Mayo Test Catalog, (Sorted By Test Name) - Mayo Medical ...

PT
9236
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)
sensitivity to coagulation deficiencies. More sensitive thromboplastins have a low ISI (1.0-1.2), whereas
less sensitive thromboplastins have a higher ISI (eg, 2.0-3.0). Calculation of the INR is as follows: INR =
(Patient's PT/mean PT of reference range)ISI where: INR = International Normalized Ratio ISI =
International Sensitivity Index Recommended INR therapeutic ranges for orally administered drugs are as
follows: Anticoagulation Intensity INR Standard Intensity 2.0-3.0 Higher Intensity 3.0-4.5 The INR is
used only for patients on stable oral anticoagulant therapy. It makes no significant contribution to the
diagnosis or treatment of patients whose PT is prolonged for other reasons. At Mayo Clinic and for Mayo
Medical Laboratories' clients, the PT test is performed with a sensitive thromboplastin (ISI 1.0+/- 0.05),
containing phospholipid and recombinantly derived TF.
Useful For: Monitoring intensity of oral anticoagulant therapy when combined with INR reporting
Screening assay to detect deficiencies of 1 or more coagulation factors (factors I, II, V, VII, X) due to:
-Hereditary or acquired deficiency states -Vitamin K deficiency -Liver disease -Specific coagulation
factor inhibitors Screening assay to detect coagulation inhibition ("circulating anticoagulants") associated
with: -Specific coagulation factor inhibitors -Lupus-like anticoagulant inhibitors (antiphospholipid
antibodies) -Nonspecific PT inhibitors (eg, monoclonal immunoglobulins, elevated fibrin degradation
products)
Interpretation: The PT test varies in its sensitivity to the activity of coagulation factors II, V, VII, and
X, and is least sensitive to decreased factor II. According to Mayo Clinic Special Coagulation laboratory
validation, using recombinantly derived thromboplastin with ISI close to 1.0, the PT begins to become
prolonged when: Factor II