Partial Thromboplastin Time (PTT), Lupus Anticoagulant
| Partial Thromboplastin Time (PTT), Lupus Anticoagulant | | | |
| Number | | 117002 |
| CPT | | 85732 |
| Related Information | | Hemostasis and Thrombosis Appendix Lupus Anticoagulant With Reflex Partial Thromboplastin Time (PTT), Activated |
| Synonyms | | aPTT-LA ; aPTT, Lupus-Sensitive ; Lupus-Sensitive aPTT ; PTT |
| 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. |
| 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.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. To
avoid delays in turnaround time when requesting multiple
tests on frozen samples, please submit separate frozen
specimens for each test requested.
Please print and use the Specimen
Collection Bulletin as a tube-filling guide. |
| Storage Instructions | | Freeze |
| Patient Preparation | | Avoid warfarin (Coumadin®) therapy for 2 weeks and heparin therapy for 2 days prior to the test. Do not draw from an arm with a heparin lock or heparinized catheter. |
| Causes for Rejection | | Gross hemolysis; clotted specimen; frozen specimen thawed in transit; tubes <90% full; improper labeling; specimen collected in tube other than 3.2% citrate |
| Reference Interval | | Established with each lot of reagent |
| Use | | The aPTT-LA is a sensitive screening test for lupus anticoagulants (LA) in patients with a history of thrombosis.6 |
| Limitations | | Due to the heterogeneity of LA antibodies, no single assay will identify all cases7 The International Society on Thrombosis and Haemostasis (ISTH) has established criteria for the diagnosis of lupus anticoagulants. (See Lupus Anticoagulant With Reflex [117892] .) The ISTH has defined the minimum diagnostic criteria for LA to include:7,8,9 - A prolonged clot time in a screening assay such as aPTT-LA and/or dRVVT
- Mixing studies indicating the presence of an inhibitor
- Positive confirmatory studies defining phospholipid dependence of the inhibitor
- No evidence of other coagulopathies through the use of specific factor assays if the confirmatory step is negative or if there is evidence of a specific factor inhibitor
Factor VIII elevations, as can occur due to acute phase reactions, can normalize a mildly extended aPTT result.6 |
| Methodology | | The aPTT-LA reagent consisting of silica mixed with phospholipid from rabbit cerebellum is mixed with the patient plasma. The silica provides a negatively-charged particulate surface for the activation of the contact pathway of coagulation. The amount of phospholipid included in the aPTT-LA reagent is diminished relative to the standard aPTT reagent. This increases the assay sensitivity for LA. Calcium chloride is then added to the sample/reagent mixture to initiate clot formation. The time to clot formation is measured photo-optically. |
| Additional Information | | Lupus anticoagulants are nonspecific antibodies that extend the clotting time of phospholipid-dependent clotting assays such as the aPTT.7,10 Unlike specific factor antibodies, LA are usually associated with venous thrombosis, pulmonary embolism, arterial thrombosis, and recurrent fetal loss.11 LA do not specifically inhibit individual coagulation factors; rather they neutralize anionic phospholipid-protein complexes that are involved in the coagulation process. Prolongation of clot-based assays is highly dependent on the sensitivity of the reagent employed. Reagents with reduced amounts of phospholipid, such as the aPTT-LA and dilute Russell viper venom time (dRVVT), have enhanced sensitivity for LA.7 Testing for lupus anticoagulant (LA) and the antiphospholipid syndrome that is associated with these antibodies is described in more detail in the Hemostasis and Thrombosis Appendix . The aPTT is sensitive to deficiency or inhibition of factors in the intrinsic pathway. These include the contact