Basic Coagulation Assays - Application and Interpretation Webinar

Hello everyone. My name is Jacob Dutcher and I'm consultant hematologist in Manchester and I'm very pleased to do this presentation about basic coagulation assays. I work in a big teaching hospital which is a referral Center for almost 2,000,000 population and we our laboratory has got the ability to do almost every known hemostasis essays. But of course we need to start with the basic test which is something which is done almost on a daily basis all around the world in our. Laboratory. Uh, we did a analysis about. Four years ago when we noticed that we do almost 2000 tests for basic coagulation screen in one week. So it's a very common test and I'd like to take you on a journey about how this test came about and how to interpret this test. So before I tell you about the test, the test which is meant to help in stopping bleeding. We should look at how the bleeding stops in all of us, so we'll try to show here this is a blood vessel. And that's one really brand factor, which is usually coiled underneath the blood vessel. And what I try to do here is to make a injury to the blood vessel and the blood might come out. So let's see what actually happens when you have an injury to a blood vessel. The first thing which happens is the blood vessel will try to constrict. So it's a bit like if you have a small hole in the blood vessel wall, it tries to close the hole by shrinking. Now that makes sense because it's useful for it to narrow the pole. We need to bear in mind that this small gap in the blood vessel cannot be. Measured using hemostatic tests because it's a, it's a function of the blood vessel wall. So for example, in when a patient has a surgery and if the surgeon makes a big hole in the blood vessel, you know, and it continues to bleed, then if you do a blood test to find out if the problem with the hemostasis, they may not be a problem. Something we need to bear in mind when we are consulted about bleeding. The second thing which happens is the one really run factor which is coiled underneath the blood vessel. Will open into the blood vessel. And this tends to happen because of the fast blood flow. So the coiled round one willibrand factor will open up. Why does it open up? It opens up because the platelets, which is moving very fast in the blood vessel has to stop here. To stop the platelets, the one willebrand factor will put its arms out and we'll stop the platelets. And this is a very important step because there's no point in the platelets stopping here or here. It has to stop here to help in the stopping the bleeding. So it's a very important step. So once the platelets are caught by the one Willebrand factor. It starts to. Help each other and form a platelet aggregate or platelet clump. So you can see the play aggregate is here and trying to close the gap in the blood vessel wall and stop the bleeding. The next step, which is very important in what we're going to discuss further. Is the clotting factors. Now we all know that the clotting factors are the key thing in forming a clot, but actually the clotting factors can only work. If it sits on top of the platelets, so we need platelets to help in the clotting, we all know that. But how does it help? By allowing a surface for the clotting factors to sit on it and then it forms a clot or the fibrin mesh as we all know. So this is a very interesting, complex and a quick process, but there are a lot of things which is happening which we need to always keep in the back of our mind before we assess someone's blood test. So I told you about how platelets form a surface for the. For the clotting factors to sit on it. So how does it do that this is a platelets? Outer surface or platelet membrane. And you can see here there is something called phosphatidylserine and phosphatidyl ethanolamine, it's called PS and PE. Which is usually in the inner part. So this bit here is the P S&P. And the phosphatidylcholine is on the outer side. PS and PE are negatively charged and PC is positively charged. Now this is a platelet which is not activated. So this is the platelet which is floating in the blood without any activation because there's no injury. But when the patient starts bleeding, the platelets are activated. And it will actually start changing and you can see the phosphatidylserine, the PS is coming outside. And this is done by certain enzymes which will move the inner bit to the outer bit. This is a very, very important. A mechanism because this was found out in in Cardiff, in Wales, in England or in the UK where they found out that if this doesn't happen the patient can bleed. So this movement of the UPS from inside the. Membrane of the platelet to the outside membrane of the platelets is a key step. And what happens then is that you can see this same thing shown in a slightly more elaborate picture here. So you got the platelets, outer membrane and you can see the different clotting factors factor 5, factor 10, Factor 2 sitting on it. So this is what I showed in my first cartoon how clot happens on top of the platelets. And this is a very important thing to bear in mind because now the coagulation system is based on cell based hemostasis. Model. The cell is the platelets and thus the clotting factors. Now this is the original