Mixing Study

Test Code

8922

CPT Codes
85610, 85730

Includes

Prothrombin Time
PTT-LA, Baseline

If Prothrombin Time is prolonged (>13.5 seconds), then an Immediate PT-Mix will be performed at an additional charge (CPT code(s): 85611).

If PTT-LA, Baseline is prolonged (>40 seconds), then an Immediate PTT-LA Mix will be performed at an additional charge (CPT code(s): 85732).
If Immediate PTT-LA Mix corrects, then an Incubated PTT-LA Mix will be performed at an additional charge (CPT code(s): 85732).

Preferred Specimen

2 mL frozen platelet-poor plasma collected in a 3.2% sodium citrate (light blue-top) tube

Minimum Volume

1 mL

Instructions

Please submit a separate, frozen vial for each special coagulation assay ordered. Draw blood in a light blue-top tube containing 3.2% sodium citrate, mix gently by inverting 3-4 times. Centrifuge 15 minutes at 1500 g within one hour of collection. Using a plastic pipette, remove plasma, taking care to avoid the WBC/platelet buffy layer and place into a plastic vial. Centrifuge a second time and transfer platelet-poor plasma into a new plastic vial(s). Freeze immediately and transport on dry ice.

Transport Temperature

Frozen

Specimen Stability

Room temperature: Unacceptable
Refrigerated: Unacceptable
Frozen: 14 days

Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)

Gross hemolysis

Methodology
Clot Detection

Setup Schedule

Set up: Sun-Thurs; Report available: 2-5 days

Clinical Significance

This test is used to help differentiate coagulation factor deficiency from inhibitor activity in patients with prolonged clotting times-prothrombin time (PT) and activated partial thromboplastin time (aPTT).

Deficiency of a coagulation factor or factors can be congenital or acquired. Congenital factor deficiency is caused by pathogenic variants of the gene that encodes the coagulation factor; acquired factor deficiency is caused by inhibitors that interfere with factor activity. This mixing study can be used to determine the cause of factor deficiency. If the clotting time of the immediate mix remains prolonged ("not corrected"), the presence of an inhibitor is suspected (ie, lupus anticoagulant, factor specific inhibitors, anticoagulant, or other non-specific inhibitors). If the clotting time of the immediate mix is below the cut-off ("corrected"), a factor deficiency is suspected. Because some inhibitors (eg, inhibitors against factor VIII) can only be detected after incubation, an incubated mix may also be included with the aPTT mixing study [1].

This test can be used to guide the investigation of coagulation disorders but has limited sensitivity and specificity [1], specifically if the baseline clotting test is minimally prolonged. Tests for coagulation factors, specific inhibitors, and lupus anticoagulants may be needed for the diagnosis.

Note that the test results may be affected by the titer and strength of inhibitors, number of the deficient coagulation factors, heparin contamination, anticoagulants, direct thrombin inhibitors, or direct Xa inhibitors [1].

The results of this test should be interpreted in the context of pertinent clinical and family history and physical examination findings.

Reference
1. Higgins RA, et al. Coagulation, anticoagulation, fibrinolysis. In: Rifai R, et al, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier Inc; 2022.

The CPT Codes provided in this document are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed. Any Profile/panel component may be ordered separately. Reflex tests are performed at an additional charge.