Hello Mr. Fritsma,
I am a pharmacy student working in an internal medicine clinic where we manage patients on warfarin. We had a question come in about how an elevated hemoglobin and hematocrit, HGB/HCT can affect INR levels as the lab is stating that as long as this patient’s hemoglobin is elevated, her INR results will not be accurate. They have apparently also been having trouble getting a reading for the past couple weeks. This patient’s last HGB = 20.1 and HCT = 62. Do you have any insight as to how these elevated values would affect the INR? Looking forward to hearing back from you.
Thank you, Kayla Brown, PharmD Candidate 2012, Drake University
Hello, Ms. Brown, and thank you for your question. Most anticoagulation clinics employ a point of care (POC) instrument such as the CoaguChek or iStat to perform prothrombin times and compute international normalized ratios (PT/INR). POC instruments are cleared to report the PT/INR when the HCT is 25%–55%. Outside that range, a standard blue-closure venous specimen is collected and accurately assayed using a plasma-based method in the “central” laboratory.
The plasma-based assay is accurate when the HCT is 65% or less. When the HCT is over 65%, the ratio of anticoagulant volume in the collection tube to plasma volume is proportionally elevated, falsely raising the PT/INR. In this case the lab scientist prepares a tube with a measured volume of 3.2% sodium citrate using this formula:
C (mL) = 0.00185 x (100-HCT) x V (mL), where…
- C = mL of 3.2% sodium citrate to be used
- HCT = hematocrit in %
- V = mL of whole blood/anticoagulant solution in the tube.