molecular biology of the platelets and it looks very funny picture. But what I've shown here is factor 210, a activated factor 10 activated factor 5. And what's shown here is the activated platelets. And I've shown you the negatively charged top part of the platelets. What tends to happen is when the clotting factors are activated, which is done by vitamin K. Which is why if you block the vitamin K by warfarin or or syndrome, this will not happen. When the clotting factors are activated, they also become negative. And if you add calcium to it, the calcium has got 2 positive signs. So it's called a divalent calcium metal. They bring these two together. So how this happens, as in the clotting factors come on top of the plate list. It's because it's brought together by calcium because of the two negative charges. So coming to the laboratory. What happens in our biglycan lab? We get the blood from a citrated tube where we remove the calcium. The blood, of course, is not clotted. And then recently, forget the blood. Why do we tend to forget the blood? Because we want to remove all the cells which includes the platelets. And we obtained just the plasma for violation test. But so far I've told you that the most important thing in forming a clot is the platelets. But actually we we do the basic coagulation screen. We actually take extra measure to make sure there is no platelets in the sample. So how will the plasma clot without platelets? Now for this, what we have to remember is what people discovered in 1900. And it's called this classic theory of coagulation. And in classic theory of coagulation, there were four things. As 123 and four four clotting factors, let's look at how those came about. So there is this very famous Greek philosopher called hypocrites. And in those days, and they used to sacrifice animals to for religious and various purposes, but he used to examine the blood. And use noted that when the blood was cooled. It clotted. So what he did is he shikshak. He shook the blood. And remove some fibres from it. And then he noticed there's no clot front, so he said that. For the clotting process to happen you need a fibre fibre generator, which is called a fibrinogen. Word comes from fibrin generator fibrinogen. And that's a clotting factor #1. Then in 1800 and 1850, lots and lots of people started studying the blood clot. And one of the things they did is they used varying scale. To measure the weight of blood clot. And they notice that if you weigh the blood clot just before the clot was formed. And after the clock was formed, the wait was higher before the clock was formed and lower when the clock was visible. So they knew that there was something in the blood. Which is making the clot and they call it prothrombin as in something which comes before thrombin or prothrombin. So they were doing a lot of studies on on on the blood clot and they found that if you add certain things and one of the things they used to add was the dead animals brains. You've promoted clotting. And they call it thromboplastin, as in plastic means liking a blood clot. So now we have clotting factor #1. Starting that number 2, clotting factor #3 and clotting factor #4 was calcium. When we do the test in the lab, this thromboplastin which is the dead animals brains is what actually replaced the platelets when we do the test. So actually for the hemostatic test instead of platelets we use thermoplastic. Which brings me to the first Test, which is prothrombin time. It was first found out in 1910. And what Armand Quick, who actually discovered PT, what he did, he said he added thromboplastin to the blood to act like platelets, and of course he added calcium. And in the patient there was normal amount of fibrinogen. So we got fibrinogen present, he added the thromboplastin, he added calcium. So only thing which is different is prothrombin. So he called it prothrombin time, or PT. So now we know that the clotting cascade is more than just four factors. We have now got 13 factors, of course. And we know that this is what happens. I'm sure all of you are aware of the clotting cascade. We have the factor 7 which binds to tissue factor which is actually the thromboplastin. It activates factor 10 which will convert both from into thrombin and thrombin. Converts 5 minutes to fibrin and a little bit of thrombin which is formed, which is why I shown with a smaller arrow will activate factor 8 and 9 and sometimes 11 and this will activate further factor 10 to create a big thrombin burst and thus the big clot. So this is what we know now. But previously there was only four factors, now we have 14 cascade. When you look at PT, it meshes. The factor 7. It's a factor of 10, the factor 2. There's also cool factor, factor 5 and factor one. And the reason for this is because we add a lot of thromboplastin, so if there is a problem with any of these factors, the PT will be affected. So what about APT? So to think about APT, we need to look at some things which happen in 1950s. In 1950s, we already knew that there was something called wow. People knew about hemophilia and they tried to use the PT to diagnose hemophilia. But they couldn't measure. Well, I didn't diagnose him aphelia by using PT. Then they did something which was quite remarkable. As you can see here, they've decreased the