In June and July, two new oral antithrombotics were released in the US: rivaroxaban, and ticagrelor, and one new anti-platelet drug, ticagrelor. Now over half our discussions have centered on interferences by or monitoring of these new drugs.
Ticagrelor (Brilinta®, AstraZeneca) joins aspirin, ticlopidine, clopidogrel, and prasugrel as another oral antiplatelet drug for use in acute coronary syndrome, as reported June 1 by theheart.org.
“Tom” contacted Fritsma Factor with his lab results and wonders if he has a factor VIII inhibitor:
- PTT: 38.5s (reference: 24.3-36.2s)
- PTT 4:1 NP: 37.2s
- After 1 h incubation PTT 4:1 NP: 38.2
- Factor VIII: 43%
- Von Willebrand factor: 55% (normal)
George responds that it is often futile to perform mixing studies when the PTT result is just a second or two longer than the upper limit of the reference interval. His opinion is that you should only perform mixing studies when the PTT result exceeds the RI limit by five seconds.
“What constitutes correction?” has several answers, George advocates for using the criterion “within 10% of the pooled normal plasma (PNP) result,” although the majority of laboratories, like Tom’s, choose “mixture PTT within the RI.”Tom can’t conclude that he possesses a factor VIII inhibitor. Most laboratories only choose to work up a factor VIII inhibitor when the factor VIII assay result is less than 30%. Further, if you possess either a factor VIII inhibitor or a lupus anticoagulant, you can’t trust the result of your factor VIII assay, as the result is based on the PTT, which is being affected by the inhibitor. However, because factor VIII inhibitors are dangerous, Tom’s physician may wish to schedule a return visit with repeat testing 12 weeks after the previous visit. She would ensure that the laboratory centrifuges plasma to be platelet-free, and uses a platelet-free PNP such as CRYOcheckTM pooled normal plasma.
Herb Crown (SLU) commented, “Our laboratory would take a similar approach but we differ on how to perform mixing studies and their interpretation. Regardless, the conclusion would probably be the same: Tom does not possess a factor VIII inhibitor. Factor VIII inhibitors generally lower the patient’s factor VIII levels to <1 to maybe 10%. I agree with George, repeat testing in 12 weeks should be strongly considered. I would not limit the testing to only factor VIII levels, but again would repeat the von Willebrand work up.
LAs lower the factor VIII levels to 40-50% but it increases to normal levels on subsequent dilutions. Rarely, does LA lower the VIII levels to 1-10%. Factor VIII levels of 40-50% in the presence of a LA are an artifact of the LA binding the phospholipid in the PTT reagent and extending the clotting time of the PTT and factor VIII assay. Your next doctor’s visit should probably include a LA evaluation as well.”
Mary Dugal, Precision BioLogic Inc Account Manager writes, “During a recent meeting, a customer of mine described the following anomaly:
‘When performing comparisons with the intention of switching to a different concentration of thrombin clotting time (TCT, TT) reagent, we noticed the results were consistently lower post-thaw on prolonged samples containing unfractionated heparin (UFH): 80 s versus 50 s; 89 s versus 34 s; and 42 s versus 24 s. Our normal TTs compared well. We used double centrifugation of samples, froze at -70°C and thawed in a 37°C water bath. When correlating with several sites using the same instrument, reagent, and thrombin concentration, the same pattern was seen.’ Mary continues that her friend found an article published in 1960 where it states that plasma is sensitive to heparin, when frozen for more than 24 hours, which results in lower thrombin time values post thaw.”
George expressed no experience with the effect of freezing on UFH, although the TT may be used to monitor UFH therapy. He guesses that despite all precautions, including double centrifugation, UFH is partially neutralized by platelet factor 4 during centrifugation, freezing, or thawing.
George posed this question to Dr. Alvin Schmaier, Professor of Hematology/Oncology, Case Western Reserve University, who responded: “We used to monitor heparin with the TCT at Michigan. However, we always ran it on a freshly obtained sample. I do not recall data on fresh-frozen and then thawed samples. I could propose various mechanism(s) for the shortening of the thawed sample, but without experimentation, I would not know the precise one.”
This remains an open question, one that I suspect has not been previously investigated, based upon Dr. Schmaier’s response.
On June 22, Pfizer/Bristol-Myers Squibb released preliminary data on apixaban (Eliquis) indicating non-inferiority to warfarin.
A subscriber describes a 49-YO female who had HGB 8.9, HCT 26.3 and a normal platelet count. However, her PT was > 250 seconds. Her PTT was 73 s. Testing a recollected sample confirmed the results. The patient was not on coumadin and that no one in her household was either.
George wrote, “My first response to results this prolonged is to distrust the specimen. In this case, confirmation by the duplicate specimen should remove my distrust, unless both specimens were collected above an intravenous line or allowed to clot. It is worth checking both specimens for their integrity, particularly as you mention no acute bleeding.
You are correct to next look for coumadin, even though the patient reports no coumadin. She could have had accidental coumadin exposure, but again, this would be accompanied with bleeding. A direct thrombin inhibitor such as argatroban, or the new oral DTI, dabigatran, could also be responsible.”
This case remains open for additional comments.
“nomorefede” comments on a case in a newborn hospital. PT: 55 s, PTT: 60 s, FG: 130 mg/dL, F II: 8%. The baby has central bleeding. It was given fresh plasma but the factor II level didn’t rise. Can you help?
George concludes the foremost possibility is vitamin K deficiency and recommends assaying at least one additional vitamin K-dependent factor, perhaps factor VII. The fibrinogen level is relatively unremarkable, as the lower limit for a newborn (1 day) is 167 mg/dL. Of course, low fibrinogen could also mean an immature or diseased liver.
