From Diane Treadway: Hi, we had a situation in our cath lab last week. The patient was on the table. They performed an ISTAT PT and obtained 13.0 seconds and 1.1 INR. The patient had had a PT drawn and run by the main lab earlier. Those results on a Stago instrument were 24.2 seconds and a 2.2 INR.
The POC coordinator told me the sample for ISTAT was drawn through the port. I asked her if they flushed the port, and she affirmed. The cath lab nurses stated the patient was not on any medication. The test was repeated by both methods, and similar results obtained. I had the POC coordinator pull the patient’s medication list. At about the time of the procedure, there was an order for heparin and bivalirudin. I feel the bivalirudin affected the Stago PT. I present this information to you for your guidance and thoughts. In this case, I felt the Stago results were more accurate. This caused the nurse to think something was wrong with the ISTAT. I explained to the POC coordinator that there were two different methods being used that did not use the same sample type. We are going to do a correlation between these two methods. Literature I found states that the INR should not differ by more than +/- 0.5. Since I am basing my opinions on articles I read, I would really value your opinion and expertise on this situation. thanks so much!
Hello, Ms. Treadway, and thank you for this interesting case. I believe you are correct in concluding the bivalirudin prolonged the PT, though the degree of prolongation is somewhat surprising. In chapter 25, Heparin Induced Thrombocytopenia, of Kitchens CS, Kessler CM, Konkle BA, Consultative Hemostasis and Thrombosis, 3rd edtion, Elsevier, 2013, Dr. Ted Warkentin writes that an advantage of bivalirudin over argatroban is “minimal prolongation of the INR.” It may be that bivalirudin’s effect on the PT may be reagent and coagulometer specific, however, as you conclude. I’m curious whether heparin and bivalirudin were administered simultaneously, whether the ISTAT and central lab specimens were collected at the same time from the port, and also what the PTT results were from the central lab specimen. As always, I invite our participants to comment with your own experience comparing POC and central lab results.