• Activated partial thromboplastin time (aPTT)
• Factor V activity
• Factor V Bethesda titer
• Prothrombin time (PT)
• PT 1:1 mix with normal plasma
• PT 1:1 mix with saline

Confirmation and characterization of factor V inhibitor

The patient should not be on anticoagulant therapy

Lt blue Sodium citrate (NaCit) - 2.7mL x 3

• Do not draw from an arm with a heparin lock or heparinized catheter
• Immediatley following collection, mix sample thoroughly by gentle inverting 8 - 10 times, to prevent clotting

1. Centrifuge for 10 minutes and carefully remove 2/3 of the plasma using a plastic transfer pipette, being careful not to disturb the cells.
2. Deliver to a plastic transport tube, cap, and recentifuge for 10 minutes.
3. Use a second plastic pipette to remove plasma, staying clear of the platelets at the bottom of the tube.
4. Transfer 2 mL of plasma into three (3) separate Screw-cap polypropylene frozen transport vial/tube - 4mL (LabCorp) labelled as Na cit plasma
5. Freeze immediately

Screw-cap polypropylene frozen transport vial/tube - 4mL (LabCorp) x 3

Frozen (preferred) – 30 days

Ambient – 4 hours

Refrigerated – NO

The factor V inhibitor (Bethesda titer) assay is performed using a prothrombin time-based system, using thromboplastin reagent.6 Serial dilutions are made of patient plasma with veronal buffered saline, then mixed with normal plasma containing close to 100% factor V activity, and are then incubated for two hours. A PT-based factor V assay using factor V-depleted plasma substrate is then performed on these incubated mixtures. Results are compared to those of incubated normal plasma. One Bethesda unit is defined as the amount of factor V inhibitor that neutralized 0.5 IU of factor V in this system. The number of serial dilutions tested is based on the anticipated level of the inhibitor.

Prothrombin Time / PT 1:1 NP

0 - 3 d:       10.7 - 15.6 sec

4 d - 6 m:   10.4 - 13.9 sec

7 m - 17 y:  10.1 - 12.7 sec

≥18 y:         9.6 - 11.5 sec 

APTT

0 - 3 d:       not established

4 d - 6 m:   23.1 - 32.5 sec

7 m - 17 y:  23.1 - 30.1 sec

≥18 y:        22.9 - 30.2 sec 

Factor V activity

0 - 3 d:       not established

4 d - 6 m:   34 - 108 %

≥7 m:  23.1 - 30.1 sec

Facator V inact assay

<0.8 Bethesda