2011 Cheat Sheet
In a December 2, 2010 post, Bob Gosselin at
UC-Davis in Sacramento suggests that the term platelet free plasma (PFP
) should be reserved for plasma with a platelet count of zero, achievable only by filtration; and that we should call plasma with a platelet count less than 10,000/mcL platelet poor plasma (PPP
George posted a December Quick Question and got the following results:
How do you define platelet free plasma (PFP )?
1. Plasma with a platelet count less than 10,000/mcL: 39 (60%)
2. Plasma with a platelet count less than 5,000/mcL: 9 (14%)
3. Plasma with a platelet count of zero, prepared by centrifugation: 6 (9%)
4. Plasma with a platelet count of zero, prepared by filtration: 11 (17%)
There are a substantial number of respondents like Bob whose definition fits the terminology. There are also a number who set the platelet count at 5,000/mcL, which isDr. Douglas Triplett’s recommendation. It looks like the PFP-PPP terminology issue is unresolved.
Cara Calvo, director of the Medical Technology program at the University of Washington reminded George of a workshop he presented with Lynn Quarles of Diagnostica Stago entitled “The Top Ten Problems in the Coagulation Laboratory.” George asked for a list of current top ten problems.
Joe Lamb, St Francis Hospital; “Are you sure you want these? I have some:”
1. Orders for a factor V when they really wanted a factor V Leiden.
2. Orders for platelet antibodies when they really want an HIT screen/profile.
3. Orders for fibrin split products, we don’t do those anymore.
4. Orders for a thrombotic profile, i.e. protein C and S and other tests, right after an event and/or while the patient is on heparin/coumadin.
5. Orders for a factor V Leiden AND an APCR.
6. Mixing studies when the PT/PTT is not prolonged.
7. Nurse draws under-filled tubes or the blood is left in the syringe too long.
“Ipaz” at Iline Microsystems asked, “For the dilute thrombin time, why is the patient plasma diluted with normal pool plasma and not with saline solution?”
George responded that the dilute thrombin time assay monitors the efficacy of DTIs dabigatran, hirudin, and argatroban. Patient plasma is first diluted (1:8 for dabigatran) using normal saline. Next, pooled normal plasma is added, then purified human α-thrombin, and the interval to clotting is recorded and compared to DTI calibrators. Diluting the patient plasma and adding normal plasma enhances assay reproducibility. The patient’s DTI acts upon a predictable level of thrombin provided in the normal plasma, and variations in patient procoagulant activity have little effect upon the clotting time.
Dr. Jean Amiral, President and Scientific Director of Hyphen BioMed and dilute thrombin time developer commented the plasma pool is used to render the assay insensitive to variable coagulation factors in tested specimen (mainly II and Fbg).
George linked an announcement from theheart.org indicating that Merck has discontinued a portion of a phase III trial of vorapaxar, an oral antiplatelet drug that blocks the thrombin membrane receptor protease activated receptor-1 (PAR-1).
George asked what is an older name for the original Russell viper venom (RVV) assay and what was its original purpose? Joe Lamb answered Stypven time, which was used when there was a prolonged PT. If the Stypven time was normal, the PT was prolonged by a factor VII deficiency, since the Stypven activates at the factor X level.
Terry Black at
Orlando Regional Medical Center is getting a lot of questions about monitoring Pradaxa (dabigatran) and it keeps coming back to the ecarin clotting time (ECT
). She asks, “Do you know of any POC
testing kit or instrument that could be used to monitor this drug’s effect?”
George provides a link to Hyphen Biomed’s new Thrombin Inhibitor assay for direct thrombin inhibitors, which Hyphen claims is linear for dabigatran measurement. He knows of no point of care instrument available for dabigatran monitoring. There are responses from Jonas Kingo, of Aniara, which markets Hyphen products, and Paul Riley, Diagnostica Stago, describing their chromogenic ecarin time.
From Debbie Moffitt, Queens Medical Center, Honolulu: A hematologist is asking for the reptilase time test to distinguish when prolonged PTT
results are due to heparin. When heparin is suspected, we use Hepzyme to try to correct the TT
. We also review the patient’s clinical condition, medications, and if the specimen was drawn through a heparin line. Does anyone have experience with reptilase, and what vendors do you use?
George responded: Stago offers a reptilase kit. Because reptilase cleaves only fibrinopeptide A from fibrinogen, as opposed to thrombin, which cleaves fibrinopeptides A and B, it is not prolonged by heparin, so when the thrombin time is prolonged and the reptilase time is normal, you’ve confirmed heparin. There is no other current use for the reptilase time and thus no reason to expend the resources to set up the assay. Your approach, neutralizing with Hepzyme plus checking the chart, is the way most of us do it, and if that doesn’t offer enough proof, you can offer the chromogenic anti-Xa heparin assay, which is definitive.
Here is Debbie’s follow-up conclusion: We have used Hepzyme for years. We always keep it on hand although we don’t have to use it often. It has been useful when there is a question of whether a sample contains heparin in cases of disputed contamination. Reconstitute with 1 mL of patient plasma, then re-test the plasma. It neutralizes up to 2 units of heparin per vial. We use commercial heparin controls and test PTT pre and post neutralization.
“Plovejoy” asks is there a reason to perform a PTT
mixing study on a patient whose initial PTT
result is slightly shorter than the reference range? Since a short PTT
would appear to indicate there isn’t a factor deficiency or a circulating anticoagulant, might there be another reason?
George responds that nothing can be gained by performing a standard mixing study on a shortened PTT. Mixing studies are designed to detect inhibitors or factor deficiencies that prolong the PTT , and are unlikely to provide any information for a shortened PTT. Short PTs and PTTs are probably the consequence of a specimen management error, howeverLippi G, Salvagno GL, Ippolito L, Franchini M, Favaloro EJ. Shortened activated partial thromboplastin time: causes and management. Blood Coagul Fibrinolysis 2010;21:459-63speculates on the possible clinical and physiological causes. While a mixing study doesn’t help, there may be an argument for factor assays that detect, for instance, elevated factor VIII, which could be associated with thrombophilia.
Vilas Hiremath, United Labs, spiked normal pooled plasma with crude Russell viper venom (RVV). The D-dimer is negative. Your comments please.
George knew of no reason to anticipate a positive D-dimer from a clotting assay on normal plasma, be it RVV, PT , or PTT. The fibrin that is the endpoint of a clotting assay is not cross-linked, as there is no opportunity for factor XIIIa to act , so it would not produce D-dimer, even if there were time for in vitro fibrinolysis.