Prothrombin Time (PT) Mixing Study

TEST: 117028
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CPT: 85610; 85611
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Synonyms

  • PT Mixing Study

Special Instructions

If the patient's hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted. Refer to Coagulation Collection Procedures for directions.

Expected Turnaround Time

2 - 3 days

Turnaround time is defined as the usual number of days from the date of pickup of a specimen for testing to when the result is released to the ordering provider. In some cases, additional time should be allowed for additional confirmatory or additional reflex tests. Testing schedules may vary.

Related Information

Related Documents

For more information, please view the literature below.

Procedures for Hemostasis and Thrombosis: A Clinical Test Compendium

Specimen Requirements

Specimen

Plasma, frozen

Volume

2 mL

Minimum Volume

1 mL

Container

Blue-top (sodium citrate) tube

Collection

Citrated plasma samples should be collected by double centrifugation. Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood to anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples, except when using a winged blood collection device (ie, "butterfly"), in which case a discard tube should be used.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternate anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes. Centrifuge for 10 minutes and carefully remove 2/3 of the plasma using a plastic transfer pipette, being careful not to disturb the cells. Deliver to a plastic transport tube, cap, and recentrifuge for 10 minutes. Use a second plastic pipette to remove the plasma, staying clear of the platelets at the bottom of the tube. Transfer the plasma into a Labcorp PP transpak frozen purple tube with screw cap (Labcorp No. 49482). Freeze immediately and maintain frozen until tested.

Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.

Storage Instructions

Freeze.

Patient Preparation

Ideally, the patient should not be on anticoagulant therapy. Avoid warfarin (Coumadin®) therapy for two weeks prior to the test and heparin, direct Xa, and thrombin inhibitor therapies for about three days prior to testing.

Causes for Rejection

Severe hemolysis; improper labeling; clotted specimen; specimen diluted with IV fluids; samples thawed in transit; improper sample type; sample out of stability

Test Details

Use

Distinguish between circulating inhibitor and coagulation factor deficiency when PT is prolonged

Methodology

Prothrombin time assay

Additional Information

A mixing study is used to study the cause of a prolonged screening test. This study can determine if the cause is a deficiency of one or more factors or an inhibitor.6 In a mixing study, platelet-free, normal plasma that is replete with all coagulation factors (near 100% activity for each) is mixed with the patient sample. For example, in a 1:1 mix, one part patient sample is mixed with one part normal plasma and the mixture is tested. In this case, the lowest possible concentration for any individual factor in the mixture would be approximately 50% (in the case of a patient with a factor concentration of zero) and the normal pool has an activity of 100%. In the absence of inhibitors, this mixture will produce a normal clotting time. When addition of normal plasma fails to correct the clotting time into the normal range, the cause of the abnormal test is likely an inhibitor. The performance and interpretation of mixing studies is described in more detail in the online Coagulation Appendices: Abnormal Screening Results and Lupus Anticoagulants.

Footnotes

1. Adcock DM, Kressin DC, Marlar RA. Effect of 3.2% vs 3.8% sodium citrate concentration on routine coagulation testing. Am J Clin Pathol. 1997 Jan; 107(1):105-110. 8980376
2. Reneke J, Etzell J, Leslie S, Ng VL, Gottfried EL. Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant. Am J Clin Pathol. 1998 Jun; 109(6):754-757. 9620035
3. National Committee for Clinical Laboratory Standardization. Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays; Approved Guideline. 5th ed. Villanova: NCCLS; 2008. Document H21-A5:28(5)
4. Gottfried EL, Adachi MM. Prothrombin time and activated partial thromboplastin time can be performed on the first tube. Am J Clin Pathol. 1997 Jun; 107(6):681-683. 9169665
5. McGlasson DL, More L, Best HA, Norris WL, Doe RH, Ray H. Drawing specimens for coagulation testing: Is a second tube necessary? Clin Lab Sci. 1999 May-Jun; 12(3):137-139. 10539100
6. Triplett DA. Coagulation abnormalities. In: McClatchey KD, ed. Clinical Laboratory Medicine. 2nd ed. Philadelphia, Pa: Lippincott Williams and Wilkins; 2002:1033-1049.

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
117028 PT Mixing Study 117030 PT sec 5902-2
117028 PT Mixing Study 117031 PT 1:1NP sec 5959-2
117028 PT Mixing Study 117032 1 Hr Incub PT 1:1NP sec 33356-7

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