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What is ADAMTS13?

ADAMTS13 stands for "A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13". If you can't remember that, just remember that it's also known as von Willebrand factor-cleaving protease. It's easy to see, though, how you could mistake it for someone's online username or handle. Or even AIM screenname if I may date myself. I wouldn't be surprised if someone has that taken as a handle on PathLabTalk.

ADAMTS13 is a important protein involved in regulating coagulation. It helps to break up unusually large von Willebrand factor monomers and vWf/platelet complexes. It interacts with von Willebrand factor to aid in the breakdown of platelet-vWf complexes that may start to form. As we know, platelets bind to vWf to form these complexes to ultimately aid in platelet adhesion at a wound site. ADAMTS13 essentially cuts through these complexes breaking them into smaller pieces, which can then be further broken down by proteases if need be. 

Without the ability to breakdown these unusually large monomers of vWf, they can continue to grow unaffected. As platelets pass through circulation they will begin to adhere and bind to these monomers increasing in size. This not only increases the risk of a large thrombus or clot forming within the vasculature, it also starts to use up platelets in the process making them unusable, were an actual bleed to form. As such you can start to have clotting issues and bleeding issues concurrently as platelets start to get used up in the every growing thrombus.

 TTP (Thrombotic Thrombocytopenic Purpura)

ADAMTS13 deficiency is well noted in TTP (Thrombotic Thrombocytopenic Purpura). A large number of patients with TTP have abnormal ADAMTS13 enzyme activity. Lowered ADAMTS13 activity can be seen in other conditions (such as Hemolytic Uremic Syndrome), however, an activity of less than 10% is highly suggestive of TTP given other clinical correlations. This deficiency can be acquired or genetic. Mutations in the ADAMTS13 gene will give rise to impaired enzyme production and function. As a congenital disease, this is known as Upshaw–Schulman syndrome. This is rare and an autoimmune (acquired) etiology is the more likely culprit of pathology. 

The large majority of ADAMTS13 deficiency is caused by an autoantibody/inhibitor antibody. These antibodies either inhibit the enzyme from functioning or by clearing the enzyme from circulation completely by tagging it for destruction. 

If TTP is suspected or known, platelet transfusions are generally contraindicated, unless under emergent necessitations. Platelet transfusions will simply add to the growing platelet/vWf complexes, potentially causing furthering clotting and additional microangiopathic pathology. 

Acquired TTP treatments? 

Plasmapheresis -- Plasma exchange procedures have changed the game for patients with acquired TTP. Decades ago prognosis was usually rather poor. Today, however, with proper and timely diagnosis many patients have a much more positive prognosis and outlook. The apheresis section of the Transfusion Medicine department will perform this procedure. Many smaller hospitals will not include an apheresis section in their Transfusion Medicine department. Larger hospitals and hospital systems are much more likely to include an apheresis program within the department. The idea behind performing a PLEX or PEX (Plasma Exchange) is simple. The apheresis machine will remove the patient's plasma (leaving RBCs and platelets in tact) and replace it with an equal amount of donor FFP. This does two important things. By removing the patient's plasma, it is removing the ADAMTS13 antibody/inhibitor. By replacing with donor FFP, you are adding  ADAMTS13 enzyme into circulation of the patient as the donors are likely to have healthy levels of ADAMTS13. 

This is not a one time procedure. Patients will generally get daily or every other day apheresis treatments until their platelet counts return to normal (usually above 150K). If there is evidence that their platelet counts are starting to drop again, more procedures may be required. 

Other treatments that can be concurrent with plasma exchanges are heavy steroids to suppress the immune system from creating more antibody/inhibitor as well as Rituximab which is a monoclonal antibody treatment that targets CD20 on the surface B cells and targets them for removal. By targeting B cells for removal, they will not be able produce antibodies towards ADAMTS13.

Duffy The Vampire Slayer?

Duffy Null (Fya-Fyb-) and Malaria

 The prevailing thought was that Duffy negative (Fya-Fyb-) phenotypes typically found throughout the African population confer a level of immunity, protection, against Plasmodium vivax (and Plasmodium knowlesi) infection typically spread through Anopheles mosquito encounters.

 Nearly 75% of African descended humans have the Duffy negative phenotype Fya- Fyb-. Some populations and regions of Africa may have an even higher prevalence of such phenotype. The significance of this is...much like sickled cells are thought to provide protection against Plasmodium falciparum infection, lacking the Duffy antigen system is thought to provide protection against Plasmodium vivax infection, considering P. vivax uses the Duffy antigen system as a vector for infection.

 Duffy negative phenotype carriers (Fya-Fyb-) will typically carry a "silent" Fy-b allele. However, it does not get expressed on red blood cells due to a mutation that causes an encoding failure within the Erythroid Transcription Factor or "GATA-1". This produces a red cell that fails to carry and express the Fy-b antigen. It is highly unlikely that a patient would develop Anti-Fyb, because Fyb would still be expressed throughout other bodily tissues. Thus, the immune system recognizes the Fyb antigen as part of 'self'. For transfusion purposes, most transfusion centers would consider the patient to be Fyb positive *for transfusion purposes* and could receive Fyb positive blood. It is very unlikely the patient would mount a reaction to Fyb+ red cells. There is an even more rare mutation that does cause complete lack of Duffy antigen throughout the body

The Duffy antigen, known, wholly as DARC or Duffy Antigen Chemokine Receptor (shouldn't that be DACR then??) acts as a receptor for the parasite to invade the cell and begin the stages of infection. For decades of time it was believed this gave full protection over Plasmodium infection. Unfortunately, this is not the full case. As time went on and more studies are taken on, it is being noticed that Fya/Fyb patients do have Plasmodium infection. If anything, it appears there could be less severe infection with noticeable asymptomatic carriers of the disease in Duffy negative patients. It is possible that with time, Plasmodium will mutate further and cause considerably more damage in Duffy negative patients. 