amount of thromboplastin. In the sample, so in this one, you have both people, both patients without hemophilia, patients with hemophilia normal thromboplastin, then they diluted the thromboplastin. And you can see if you dilute the thromboplastin a lot. Then you could diagnose hemophilia. Which is different to the test done in and person without hemophilia. So they call it partial thromboplastin because what we've done is you reduce the thromboplastin, which means that you have less factor 7 mediated 10 activation, which means factor 8 becomes more important. And if you go back to our. Chart here. This bit here becomes less important because we have less of that, and that becomes more important, which is of course why we diagnose hemophilia. But the problem in using partial from the Boston is because it can take a long time and we might only get the results by the end of the day when we start in the morning. And that's not practical. So what they did is they used something to activate it and as we all know it could be calling logic, acid or silica. So the word APT comes from activated. Harshu thromboplastin time. And that could actually diagnose patients with factor 8 and factor 9 deficiency as you can see here. So for APT, you can use for diagnosing deficiency of that and that. So now we understand how these things came about. Let's look at one other thing which commonly comes up for discussion in patients with when we do the PT and APTT tests, this is the lupus anticoagulant. Now we all know that in lupus anticoagulant, which is otherwise called the anti phospholipid antibodies, the phospholipid is another name for the tissue factor or the thromboplastin. So it doesn't work. This part doesn't work. We usually do the PT and aptt to look at the factor 7 and all the other clotting factors. But in lupus anticoagulant the problem is with the phospholipid or the tissue factor which is inhibited by. The antibodies. So why is aptt affected by lupus anticoagulant is because this is a problem because tissue factor is blocked by the lupus anticoagulant and the aptt will be prolonged. But remember in PT the amount of tissue factor is much higher. It's not diluted, that's why PT is less affected by lupus anticoagulant. But if the lupus anticlines very strongly positive, then the PT also can be affected by the test. So This is why when we have loop as anticoagulant present in the patient both especially the APTT and the PTS affected. To remove all the phospholipid, in the case of lupus anticoag testing, we use double centrifugation and this to make sure absolutely everything has been removed so that there's no interference. And then as we all know, we use the dilute Russell Viper Venom and what it does is actually it directly activates the factor 10, so it actually removes that step. Where the tissue factor and is needed and it'll go directly to factor 10. This is why typically the Russells Viper venom test or for that matter eccrine test and other tests are used. So there are some variations with respect to PTSD we need to bear in mind. So we know that the PT depends on the trumpet blastin or the tissue factor. Different manufacturers uses different thromboplastin reagents with different sensitivities to coagulation factors. This is actually one of the biggest problems in in the PT test all over the world. If it was the same reagent used with the same sensitivity, it would have been the same everywhere, but unfortunately there are several different variations. This is why even in the same country, the PT results could change when different laboratories depending on which reagent that's being used. And This is why we always have to create a normal range for each laboratory, because this is changing reagents. But was one situation where this is a problem because if a patient is taking a drug like warfarin or syndrome? And they go to a different hospital or different part of the country or different country itself because of the different thromboplastin reagents. Their results become a problem. And then we'll have to change the dose of the tablet based on the results, which could be falsely done and can cause problems to the patient. Which brings me to the INR or the international normalized ratio. And this is the ratio, so it's a number. And it was created to stop the variation in the laboratory test. And how do we do that? This is a mathematical formula. But this is the patient's prothrombin time. And the normal prothrombin time is the average of the mean of 20 healthy people of both sexes tested at the laboratory. So patients and the normal PT. And there's one other thing which people forget because everybody thinks I NR is a ratio of the patients PT and the overpay the normal PT, but that's not true. It's in addition to that there's an ISI, what is ISI or international sensitivity index. Is something supplied by the manufacturer. Again to account for these variation thromboplastin, but the important thing for you to remember is derived from warfarin patients. Stable on warfarin. This is not from individuals who's not taking warfarin. In other words, the ratio of PT. And Ayanna are not the same. I notice that in many laboratories all over the world, patients actually get PT done. And they use a ratio, but it's not the same as ianr. Remember INR should