2012 Cheat Sheet

Sue Allen asked about a previous post that discussed the correlation between the Abbott i-STAT ACT , ITC Hemochron 401 ACT , and the Hemochron Jr. ACT. She wondered if we ever received any responses. She is currently comparing instruments and the methods do quite well at the lower ranges but at the 300–350 second levels they differ by as much as 100 seconds.

George indicates that the longer ranges of the ACT are important to anesthetists in cardiac surgery for monitoring heparin. We’ve received no additional responses as of July 11, and the topic remains open.

Axnzak:” wants to learn what calculation you use to determine mixing study results, both PT and PTT , when comparing results to patient reference ranges. George linked the questioner to a recent post, “Mixing Studies: 4:1?“  and a Quick Question. George advocates for the Rosner Index as being the most consistent and scientifically defensible approach.

Crystal Azavedo is looking for Destiny Max users who run the quantitative D-dimer. She’s considering adding it, but hasn’t heard from any northeast US users that have put DD onboard. George invites all Destiny Max operators to respond to this open question.

Frequent contributor Dave McGlasson asks about normalizing the DRVVT ratio using the laboratory’s MRI in accordance with a Zhang reference that states, “The normalized ratio is recommended to correct for differences in instrument-reagent combinations and to improve discrimination between normal and low-positive LA samples.”

Dr. Emmanuel Favaloro (New South Wales) responds: “We have always used normalised ratios. It doesn’t make sense to me not to, since the baseline values for the screen and confirm are invariably different. Moreover, they will change slightly with different batches of reagents and different normal pools. On the other hand, if you have established your normal ranges based on non-normalised data you might be OK.”

George asks participants, “Do you know about normalization of the DRVVT ratio, do you use it, do you advocate for it, and what advantages does the ratio provide?”

Vanessa Chan (Toronto) responds that her group is aware of the DRVVT normalization ratio and the recommendations to use it, however, she currently is not. There are a couple of reasons. When they validated cut-offs, they used about 40–50 normals and calculated them using the 99th percentile. When they switch lot numbers of PPP , lupus sensitive PTT , and DRVVT reagents; they found that the QC ranges do not shift very much and so they have always kept the same cut-offs to date.

Vanessa continues, “We took a look retrospectively at 200 samples and calculated a normalized ratio for each step of our testing: Lupus sensitive PTT , DRVVT screen and if applicable, DRVVT screen and confirm ratio. The normalized ratio cut-offs were calculated based on our initial validation samples. We found only one sample where, using the ratio, we would have continued on with a confirmatory step. On the other hand, there were a lot of samples where the mixing corrected and we did not continue with confirmation testing.

The value of the normalized ratio would be that it does correct for slight differences in PPP and reagent lots but at the same time, it also adds an extra calculation or two or three which increases the chance of error and adds to workload.

Marisa B. Marques, MD, who directs the transfusion service and special coagulation laboratory at the University of Alabama at Birmingham (UAB) Hospital asked George to find out what medical centers are using anti-Xa instead of PTT to monitor unfractionated heparin. She would like to hear their input and experiences in terms of patient care. We have no responses as of July 11, the topic is still open, although George was able to put her in touch with Paul Riley, PhD at Diagnostica Stago, who maintains such a list.

George points out a thought-provoking article: Repesse Y, Dimitrov JD, Peyron I, et al. Heme binds to factor VIII and inhibits its interaction with activated factor IX. J Thromb Haemostas 2012;10:1062–71. The authors do not speculate on in vitro specimen hemolysis, however their findings could provide additional support for rejecting hemolyzed specimens.

George provides a summary of our June, 2012 Quick Question:
To perform the chromogenic anti-Xa heparin assay, how many separate reference curves do you maintain for unfractionated (UFH ), low molecular weight (LMWH ) heparin and fondaparinux?
a. One hybrid curve for UFH and LMWH , same curve (computed) for fonda: 7 (16%)
b. One hybrid curve for UFH and LMWH , a separate curve for fonda: 11 (25%)
c. One reference curve for each heparin: UFH , LMWH , and fonda: 22 (50%)
d. We don’t perform the chromogenic anti-Xa heparin assay: 4 (9%)

Most of us have chosen to stay with three curves rather than take advantage of the various manufacturers’ combined (hybrid) curve that may be used to measure either UFH or LMWH. George asks whether those who do not use the hybrid curve find it necessary to confirm the type of heparin being used before running the assay.

George references a definitive article by Favaloro, et al, in the June, 2012 JTH that describes the three methods currently available to assay von Willebrand factor activity: ristocetin cofactor (VWF :RCo), collagen binding (VWF :CB), and a monoclonal antibody-based immunoassay (VWF :Act ). Based on this article, this month’s Quick Question asks which VWF activity assay method you use in your laboratory. The list does not include the standard VWF concentration immunoassay that is based on polyclonal antibodies, von Willebrand factor antigen (VWF :Ag) nor does it include the factor VIII (FVIIIC) activity assay. Also, the ristocetin cofactor assay (VWF :RCo) is divided between the more traditional platelet aggregometry-based assay and the automated assay.

George asks that if you use two VWF activity assays, for instance, if you may run concurrent VWF :RCo and VWF :CB for confirmation, please comment. This topic remains open for your response.

George comments that guidelines for Coumadin, aspirin, dabigatran (Pradaxa), rivaroxaban (Xarelto), and apixaban (Eliquis) are changing rapidly, favoring the new oral anticoagulants in most cases of atrial fibrillation and provides a link to conference presentations.

Marisa Marques, MD from UAB forwarded this question: “One of our pharmacists asked if there was a maximum dose for the heparin bolus or infusion rate for obese patients since our current recommendations are based on body weight. I haven’t been able to find a recommendation to cap dosing for these patients.”

George provided these doses from the 2012 American College of Chest Physicians Guidelines:

  • Venous thromboembolic disease: 80 units/kg bolus, 18 units/kg/hour
  • Acute coronary syndrome: 60–70 units/kg bolus, maximum 5000 units, 12–15 units/kg/hour, max 1000 units/kg/h
  • If given with thrombolytic agents: 60 units/kg bolus, 12 units/kg/hour, maximum 1000 units/kg/hour

The authors do not discuss a maximum dosage other than the limits given. They do recommend regular monitoring to ensure that the dosage is within the therapeutic range. Likewise, they recommend monitoring subcutaneously administered low molecular weight heparin in patients who are either obese or underweight.