More studies are still needing to look into this play between Duffy and Plasmodium. Don't try to load up on Duffy negative blood before traveling to endemic areas... that would be silly.

Platelet Additive Solution (PAS) Platelets and Their Variations

 What's the difference between all of the different types of PAS platelet variations? Maybe you're only used to seeing one type, such as PAS-C, but there are other variations out there. 

What are Platelet Additive Solution (PAS) platelets?

Well by actual volume, a whole-blood derived or apheresis platelet collection consists of mostly plasma. The platelets are suspended in the donor's plasma, ready for transfusion. To say "the blood type on a bag of platelets doesn't matter, just give the patient whatever" is not exactly true. I've heard this sentiment echoed in smaller hospitals and smaller blood banks where platelet inventory may not be as well stocked. You wouldn't think to give ABO incompatible plasma to a patient...right?

PAS Platelet Advantages

The major advantage to PAS platelets is the ability to transfuse out of type/ABO incompatible platelets to patients with a largely decreased risk of transfusion reaction. To accomplish this, 60-70% of the plasma is removed from the platelet donation and replaced with an isotonic crystalloid additive. This additive is devoid of ABO antibodies and other plasma proteins that could potentially cause an allergic reaction. ABO antibody titers are shown to be significantly reduced following this process. If your facility performs ABO titers on platelets already, perhaps PAS platelets can free up extra time in your daily process by no longer requiring titers to be performed on in-house platelet inventory. In many studies, HLA antibodies that were detected in non-PAS platelets, were now undetectable after undergoing PAS addition. This could potentially lower the risk of TRALI in patients as well. 

All of the additives contain normal saline (NS) for osmotic balance. 

PAS-A -- Citrate, Phosphate, Potassium 

PAS-B (T-SOL,SSP, PAS II) -- Citrate, Acetate (Removed Phosphate, K+, and added Acetate) Acetate was a key addition, as it helped with platelet shelf stability. It works in tandem with platelets active metabolism to ensure lactic acid does not build up during storage, as this would be detrimental. It does this by offering acetate as an energy source rather than glucose, as glycolysis would result in lactate, lowering pH levels. This product is not licensed in the USA.

PAS-C (PAS III/Intersol) -- Citrate, Phosphate, Acetate (Added phosphate). The re-addition of phosphate was key in adding extra buffering capability to the platelet solution. This keeps the pH of the solution stable as shelf life increases and the platelet storage lesion increases. Additionally this is helpful if the platelets were to undergo pathogen-inactivation, such as through the Intercept method. The Intercept method uses a compound known as Amotosalen-HCl. This could potentially acidify the environment, thus phosphate acts as a buffer to keep pH within a safe limit. An overly acidic environment would be detrimental to the platelets storage health. PAS-C is currently the most commonly used platelet additive solution in the US.

PAS-D (Composol PS) -- Citrate, Acetate, Magnesium, Potassium, Gluconate. The addition of Mg and K+ has shown diminished storage legion effects over the course of the platelet's shelf life and has a positive effect on inhibiting platelet aggregation/activation. pH was more stable as well as lactate production was lowered. Gluconate was added for its chelating abilities. It is thought that reducing the build of of calcium could result in lowered platelet activation and potentially even recover from early activation stages. Calcium is reduced by binding (chelating) with gluconate, usually in the form of Sodium gluconate. 

PAS-E (PASIIIM/SSP+/T-PAS+) -- Citrate, Phosphate, Acetate, Magnesium, Potassium. What does SSP even stand for? It seems to be Safe (Platelets Through) Saved Plasma and is a trademark by MacoPharma. Manufacturers of PAS-E allow up to 80% plasma replacement with this product. 

PAS-F (PlasmaLyte A, Isoplate) -- Acetate, Magnesium, Potassium, and Gluconate. This link provides a look into the actual packet insert of a platelet additive solution with all indications, warnings, etc. 

PAS-G (M-SOL) -- Citrate, Phosphate, Acetate, Mg, Potassium, and Glucose. Although most civilians likely have never needed to deal with cryopreserved (frozen) platelets, they do still exist and perhaps better formations and technology can bolster more widespread usage of the product. Platelets with PAS-G added, saw the best platelet recovery percentage post thaw. 

Platelet Additive Solution Table

Although many PAS products share the same composition, the concentration of these additives may differ per dose.

Why not 100% PAS and no plasma? Well we're getting there! There is no perfect additive solution on the market today. Current in-use technologies require roughly 30% plasma to remain in solution. Anything less and platelets begin to suffer from quality issues. Newer PAS formulations are allowing for less and less plasma to be kept however. It is postulated that with the addition of glucose and other constituents such as bicarbonate (for buffering purposes), we can get very close to removing 100% of plasma from the platelet concentrate.