only be used for patients on warfarin. So let's look at how to go about assessing a prolonged PT and a PTT. First of all, we need to look at some details, some some more information about the patient who's at the test? Does the patient have any history of bleeding? Sometimes when the referring person sends the sample, there's no details given on the form. Sometimes it's sent by a special a physician or or a referral unit who is not specialized in as understanding and interpreting these results. It could be coming from a different institution which might be a distance and on different vehicles and there may be delays in transport. In these situations when you get the sample and you get a prolonged result, we need to be extremely careful to avoid and exclude pre analytical variables which you heard from the previous speakers. And in such situations, if you have a doubt that this might be because of some of some of these not a laboratory related issue that we need to ask for repeat sample. On the other hand, if the patient has history of bleeding, the samples come from your own very good hematologist in your hospital. With no delay in transport, you can go to the next step, which is mixing studies. So if we PT actually is prolonged without any other issues and the patient is bleeding, the likely causes are warfarin anticoagulation or the newer or the direct oral anticoagulants, vitamin K deficiency, liver disease. And this is in the order of frequency because factor 7 deficiency is very rare. And factor 7 inhibitors even rarer and there are very rare other things go factor 2/5 and 10 deficiency. On the other hand, the ptes prolong the patient is not bleeding. It's got no history of bleeding. Rarely it could be due to heparin, although many reagents have got. They happen neutralising agents, so it's unusual for prolonged PT. Some of the other laboratory, uh, pre analytic variables like hematocrit could be a high, in which case that could affect the PT if the sample is hemolyzed and is lipemic. Although Lipemia is not usually a big problem because we tend to centrifuge and hopefully remove most of the lipids. If there is none of this and the patient is not bleeding and the PT is prolonged, it's just a strong lupus anticoagulant. What about prolonged PT? Once again, if the patient is bleeding, it could be due to heparin and some types of doacs, especially the double getra. The other courses which I think most of you would be aware of is factor 8 or 9 deficiency. Rarely, in fact 11, in fact 12 deficiency and there's some rare extremely rare contact factor deficiency. In some cases this could be inhibitors or antibodies to these factors as well. What about if an appetite in the patients not bleeding and we also need to check exactly the same things as with respect to PT? But if all of them are not an issue, then it's almost certainly due to lupus anticoagulant. So let's look at mixing studies and what I've done. Here is how we do it. This is a patient with prolonged PT or APTT. Which means that they might have low clotting factor and this is a sample from a normal person with normal clotting factors and when we mix these together. If these two are mixed and the Peter appetite is normal, that means there's a decrease in the clotting factors. OK. So it's clotting factor deficiency because after mixing it's gone to normal. However, after mixing is still prolonged. These are just two things, either it's an inhibitor. Or something a loop assigned quickly because you remember it stops the phospholipid working. So the mixing studies are helpful in finding out if someone's got a clotting factor deficiency. Because the abnormal result will become normal after mixing. Or we can find out they go loop is anticoagulant because they have normal result will still be abnormal after mixing or they could have an inhibitor to one of the clotting factors. In which case it will be prolonged. But the differentiating between lupus and non lupus is if you incubate the sample. In the case of lupus it can change. Some simple things just repeat what might have already been said. We need to make sure that the blood collection tube has got the right amount of citrate, because if there is more citrate it can overestimate PT and aptt and underestimate fibrinogen. This is a very common problem and the the tube should be adequately filled at ideally up to the mark on the tube. Otherwise it can cause prolonged clotting times and we actually start treating the patient differently. This is something, uh, also, uh, quite an important thing to do, and how we should ideally mix is just to move it up and down slowly for about 10 times is the ideal way. Now in our institution we actually teach the people who take the blood sample to make sure these are done correctly to avoid repeating the test. Now this is something which I've heard, although it's not very common in our trust, but it's I've heard something which tends to happen where the patient sample is taken in, usually the purple EDTA bottle and transferred to the coagulation bottle. If that happens, it prolongs speech in aptt, and if you go on to do additional tests you can reduce factors five and eight. And it means that you have an inhibitor and actually there's no inhibitor. So we just need to bear that in mind. Sometimes it's heparin