Do Elevated Hematocrits Prolong the PT/aPTT?

Melissa Austin; Chris Ferrell; Morayma Reyes

doi:10.29074/ascls.26.2.89

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Melissa Austin, MD⇑
1. Departments of Pathology and Laboratory Medicine, University of Washington Medical Center, Seattle, WA
Chris Ferrell, MT(ASCP)
1. Department of Laboratory Medicine, Harborview Medical Center, Seattle, WA
Morayma Reyes, MD, PhD
1. Department of Laboratory Medicine, University of Washington Medical Center, Seattle, WA

Address for Correspondence: Melissa Austin, MD, Resident in Anatomic and Clinical Pathology, Departments of Pathology and Laboratory Medicine, University of Washington Medical Center, Seattle, WA, (206) 598-6400, mcaustin{at}uw.edu

Abstract

The Clinical and Laboratory Standards Institute guidelines require special processing of whole blood specimens with hematocrits greater than 55% due to the possibility of spurious prolongation of routine coagulation studies (PT, aPTT). As samples with hematocrits above 60% are rare at our institution, our study seeks to determine the effect of relative citrate excess on routine coagulation studies in samples with hematocrits of 60% to determine whether special processing is necessary. A calculated volume of 3.2% citrate was added to 1 mL aliquots of 40 whole blood samples in citrated tubes from adult patients to simulate a hematocrit of 60%. A dilutional control was created by adding an equivalent volume of saline to a separate 1 mL aliquot. Routine coagulation studies (PT, aPTT) were run on both samples on the STA Compact Analyzer in accordance with manufacturer instructions. While a paired Student's t-test demonstrated a clinically significant change in both PT and aPTT with the addition of citrate (p = 0.0002 for PT and p = 0.0234 for aPTT), clinical management would not have been altered by any observed change. More interestingly, we observed a shortening of 27/40 PTs and 23/40 aPTTs rather than the expected prolongation. Based on our data, no adjustment of citrate volume appears to be necessary in samples with hematocrits less than or equal to 60%.

ABBREVIATIONS: CLSI - Clinical and Laboratory Standards Institute, PT - prothrombin time, aPTT - activated partial thromboplastin time, CBC - Complete Blood Count