Lab Dept:


Test Name:


General Information

Lab Order Codes:



Protime Mixing Study; Prothrombin Time Mixing Study; Simple PT Mixing Study

CPT Codes:

85611 - Prothrombin time: substitution, plasma fractions, each

Test Includes:

PT 1:2 dilution reported in seconds.


Test Indications:

Useful for detecting the presence/absence of an inhibitor or the absence of specific coagulation factors.

Lab Testing Sections:


Phone Numbers:

MIN Lab: 612-813-6280

STP Lab: 651-220-6550

Test Availability:

Daily, 24 hours

Turnaround Time:

4 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. ● Specimen must arrive within 30 minutes of collection.
● Indicate when specimen is drawn from a line or a heparin lock.


Specimen Type:

Whole blood


Light Blue top tube (Buffered Na Citrate 3.2%)

Draw Volume:

2.7 mL blood

Processed Volume:

1 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 4 hours. ● Do not refrigerate.
● If not received in our lab within 4 hours of collection, sample must be centrifuged and *platelet-poor plasma removed from cells and transferred to an aliquot tube being careful not to disturb the cell layer. Centrifuge the plasma a second time and transfer into a clean aliquot tube being careful not to include any residual platelets on the bottom of the 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: Spin sample collected in blue top tube for 5 minutes on the Stat Spin centrifuge, remove plasma and transfer to a 4 mL BCS sample cup. Spin remaining plasma again for 5 minutes in the Stat Spin Centrifuge. Transfer plasma to a new BCS sample cup for analysis leaving approximately 200 uL in the bottom of the original cup to discard.

Test within:
● Four (4) 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, Overfilled tubes, Specimen more than 2 hours old, Mislabeled or unlabeled specimens


Reference Range:

If an inhibitor is present the PT will not correct into the normal range. If the patient is lacking specific coagulation factors the PT will demonstrate significant correction.

Critical Values:



Correction reactions may be difficult if the patient’s PT is only modestly prolonged. Samples drawn through line or heparin lock will not correct giving the impression of an inhibitor.


PT based correction of patient’s plasma with normal plasma.


Current anticoagulant therapy.


Harmening DH (1997) Clinical Hematology and Fundamentals of Hemostasis


7/19/2006: Removed statement, "Do not transport by pneumatic tube".
12/14/2010: Updated processing information.
8/19/2013: Updated specimen stability information.
9/15/2014: Added Off Campus collection info.