contamination, which can also affect these things. And lastly, if you got clogged in the samples, if it's not being mixed properly, it again affects the results. Sometimes they transfer the sample from a clotting tube which is sent for biochemistry analysis where there will be no fibrinogen because it's serum means that the plot has already been formed, so that also is an issue. So we just need to bear in mind that the bottle is correct, otherwise some of these results will be affected. What about transport? Because I know the wonderful part of the world in Latin America. The it's huge and there's lots of, uh, what nice place to see, but not many laboratories in certain of the remote area, so the transport could be an issue. But we need to bear in mind the PT and APTT should be done ideally within 4 hours. And if it's in the case of unfractionated heparin, aptt should be done in within one hour. Because the longer you wait the platelets which are still there because you have not yet centrifuges, a sample, you can actually release platelet factor 4 and inhibit the heparin. It should not be transported refrigerated as an on ice or high temperature. These are things. Sometimes it's done, especially when they need a long transport, and I've also heard a situation where the blood sample was left in the back of a car and it was affected by the hot sun. This especially is a problem with factor 5 and factor 8, which also means that it can affect the aptt. If there is a long delay expected, we should make sure that the certification is done in a smaller unit and the plasma separated and frozen. And then you can transport the frozen plasma. Once the person sample has reached the tertiary Referral Center lab, that can be rapidly thawed in a 36 degree water path for five to 10 minutes. I'll finish off with another common situation where PTT, aptt are often done, which is the preoperative patient. So you should consider doing PTNR if the patient is on warfarin, if they got liver disease and if you think that the patient might have vitamin K deficiency. In a person without history of bleeding, it's unlikely that they will have a factor 7 deficiency or any other deficiency in that situation. And a PT in the similar way to PT. You need to think about IV heparin as a cause of it. Of course person could have him if Levich was not previously diagnosed or one Willebrand disease, and rarely because of the strong lupus anticoagulant could be Andy Phospholipid syndrome, but that should not be a problem in proceeding with surgery. Now in the current day and age, one of the biggest issues is too much blood test being done as routine blood work routine preop screen in for minor procedures. There's not many data on whether we should use. PTA, PTA to to monitor director Orlando Coagulans because it doesn't give much meaningful information and certainly doesn't help with low market heparin or thromboprophylaxis. And with that, I finished my talk and I would like to say gracias por escuchar. Thank you.

10 150 100 50- 1.0 0.1 0.01 200 20 Basic Coagulation assays Application and interpretation Jecko Thachil Consultant Haematologist, Manchester Royal Infirmary For the Educational Program on Hemostasis, 2022 Unifying the Hemostasis Pathway in Clinical Diagnosis in Latin America How does bleeding stop ? Platelet procoagulant surface PC outside PS and PE inside Phosphatidylserine(PS) Phosphatidylethanolamine (PE) Phosphatidylcholine (PC) Transfusion Medicine Reviews 34 (2020) 108-116 Extracellular vesicle release Phosphatidylserine externalisation Prothrombinase VaH VaL Xa OVERVIEW OF HEMOSTASIS AND THROMBOSIS James C. Fredenburgh and Jeffrey I. Weitz "Cell-based Haemostasis model" PT Xa- Va Activated platelets Activated coagulation factors (glu > gla done by vitamin K) Calcium James C. Fredenburgh and Jeffrey 1. Weitz But what happens in the coagulation lab ? We centrifugate the blood to remove the cells and the platelets, to obtain plasma for the coagulation tests We ensure there are no platelets left in the sample Then how will the plasma clot !!! Classic theory of coagulation Thrombo- plastin 3 Prothrombin Thrombin 2a Ca Fibrinogen Fibrin The origin of Clotting Factors Hippocrates - blood from sacrificed animal clotted on cooling. But if the blood was shaken before cooling and fibres removed, no clot formed Clotting requires a fibrin generator (fibrinogen) -- 1 During experiments, blood clot weight was measured -1 just before clot occurred and + when clot was visible - prothrombin - 2 Many substances promote clotting - especially dead animals brains; it was called thromboplastin - 3 . Its this thromboplastin which replaced the platelets in the haemostatic tests Prothrombin Time (PT) Thrombo plastin Fibrir Am J Clin Pathol. 1974;62(5):670-2 . A potent thromboplastin is added to act like platelets - PT reagent . Adequate fibrinogen present in the plasma The only variable is "prothrombin" 4 factor theory to Clotting Cascade Factor VII + Tissue Factor (thromboplastin) Factor X So PT measures Why did we need APTT? HEMOPHILIC Clotting Time - Seconds J Thromb Haemost 2003;1: 226770. 