In adults, plasma may be expected to raise the deficient factor to no more than 30% activity, as the volume of plasma may induce circulatory overload. I suspect the rise in a newborn may be smaller.
Which antithrombotic monitoring problem happens the most often in your laboratory?
- a. Unexplained INR variation: 14 (29%)
- b. Trouble correlating lab INR with POC INR: 4 (8%)
- c. Heparin; is it UFH, LWMH, or pentasaccharide: 7 (14%)
- d. Need a way to monitor antithrombins like argatroban: 16 (33%)
- e. Need a way to monitor direct anti-Xa like rivaroxaban: 8 (16%)
INR variation continues to create difficulties that may be related to treatment and diet or inter-laboratory variation. We don’t have a lot of trouble correlating lab to point-of-care instrument results. A rising issue asks how to monitor dabigatran and rivaroxaban. Dabigatran is a direct thrombin inhibitor, rivaroxaban a direct anti-Xa inhibitor. Physicians are asking laboratories to monitor for a variety of reasons, and we are having to develop new assays.
The US FDA cleared rivaroxaban on July 1, 2011, the first oral direct anti-Xa anticoagulant. The anticoagulant is cleared for DVT prophylaxis in patients with THR or TKR surgery.
Dabigatran was approved in 2008 in Europe and Canada for prevention of venous thromboembolic disease following hip- or knee-replacement surgery and cleared by the US FDA for prevention of ischemic stroke in atrial fibrillation in October, 2010. Our latest Quick Question asks, how do you intend to monitor it, and asks for comments.
From a subscriber: “After a limited search, I can find no information supporting the theory of drawing a discard tube before any coagulation test. What are your thoughts?”
George agrees there is no need to collect a discard tube before collecting a tube for any coagulation test, including platelet function testing. This is documented in the CLSI guideline H21-A5, Fifth Edition.
A follow-up comment from Dr. Emanuel Favaloro: George is correct that you do not need to draw a discard tube before any coagulation test UNLESS you happen to be using a butterfly or variable tubing blood collection system. In this case a discard tube is required since the air in the tubing will result in an under-filled first tube, which should not be used for any coagulation studies.
Anesthetists use the ACT to monitor UFH during coronary bypass graft surgery. UFH is raised to 2.5 units/mL during surgery. Most cardiac ORs are equipped with a Hemochron®, which provides a semiautomatic ACT.
Prof. Bernadette Rodak, Indiana University, asked about the target therapeutic range. Dave McGlasson and Laura Taylor answer that anesthetists target 1.5-2.0 X the baseline, for instance 165-300 seconds. The UFH needed to reach and maintain a certain ACT varies as does the body’s clotting potential at that ACT. After surgery, the ACT may be maintained within a narrow range (for instance, 175-225 seconds) until the patient has stabilized.
“SWagner” asks George to describe how to calculate and validate a reagent-analyzer specific PT reagent ISI using Stago’s Etaloquick®. The insert contains these reference values:
- spec#1 81.0%
- spec#2 29.0%
- spec#3 20.0%
What is this % and how do I get it back to a PT in seconds?
George went to colleague Stephen Duff, Co-CEO of Precision BioLogic Inc, as he has done extensive work on PT calibrators. George asks, “are you are literally required to establish an instrument-reagent specific ISI, or must you simply verify the accuracy of the INR with the current reagent ISI-instrument specific combination?” CLSI Guideline H54-A, of which Mr. Duff is a co-author, recommends the laborious local ISI calibration only if the accuracy of the INR cannot be verified locally. George recommends you first determine if you want to do a full calibration or verification, then follow the instructions in H-54.
The US FDA released ticagrelor July 20. This is the fourth oral anti-platelet drug designed to be used with low-dose aspirin, the others are ticlopidine, clopidogrel, and prasugrel.
From John W. Scroggins, MD: An 81-YO with bowel obstruction/colitis is taking dabigatran 150 mg BID for chronic atrial fibrillation, coronary artery disease, and questionable history of stroke. Her coagulation studies are prohibiting intervention. PT is 170 seconds, PTT is 249 seconds, fibrinogen is 30 mg/dl, FDPs were 10.
George responds that dabigatran is cleared for afib patients over 75-YO, though the bleeding risks rise with age and in renal insufficiency. The patient’s PT and PTT seem markedly prolonged for a standard dose of dabigatran. It may be necessary to further analyze for a coagulopathy or for renal insufficiency. If the FG was assayed using the clot-based Clauss method, it cannot be trusted, as dabigatran, a direct thrombin inhibitor, interferes with the result.
Follow-up from Dr. Scroggins: Unfortunately, the patient passed away so I am not asking patient management questions-only trying to understand if dabigatran caused her coagulopathy. She definitely had acute renal insufficiency; GFR was estimated at 20-30, and this made me concerned about dabigatran accumulation. She received 2 units of FFP and 4 cryoprecipitate “5 packs” overnight resulting in the same PT >170 s and PTT >249 s this morning. Platelets remained normal and stable. A 1:1 mixing study showed PT correcting to 111 s and PTT correcting to 145 s. Unfortunately, she had a fulminant bowel pathology that was unable to be acted upon due to her coagulopathy. It could have been severe DIC, but I am just trying to understand dabigatran better.
From George: I’ll avoid speculating over this specific case, but the manufacturer indicates dosage should be reduced in kidney disease. Nevertheless, renal insufficiency may not account for her markedly prolonged results.