Clinics and Departments

Laboratory Services

Lab Dept:


Test Name:


General Information

Lab Order Codes:



APTT Mixing Studies; APTT Correction Studies; Mixing Study, PTT; Inhibitor Screen; Differential PTT; PTT Substitution

CPT Codes:

85732 – Thromboplastin time, partial (PTT); substitution, plasma fractions, each

Test Includes:

PTT 1:2 dilution reported in seconds.


Test Indications:

Useful for detecting the presence or 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 Type:

Whole blood


Light Blue top tube (Buffered Na Citrate 3.2%)

Draw Volume:

2.7 mL blood

Processed Volume:

Minimum: 1 mL plasma


● A clean venipuncture is essential.
● Mix thoroughly by gentle inversion.
● Deliver immediately to the laboratory.
● Refrigerate specimen if there is a delay in transport of 30 minutes or more.

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 24C.
● Four (4) hours as plasma that has been separated from cells by centrifugation when stored 2 to 8C or 18 to 24C..
● Two (2) weeks when stored -20C.
● Six (6) months when stored -70C (rapidly frozen).

● Plasma must be frozen if testing cannot be completed within four (4) hours.
● Frozen plasmas are thawed at 37C for three (3) minutes, test immediately.

Patient Preparation:


Sample Rejection:

Improper tube; clotted sample; underfilled tube; mislabeled or unlabeled specimens


Reference Range:

If an inhibitor is present the PTT will not correct into the normal range with pre/post incubation. If the patient is lacking specific coagulation factors the PTT will demonstrate significant correction.

Critical Values:



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


PTT 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".
1/5/2011: Updated specimen processing information.
8/20/2013: Updated specimen stability information.

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