0.001 0.0001 Thromboplastin Concentration Antihaemophilic factor identified but no easy way to measure PT not helpful "Partial" thromboplastin - reduction in thromboplastin reduces VII mediated Xa activation, making VIII more important - PTT "Activated" using kaolin, elagic acid etc APTT and intrinsic factors XI/XII Lupus anticoagulant > Why APTT is affected by LA > Why PTS is affected by strong LA . Double centrifugation is very important to remove all phospholipids Russell's viper venom contains potent activator of Factor X which in the presence of phospholipid initiates clotting (unaffected by factor deficiency) PT variations PT depends on the thromboplastin (tissue factor) Different manufacturers use different thromboplastin reagents with differing sensitivities to the coagulation factors* This is why PT results from the same individual differ in different laboratories This is also why we need to create a normal range for each laboratory using the thromboplastin used by the lab But there was one situation where this different PT results became a problem - patients taking vitamin K antagonists like warfarin or sinthrome INR To overcome the interlaboratory variation, the international normalized ratio (INR) was introduced Thromb Haemost 1985;53:155-6 J Thromb Haemost. 2004;2(6):849-60 ISI INR= PT test Normal PT is the mean of the PT of at least 20 healthy subjects of both sexes tested at the performing laboratory normal PT normal International Sensitivity Index (ISI) value is supplied by the manufacturer to account for the sensitivity of the thromboplastin and is derived from warfarinised patients PT ratio & INR are not the same . INR developed only for warfarin patients Prolonged PT/APTT Patient has no history of bleeding No details given on the request form Sample sent by non-specialist Sample received from a different institution Delays in transport possible Take extra precautions to exclude pre-analytical variables If in doubt about an abnormal results, ask for a repeat sample Sample sent by haematologist from the same institution No delay in transport Proceed to step 2 mixing studies Prolonged APTT interpretation most commercial PT reagents contain a substance capable of neutralizing approximately 2 U/mL of heparin Patient is bleeding Likely causes Warfarin anticoagulation DOACs Vitamin K deficiency Liver disease Factor VII deficiency or inhibitor Factor II (2), V (5) or X (10) deficiency Patient is not bleeding Checklist Could there be heparin contamination? Is the haematocrit normal? Is the sample haemolysed? Is the sample lipemic? Strong lupus anticoagulant Heparin/ some DOACs Factor VIII, IX, XI or XII or HMWK/PK deficiency or inhibitor Patient [Normal 3 - incubated for 2 hours 4 - non-incubated APTT Results Tube 3 Tube 4 Tube 1 Tube 2 [Incubated [Immediate [Test mix of plasma] INTERPRETATION and Test - Normal Prolonged - Clotting factor deficiency Prolonged Prolonged Normal - Immediate acting inhibitor - Time- dependent inhibitor Normal Prolonged Prolonged Normal Normal Prolonged Prolonged Pre-analytical variables - collection Citrate tubes should be 105 mM sodium citrate (3.2%) 3.8% citrate overestimate PT & APTT and underestimate fibrinogen Semin Thromb Hemost. 2008;34:612- 634 . Tubes should be adequately filled - 90% of the total volume Underfilling may cause falsely prolonged clotting times due to the excess citrate Thorough mixing to avoid a clot (mzy develop slowly and not always apparent) If the bottle was wrong KEDTA Prolongs PT and APTT Falsely low factors V and VIII False impression of inhibitors to factors V and VIII 2ml Laboratory Medicine, Volume 43, Issue 2, February 2012, Pages 1-10, CLSI document H21-A5. Wayne, PA: Clinical and Laboratory Standards Institute; 2008. TES TEST Heparin contamination Prolongs PT, APTT, and TT (usually TT raised >APTT raised >PT raised) Reduced factors especially VIII, IX, XI, XII Clot in the sample False prolongation of PT, APTT > False low factor levels or false high factor VII Serum - No fibrinogen, so no clot in PT, APTT, or TT Pre-analytical variables - transport Ideally, PT and APTT should be done within 4 hours of collection APTT for unfractionated heparin monitoring within 1 hour to limit heparin neutralization by platelet I> factor 4 released in vitro Int J Lab Hematol. 2021;43:1272-1283 Samples should neither be transported refrigerated (e.g ., on ice) nor at high temperature (e.g ., as vehicular transportation) Transport delays affect in particular the labile factors V and VIII If expected delays, local centrifugation and separation of plasma followed by freezing and frozen transport of the plasma be considered . Previously frozen samples should be rapidly thawed in a 37C water bath for 5 to 10 minutes When to order coagulation tests (PT/INR and aPTT) in a preoperative patient Consider PT/INR Top five reasons NOT to order PT/INR or aPTT Vitamin K antagonist (warfarin) 1. Routine blood work Patients at risk for Vitamin K deficiency (e.g. malnutrition, fat soluble vitamins, 2. Routine pre-op screen in a low risk non- cholestasis, prolonged antibiotics) cardiovascular surgery patient 3. Monitoring of direct oral anticoagulants Consider aPTT (DOAC) Planned intraoperative IV heparin (e.g. 4. Monitoring of low molecular weight cardiac or vascular surgery) heparin (LMWH) Suspected hemophilia A/B, factor XI deficiency, severe von Willebrand disease 5. Monitoring of thromboprophylaxis · Suspected antiphospholipid syndrome Res Pract Thromb Haemost. 2020;4:761-773 Gracias por escuchar