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Do Plastic Tubes Affect INRs?

From Kim Kinney at Clarian:

I got a call from an area lab that is having some issues with INRs. These are all being drawn in BD 2.7 mL plastic tubes. They are getting large discrepancies in INRs after an initial spin. They got a call from an ambulatory area questioning a 4.23 INR. When they spun the sample again and repeated the INR was 1.63. They had the patient redrawn and the INR was 1.6. This has happened several times. It always starts elevated and re-spinning brings it down They are switching to glass tubes. They have done all sorts of studies, turning up nothing. They are now double spinning everything. Have you heard of anything with plastic tubes?

Thanks, Kim

Hi, Kim. The UAB lab has used 2.7 mL BD blue-closure tubes for years with no problems. I’d like to learn what others have seen.

Comments (1)
Nov 9, 2009 12:25pm

This is an interesting problem. There are a number of thing
This is an interesting problem. There are a number of things that come to mind and none of them explain the descrepiancy between the first spin and second spin results. These tubes, as well as the 1.8 ml tubes have a smaller inside diameter. The technician performing the phlebotomy should take special care that the air bubble in the tube moves from one end of the tube to the other when mixing the blood. This mixing step was much easier with the 4.5 ml glass tubes and if the same technique is used for the 2.7 ml tube, there will be insufficient and incomplete mixing of the sample. I would also verify that the tubes are completely filled. That does not completely explain the problem expressed by Kim. In Kim’s situation, I suspect there may be an instrument problem and would check the sample level detection system on the instrument. I would also verify there are no bubbles in the sample.

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