Lab Dept:


Test Name:


General Information

Lab Order Codes:



PT; Protime; Prothrombin Time and INR for patients receiving oral anticoagulants, ie, Coumadin/Warfarin

CPT Codes:

85610 - Prothrombin time

Test Includes:

PT reported in seconds and INR


Test Indications:

Useful for monitoring or adjusting the amount of Coumadin for a patient on oral anticoagulant therapy.

Lab Testing Sections:


Phone Numbers:

MIN Lab: 612-813-6280

STP Lab: 651-220-6550

Test Availability:

Daily, 24 hours

Turnaround Time:

2 hours

Special Instructions:

● Patient’s with hematocrit levels >55% must have a special tube made to adjust for the hematocrit; contact the laboratory for special tube.| ● Depending on the patient's hematocrit, up to 3 assays can be determined using one 1.8 mL light blue top tube.


Specimen Type:

Whole blood


Light Blue top (Buffered Na Citrate 3.2%) tube

Draw Volume:

1.8 mL or 2.7 mL blood

Processed Volume:

1 mL (Minimum: 0.5 mL) plasma


● A clean venipuncture is essential, avoid foaming.
● Entire sample must be collected with single collection, pooling of sample is unacceptable.
● Capillary collection is unacceptable.
● Patient’s with a hematocrit level >55% must have a special tube made to adjust for the hematocrit; contact lab for a special tube.
● Mix thoroughly by gentle inversion. Deliver immediately to the laboratory at room temperature via courier or pneumatic tube.

Off campus collections:
● Must be tested within 24 hours. ● Do not refrigerate.
● If not received in our lab within 24 hours of collection, sample must be centrifuged and *platelet-poor plasma removed from cells and transferred to an aliquot tube. Freeze at -20°C and deliver to the lab on dry ice within 2 weeks.
*Validation of your lab’s centrifuge for platelet poor plasma is required.

Special Processing:

Lab staff: Centrifuge in Stat Spin for 5 minutes or 10 minutes at 3000 rpm at room temperature. For primary tube testing, leave plasma on cells OR remove plasma and place in a 4 mL plastic cup; allow for 100 uL of deadspace.

Test within:
● Twenty-four (24) hours when stored  as plasma remaining in the capped tube above the packed cells 18 to 24°C.
● Four (4) hours as plasma that has been separated from cells by centrifugation when stored 2 to 8°C or 18 to 24°C.
● Two (2) weeks when stored -20°C.
● Six (6) months when stored -70°C (rapidly frozen).
● Plasma must be frozen if testing cannot be completed within four (4) hours.
● Frozen plasmas are thawed at 37°C for three (3) minutes, test immediately.

Patient Preparation:


Sample Rejection:

Improper tube; clotted specimen; underfilled tubes; mislabeled or unlabeled specimens


Reference Range:


All ages

INR Therapeutic Ranges for Oral Anticoagulants

Prophylactic dose:

1.4 – 1.9

Standard dose:

2.0 – 3.0

High dose:

2.5 – 3.5

Note: Extremely high results will be reported as PT >120.0, INR >9.0

Critical Values:

INR: <1.4 or >3.5


Recombinant human tissue factor combined with synthetic phospholipids (thromboplastin)


Harmening DH (1997) Clinical Hematology and Fundamentals of Hemostasis


8/19/2013: Updated specimen stability information.
12/18/2013: Updated specimen stability information.
9/15/2014: Added Off Campus collect info.