factors; high molecular weight kininogen (HMWK); prekallikrein; and factor XII along with procoagulant factors XI, IX, VIII, X, V, prothrombin, and fibrinogen.10,12,13,14 Nonspecific, lupus-type anticoagulants can also extend the aPTT, but the more sensitive aPTT-LA test should be used to screen for this condition. The aPTT can be prolonged when the activities of any of the factors of the intrinsic pathway are significantly diminished. An extended aPTT can be seen in acquired deficiencies of intrinsic factors II, IX, and X that result from vitamin K deficiency or the use of oral anticoagulants that block vitamin K-dependent production of procoagulant factors. These conditions also affect the level of factor VII, an extrinsic pathway factor. Since factor VII has a short half-life relative to the vitamin K-dependent factors of the intrinsic pathway, vitamin K-dependent factor deficiency can often result in an extended PT with a normal aPTT. Consumption coagulopathy, such as disseminated intravascular coagulation (DIC), can produce an extended aPTT due to depletion of intrinsic factors. The aPTT can also be extended in conditions that reduce the production of procoagulant factors (ie, severe liver disease or malnutrition). Inhibitors, both factor specific and nonspecific, can also prolong the aPTT. A description of the many potential causes of an extended aPTT is described in more detail in the Hemostasis and Thrombosis Appendix . |
| Footnotes | | - Adcock DM, Kressin DC, and Marlar RA, “Effect of 3.2% vs 3.8% Sodium Citrate Concentration on Routine Coagulation Testing,” Am J Clin Pathol, 1997, 107(1):105-10.
- Reneke J, Etzell J, Leslie S, et al, “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, 109(6):754-7.
- “National Committee for Clinical Laboratory Standardization: Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays; Approved Guideline,” Third Edition, Villanova: NCCLS Document H21-A3:11(23), 1999.
- Gottfried EL and Adachi MM, “Prothrombin Time and Activated Partial Thromboplastin Time Can Be Performed on the First Tube,” Am J Clin Pathol, 1997, 107(6):681-3.
- McGlasson DL, More L, Best HA, et al, “Drawing Specimens for Coagulation Testing: Is a Second Tube Necessary?” Clin Lab Sci, 1999, 12(3):137-9.
- Van Cott EM and Laposata M, “Coagulation,” Laboratory Test Handbook With Key Word Index, Jacobs DS, DeMott WR, and Oxley DK eds, Hudson, OH: Lexi-Comp, 2001, 327-58.
- Brandt JT, Triplett DA, Alving B, et al, “Criteria for the Diagnosis of Lupus Anticoagulants: An Update. On Behalf of the Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibody of the Scientific and Standardization Committee of the ISTH,” Thromb Haemost, 1995, 74(4):1185-90.
- Alving BM, “The Antiphospholipid Syndrome: Clinical Presentation, Diagnosis and Patient Management,” Consultative Hemostasis and Thrombosis, Kitchens CS, Alving BM, and Kessler CM, eds, Philadelphia, PA: WB Saunders Co, 2002, 181-96.
- Levine JS, Branch DW, and Rauch J, “The Antiphospholipid Syndrome,” N Engl J Med, 2002, 346(10):752-63.
- Triplett DA, “Coagulation Abnormalities,” Clinical Laboratory Medicine, 2nd ed, McClatchey KD, ed, Philadelphia, PA: Lippincott Williams and Wilkins, 2002, 1033-49.
- Bick RL, “Antiphospholipid Thrombosis Syndromes,” Clin Appl Thromb Hemost, 2001, 7(4):241-58.
- Roberts HR and Escobar MA, “Less Common Congenital Disorders of Hemostasis,” Consultative Hemostasis and Thrombosis, Kitchens CS, Alving BM, and Kessler CM, eds, Philadelphia, PA: WB Saunders Co, 2002, 57-71.
- Adcock DM, Jensen R, Johns CS, et al, Coagulation Handbook, Esoterix Coagulation, 2002.
- Cohen AJ and Kessler CM, “Hemophilia A and B,” Consultative Hemostasis and Thrombosis, Kitchens CS, Alving BM, and Kessler CM, eds, Philadelphia, PA: WB Saunders Co, 2002, 43-56
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