2010 Cheat Sheet
“McDowekm” performs anti-Xa
heparin testing on a hybrid curve using Stago’s Rotachrom reagent. If a specimen is lipemic, it can falsely raise the anti-Xa
result. Can lipemic specimens be ultra-centrifuged to obtain a more accurate Anti-Xa
George responds that it would be helpful to review the data that indicate lipemia is raising the anti-Xa results. When performing the Rotachrom assay according to published protocols using the hybrid curve, the original specimen reaches a final dilution, counting diluents and reagents, of 1:22. Given this dilution, and according to the manufacturer’s package insert, interference will occur only when the original triglyceride concentration rises to above 360 mg/dL. Also, lipemia would be expected to reduce, not raise the anti-Xa result. This post generated three comments.
Stacy Askvig at
the University of North Dakota Medical Center asks, “I do only PTs
, and my references seem to give me conflicting interpretations. Our patient has a prolonged PTT
and DRVV, both are shortened in mixing studies:
PTT-LA 107 seconds, mix 79.5 (reference interval 0-50)
DRVV 73 seconds, mix 56.7 (reference interval 0-44)
George responds the PTT and DRVVT are both prolonged and do not adequately correct in the mixing studies. This is presumptive evidence for the presence of a lupus anticoagulant (LA ). To confirm an LA Stacy would need to next perform a phospholipid neutralization step using a high phospholipid reagent.
“Skierktc” wonders if anyone screens with the TT
plus protamine sulfate to rule out heparin in the specimen. If yes, what is their policy for continuation of the study?
George responds that when you encounter a prolonged PTT that is not corrected by NP , perform a TT to determine if the patient is receiving heparin. Heparin will prolong the TT from its normal mean to 40 seconds or longer. Once you have concluded heparin is present, treat the specimen with Hepsorb or Hepzyme, then repeat the mixing study.
On 11/19/10, Pfizer and Bristol Myers Squibb discontinued the APPRAISE-2 phase 3 trial of Apixiban in high-risk patients with acute coronary syndrome. Apixiban bleeding risks were not offset by its antithrombotic benefit in the high-risk patient population. At least two trials appear to show that Apixiban, which is a direct anti-Xa oral antithrombotic, is effective at stroke prevention in atrial fibrillation. The oral direct anti-thrombin drug Dabigatran (Pradaxa, Boehringer-Ingelheim) was released by the US FDA for stroke prevention in October.
We recently posted a brief survey asking laboratory scientists to characterize the sensitivity of their “routine” APTT assay for LA. This is a new three-question follow-up survey for physicians. The APTT is used to monitor anticoagulant therapy, screen for coagulation factor deficiencies as a means to predict risk of bleeding, and to screen for LA , which may be associated with thrombosis and or recurrent miscarriage. A number of APTT reagents are available that are highly, moderately, or minimally sensitive to lupus anticoagulants. We propose to learn which of these three applications of the APTT are of most importance to clinicians, and to determine clinician understanding of APTT reagent LAC sensitivity. Investigators are Ellinor Peerschke, PhD, Elizabeth Van Cott, MD,Dorothy (Dot) Adcock-Funk, MD, and Marisa B. Marques, MD. Ankush Randhawa and George Fritsma assisted with survey development.
Dan Hood asks, “What testing do you recommend for Plavix effect for patients preoperatively?”
George indicates that Plavix suppresses platelets by blocking ADP receptors. The reference method for Plavix effect is platelet aggregometry using ADP as the agonist. However, if your laboratory is not performing aggregometry, you may want to use the Accumetrics VerifyNow P2Y12 or the Helena PlateletWorks ADP Kit. Further, DiaPharma distributes the Multiplate 5.0 Analyzer, which is currently under FDA review. Joe Lamband Donna Lawler add comments recommending the Thromboelastograph.
G Pihan from Beth Israel Deaconess asks, “What is the incidence of false positive hexagonal phospholipid neutralization tests in the presence of a strong (400-800 BU
) factor VIII inhibitor?”
George answers the hex-phase phospholipid is the most specific platelet membrane surrogate for identifying lupus anticoagulants when testing in a PTT system. There are no data published on the incidence of false positives, although it is possible that a high-titer factor VIII inhibitor could cross-react in the Staclot-LA system.
The ISTH requires that we use at least two parallel testing systems to confirm LA , and most of us use a PTT-based kit and a dilute Russell viper venom (DRVVT )-based kit. In instances when the DRVVT result is negative but the PTT-based hex-phase phospholipid result is positive for LA , the lab should follow up with a factor VIII assay to rule out factor VIII deficiency or a factor VIII inhibitor.
Bob Gosselin writes, “…the abbreviation PFP
is for platelet-free plasma–PFP
usually requires filtration. Kaolin clotting time requires platelet free plasma for testing. However lupus anticoagulant testing using DRVVT
requires platelet poor plasma, which is defined as being less than 10,000/mcL, and there is ample evidence to suggest that filtration is not recommended for non-KCT testing.”
George answers that he was the process of developing a new quick question on the subject of PPP/PFP to learn what PFP means to our participants. The Quick Question is now posted; look for a summary on January 14.
Carrie was working on a hemostasis project for school and asks, “What if a patient has an abnormal PT and PTT that are not corrected by mixing study?”George answers that the combination of a prolonged PT and PTT that do not correct when mixed with normal control plasma lead to the presumption of the presence of a lupus anticoagulant (LA ), sometimes called a non-specific inhibitor. To confirm, you would follow up with a standard LA profile. A far less common possibility is a specific coagulation factor inhibitor.
George provides a link to a Heartwire summary of 2010 clinical trial reports on dabigatran (Pradaxa, Boehringer-Ingelheim) and rivaroxaban (Xarelto, Bayer and Johnson & Johnson).