarify that for me that would be great.

A. Yes! That’s exactly why they are prolonged! In disseminated intravascular coagulation (DIC) there’s a ton of clotting going on – so the platelets and coag factors are getting used up. As the coag factors get used up, the PT (prothrombin time), PTT (partial thromboplastin time) and TT (thrombin time) go up. You also see increased FDPs (fibrin degradation products) – but that’s an incredibly sensitive test, best used for other purposes.

By the way, Ed’s Pathology Notes has a way to remember the seriousness of DIC – he calls it “Death is Coming.”

Coagulation tests in 500 words or less

First, a quick review of how a blood clot is formed

Here’s how you make a clot: 

  1. Constrict the affected blood vessel
  2. Form a platelet plug
  3. Form fibrin (using the coagulation cascade) to seal up the platelet plug 

If you think back to the basics of the coagulation cascade, you might recall that there are two arms – an extrinsic arm and an intrinsic arm – which come together in the final common pathway, which ends up turning fibrinogen into fibrin. When somebody is bleeding, and you think it’s due to a coagulation problem (as opposed to a platelet problem), it’s helpful to know what part of the cascade is screwed up. That helps you figure out what’s wrong with the patient (is it hemophilia? or liver disease? or coumadin overdose?).

The two main coagulation lab tests

There are two main tests for evaluating the cascade: one for the extrinsic arm (the PT/INR) and one for the intrinsic arm (the PTT). There are other tests too – but those will have to be for another post.

1. The PT/INR

The prothrombin time (PT) is performed by adding thromboplastin to the patient’s plasma, and seeing how long it takes to make fibrin (as soon as you see the first strands of fibrin form, the test is over, and you measure the result in seconds.

Thromboplastin contains a tissue-factor-like substance, and therefore it just measures the extrinsic pathway. The annoying thing about thromboplastin is that its strength varies considerably from manufacturer to manufacturer. So the PT done at one hospital (using manufacturer A’s thromboplastin) will be significantly different than the PT done at another hospital (using manufacturer B’s thromboplastin). Dumb.

To deal with this annoying issue, someone figured out how to make some mathematical calculations that take into account each manufacturer’s thromboplastin strength. Now, you just do your PT assay, add on those mathematical calculations, and you wind up with something called the International Normalized Ratio (INR). This means that you can do the PT with anybody’s thromboplastin, and you’ll wind up with a result (the INR) that removes that variability. Super important. 

2. The PTT

The PTT, or partial thromboplastin time, is performed by adding just some phospholipid to the patient’s plasma and waiting to see how long it takes to form fibrin. You have to add in a little “contact factor” like kaolin to activate XI to XIa – but other than that, that’s it!

It’s called the “partial thromboplastin” time because initially, it was found that by adding a part of thromboplastin to a test tube, you could activate fibrin formation. It turns out that the part of thromboplastin people were adding was just phospholipid, and that thromboplastin consists of both phospholipid and tissue factor. This test measures the intrinsic pathway, which is that arm of the cascade involving factors XI, IX, VIII and the final common pathway.

 

Are HUS and TTP related?


Q. Is there a relationship between hemolytic-uremic syndrome and thrombotic thrombocytopenic purpura? I see them lumped together a lot. (more…)