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Prothrombin time: one tube or two

B P Yawn¹, C Loge, J Dale

¹Olmstead Medical Group, Rochester, Minnesota 55904, USA.

American Journal of Clinical Pathology

|June 1, 1996

Summary

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Drawing a second blood tube for prothrombin time (PT) testing is not clinically necessary. This study found no significant difference in PT or international normalized ratio (INR) results between first and second tubes in outpatients.

Area of Science:

Clinical Laboratory Science
Hematology

Background:

Current guidelines recommend using the second blood tube for coagulation studies.
This practice may inconvenience patients on anticoagulation therapy, requiring discarding of the first tube.

Purpose of the Study:

To compare prothrombin times (PTs) and international normalized ratios (INRs) from first and second blood tubes.
To determine the clinical significance of using a second tube for PT testing.

Main Methods:

Prospective study involving 343 outpatients undergoing physician-ordered PT tests.
Paired first and second blood tubes were collected and analyzed for PT and INR.

Main Results:

No statistically significant differences were observed in PT or INR values between the first and second evacuated tubes.

Related Experiment Videos

The average difference in INR between tube 1 and tube 2 was 2% (SD 1.1%).

Conclusions:

The use of a second blood tube for routine prothrombin time testing in this outpatient sample is not clinically justified.
Current recommendations for discarding the first tube may be unnecessary for PT/INR analysis.