Sorted By Test Name - Mayo Medical Laboratories

PT
9236
C, protein S, or antithrombin III deficiency or the factor V Leiden (R506Q) mutation.(3) Women PT
G20210A carriers who receive oral contraceptive therapy also are at increased risk for VTE, particularly
cerebral venous sinus thrombosis. Recurrent venous thromboembolism is increased 2-fold among PT
G20210A carriers, especially among carriers of both the PT G20210A and factor V Leiden (R506Q)
mutations.(4) An association between the PT G20210A allele and arterial occlusive disease (eg, coronary
artery disease and myocardial infarction) is uncertain(2); currently, evidence suggests a possible
association. This test is a direct mutation analysis of patient blood leukocyte genomic DNA for the PT
G20210A gene mutation. At present, there are no other methods of detecting this VTE risk factor.
Useful For: Direct mutation analysis for the prothrombin (PT) G20210A allele in patients with
documented venous thromboembolism, deep vein thrombosis, or pulmonary embolism.
Interpretation: The interpretive report will include specimen information, assay information,
background information, and conclusions drawn from the test results (normal, heterozygous prothrombin
(PT) G20210A, homozygous PT G20210A).
Reference Values:
Negative
Clinical References: 1. Poort SR, Rosendaal FR, Reitsma PH, Bertina RM: A common genetic
variation in the 3'untranslated region of the prothrombin gene is associated with elevated plasma
prothrombin levels and an increase in venous thrombosis. Blood 1996;10:3698-3703 2. Ferraresi P,
Marchetti G, Legnani C, et al: The heterozygous 20210 G/A prothrombin genotype is associated with
early venous thrombosis in inherited thrombophilias and is not increased in frequency in artery disease.
Arterioscler Thromb Vasc Biol 1997;17:2418-2422 3. Makris M, Preston FE, Beauchamp NJ, et al:
Co-inheritance of the 20210 A allele of the prothrombin gene increases the risk of thrombosis in subjects
with familial thrombophilia. Thromb Haemost 1997;78:1426-1429 4. De Stefano V, Martinelli I, Manucci
PM, et al: The risk of recurrent deep venous thrombosis among heterozygous carriers of both factor V
Leiden and the G20210A prothrombin mutation. N Engl J Med 1999;341:801-806
Prothrombin Time, Plasma
Clinical Information: The prothrombin time (PT) represents the time elapsed between 1) addition of
a standardized mixture of tissue thromboplastin and calcium to citrate anticoagulated plasma and 2)
detection of clot formation, representing fibrin polymerization resulting from the generation of thrombin
which proteolytically transforms fibrinogen to fibrin. Tissue thromboplastin is a mixture of phospholipid
vesicles and tissue factor (TF), a protein cofactor. Tissue thromboplastins have traditionally been prepared
from animal tissue extracts (brain, placenta, lung), however the recent availability of recombinantly
derived human TF combined with purified phospholipid mixtures allows preparation of well-defined
tissue thromboplastin with several potential advantages. Together with phospholipid, TF forms a complex
with coagulation factor VII/VIIa (activated factor VII), providing an enzyme-cofactor complex which, in
the presence of ionic calcium, activates proenzyme coagulation factor X to the enzyme factor Xa. Factor
Xa, in turn, forms a complex with phospholipid, calcium, and activated factor V (Va, a protein cofactor)
to form prothrombinase, which hydrolyzes factor II substrate (prothrombin) to the active coagulant
enzyme thrombin. Thrombin hydrolyzes fibrinogen (factor I) by cleaving specific peptides
(fibrinopeptides A and B), to form fibrin monomer, which assembles into fibrin polymers (a clot). The PT
is not sensitive to deficiencies of coagulation factors VIII, IX, XI, XII ("intrinsic pathway" factors), or
factor XIII, although the TF/VIIa complex can activate factor IX (in addition to factor X). A prolonged PT
indicates deficiency of 1 or more coagulation factors (I, II, V, VII, or X) or the presence of a coagulation
inhibitor. The PT is the most common test used for monitoring oral anticoagulant therapy (warfarin or
Coumadin, and congeners).Oral anticoagulants reduce the activities of the 4 vitamin K-dependent
procoagulant factors (factors II, VII, IX, and X), and the PT is sensitive to 3 of them. The PT requires
standardization because there are numerous thromboplastins and coagulation testing instruments, and they
all vary in their responsiveness to the concentrations or activities of coagulation proteins. The
International Normalized Ratio (INR) is a method of standardizing PT reporting for monitoring the
intensity of oral anticoagulant therapy. The INR is the ratio of the patient's PT to the laboratoryâ€s
mean normal (reference) PT. The International Sensitivity Index (ISI) is an experimentally derived
measurement, usually provided by the thromboplastin manufacturer, reflecting thromboplastin (and PT)
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