Having validation data to support the use of age-adjusted D-dimer cutoffs with the D-dimer assay your laboratory uses is a must, and know well the limitations of point-of-care prothrombin time/INR testing.
1. The D-dimer test has attracted attention of late because age-adjusted D-dimer cutoffs are part of an American College of Physicians clinical guideline for ruling out acute pulmonary embolism.
The ACP guideline authors took an "algorithmic approach to all steps of pulmonary embolism diagnosis, starting with assigning a clinical prediction score," said Dr. Moser, an assistant professor of pathology at Saint Louis University School of Medicine. In general, they recommend patients with a low or intermediate pretest probability of pulmonary embolism undergo D-dimer testing. Those who have a high pretest probability should proceed to imaging.
"Contained within this overall document," Dr. Moser said, "the ACP guideline tells us that clinicians should use age-adjusted D-dimer thresholds, defined as the patient’s age times 10 ng/mL, rather than a generic cutoff of 500 ng/mL, in patients older than 50 years to determine whether imaging is warranted."
2. Point-of-care PT/INR testing has been in the hot seat for a number of reasons, among them a highly publicized recall of an INR monitoring system and other FDA-related issues.
Dr. Russell Higgins, MD, associate clinical professor, Department of Pathology and Laboratory Medicine, University of Texas Health Science Center at San Antonio, and medical director of UHS Pathology Services, encouraged the CAP17 audience to think of the laboratory INR as different from a point-of-care INR, which he noted has unique features. "First of all, we use whole blood in point of care often through a fingerstick. As the blood is traveling through that wound, it is exposed to tissue factor and the clotting process is beginning. So you really have to get that drop of blood on the meter as quickly as possible so you get the right answer. That’s something we don’t deal with in the laboratory."
Dr. Higgins presented a vignette involving a 45-year-old male patient who was bridged from low-molecular-weight heparin to warfarin. The patient had a point-of-care INR result of 5.2. The central lab INR was 2.7.
Low-molecular-weight heparin was the likely culprit. Dr. Higgins said the POC devices may not contain heparin neutralizing substances: "Central lab INRs do contain substances that neutralize heparin, usually up to one unit per mL, and they can be used for bridging warfarin from heparin therapy. But the point-of-care devices shouldn’t be used in this way." He also stressed that point-of-care or laboratory INRs cannot be used to monitor any of the direct oral anticoagulants.
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