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UW Health SMPH
American Family Children's Hospital

UW Hospital and Clinics Lab Test Directory

Test Name: APTT Inhibitor Screen

Test Code(s): APTI / HCAPTI

CPT Code(s): 85730 x2, 85732 x2

Test Description: Included In: May be included in a Lupus Anticoagulant evaluation.

Methodology: Mechanical Clot Detection

Clinical Significance: Also known as a "mixing study". An inhibitor screen is performed to explain a prolonged APTT (greater than 10 seconds above reference interval).

Days Performed: Mon-Fri, dayshift - day varies. Specimens are accepted at any time. Testing required during off hours must be approved by a Coagulation physician. Call 263-5005 before collection.

Turnaround Time: Stat: 1 working day.
Routine: 4 - 7 working days.

Specimen Requirements

Specimen: Blood

Patient Preparation: Patient MUST be off heparin for at least 72 hours and off a direct thrombin inhibitor for 24 hours prior to collection.

Collection Instructions: Do NOT draw through a line containing heparin. The venipuncture MUST be atraumatic to avoid hemolysis and contamination with tissue factors and platelets. Tube must be allowed to fill completely.  

Please indicate patient history and if patient is receiving anticoagulant(Coumadin/ Heparin/Vitamin K antagonist), direct thrombin inhibitor or fibrinolytic agent therapy and duration on request form.

Collection Container: Preferred: Light blue top (3.2% NaCitrate)

Collection Volume: Preferred: Four full 3.5 mL tubes
Pediatrics: 2 full pediatric tubes filled to mark

Sample Analyzed: Plasma

Volume Required: Preferred: 5 mL
Pediatrics: 1.5 mL

Specimen Processing: Ultracentrifuge sample and label "ULT". If an ultracentrifuge is not available, centrifuge specimen to yield platelet poor plasma (platelet count should be 10 K/L or less). Separate plasma and transfer to plastic tube and spin again. Transfer this plasma to clean plastic tube or vial. Indicate on tube "spun twice". Freeze plasma within 4 hours of collection at -20C or below.

Specimen Transport: Transport whole blood specimen at room temperature to UWHC Core Laboratory (B4/220) within 4 hours of collection. See Specimen Processing for instructions if transport criteria are not met.  Transport frozen plasma aliquot on dry ice to UWHC Core Laboratory (B4/220).

Unacceptable Criteria: Specimens collected from patients who have received heparin within 72 hours of collection or direct thrombin inhibitor within 24 hours of collection are not acceptable.  Hemolyzed specimens, clotted specimens, partially filled tubes and whole blood specimens greater than 4 hours old when received by the laboratory are not acceptable. Test cannot be done if patient is receiving heparin.

Stability: Ambient: 4 hours on spun tube
Refrigerated: Not acceptable
Frozen: 2 weeks at -20C; 6 months at -70C


Reference Interval:

18 years and up: Negative for an inhibitor

Additional Information:

The presence of an inhibitor can be ascertained by performing an APTT before and after the incubation of a 1:1 mixture of patient plasma and normal plasma. Lack of correction of the APTT mixture into the reference range indicates the presence of an inhibitor (reported as Positive). Correction of the APTT mixture into the reference range indicates a factor deficiency (reported as Negative).

Mixing Studies Inhibitor Screen APTT PTT Inhibitor Screen

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