Test ID: PT
Prothrombin Time, Plasma
Secondary ID 
A test code used for billing and in test definitions created prior to November 2011
NY State Approved Indicates the status of NY State approval and if the test is orderable for NY State clients.
Useful For Suggests clinical disorders or settings where the test may be helpful
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 prothrombin time inhibitors (eg, monoclonal immunoglobulins, elevated fibrin degradation products)
Method Name A short description of the method used to perform the test
Electromagnetic Viscosity Detection System
Reporting Name A shorter/abbreviated version of the Published Name for a test; an abbreviated test name
Aliases Lists additional common names for a test, as an aid in searching
Prothrombin Time, Plasma
Protime
PT
Specimen Type Describes the specimen type needed for testing
Specimen Required Defines the optimal specimen. This field describes the type of specimen required to perform the test and the preferred volume to complete testing. The volume allows automated processing, fastest throughput and, when indicated, repeat or reflex testing.
Platelet-Poor Plasma
Collection Container/Tube: Light blue top (3.2% sodium citrate)
Submission Container/Tube: Plastic vial
Specimen Volume: 0.5 mL
Specimen Stability Information: Frozen
Whole Blood (Available to local accounts only)
Container/Tube: Light blue top (3.2% sodium citrate)
Specimen Volume: 4.5 mL
Specimen Stability Information: Ambient
Collection Instructions: Protime must be completed within 24 hours of draw time.
Specimen Minimum Volume Defines the amount of specimen required to perform an assay once, including instrument and container dead space. Submitting the minimum specimen volume makes it impossible to repeat the test or perform confirmatory or perform reflex testing. In some situations, a minimum specimen volume may result in a QNS (quantity not sufficient) result, requiring a second specimen to be collected.
Plasma: 0.5 mL
Reject Due To Identifies specimen types and conditions that may cause the specimen to be rejected
| Hemolysis | NA |
| Lipemia | NA |
| Icterus | NA |
| Other | NA |
Specimen Stability Information Provides a description of the temperatures required to transport a specimen to the laboratory. Alternate acceptable temperature(s) are also included.
| Specimen Type | Temperature | Time |
|---|---|---|
| Plasma Na Cit | Frozen (preferred) | 30 days |
| Ambient | 24 hours |
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
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 laboratory’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 to 3.0
-Higher Intensity: 3.0 to 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.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
PROTHROMBIN TIME
9.5-13.8 seconds
INR
0.8-1.2
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.
Interpretation Provides information to assist in interpretation of the test results
The prothrombin time (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 <27% (normal range: 75% to 145%)
-Factor V <47% (normal range: 70% to 165%)
-Factor VII <42% (normal range: 65% to 180%)
-Factor X <34% (normal range: 70% to 150%)
Mixing studies with normal plasma (ie, adding various proportions of normal pooled plasma to patient plasma) are useful in initial evaluation of prolonged PT when the cause of a prolonged PT is unknown (eg, not attributable to known oral anticoagulation or known coagulation factor deficiency):
-Typically an equal volume mixture (1:1) of patient and normal plasma shortens the prolonged PT into the normal (reference) range when there is a deficiency of 1 or more of the clotting factors (I, II, V, VII, X). Failure to normalize the PT in 1:1 mixing suggests presence of an inhibitor (eg, specific factor inhibitor, lupus-like anticoagulant or antiphospholipid antibody, non-specific inhibitor).
-Typically the addition of patient plasma of 1/10 or 2/10 volume of normal plasma shortens the prolonged PT, at least halfway toward the upper normal range, when there is a deficiency of 1 or more relevant coagulation factors. Inhibition is implied by failure to significantly shorten the PT.
-Additional coagulation testing may be needed to define the cause of an unexplained prolonged PT (eg, other clotting time tests, coagulation factor assays, testing for presence of a lupus-like anticoagulant). Mixing studies and such additional testing may be included in consultative testing (Coagulation Consultation).
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Not useful for detecting deficiencies of coagulation factors that have no influence on the PT test (eg, factors VIII, IX, XI, XII, XIII). International Normalized Ratio (INR) reporting of the PT is useful only for monitoring intensity of stable oral anticoagulant therapy. The activity of coagulation factor V (labile factor) typically may be 10% to 20% lower in frozen-thawed plasma specimens than in fresh specimens, even under optimum conditions of processing and transportation, or may be even lower if these conditions are suboptimal, and may lead to a falsely prolonged PT.
In an occasional individual, a more marked decrease of factor V activity may occur with freeze-thaw of plasma. Hence, the PT of frozen-thawed plasma potentially may be slightly more prolonged than would be observed in testing of fresh specimen. Frozen plasma kept on dry ice (solid carbon dioxide) in closed shipping containers may have spurious prolongation of PT, due to acidification of plasma by absorbed carbon dioxide. Exposure of thawed plasma to a normal atmosphere for a few minutes usually eliminates this spurious effect.
The PT is much less sensitive to heparin effect than is the activated partial thromboplastin time (APTT), and therapeutic concentrations of heparin usually cause only slight prolongation of the PT (< or =1 sec with normal plasma, thromboplastin ISI 1.0, heparin < or =1 microgram/mL).
Supportive Data
Effective January 26, 2011, Mayo's prothrombin time (PT)
measurements were changed. Specifically, the PT
reagent was changed from Dade Innovin to the HemosIL RecombiPlasTin 2G reagent. PT results using HemosIL RecombiPlasTin 2G reagent are, on average, 15% more prolonged.
- With the new reagent, the lower limit of PT
on the Stago STAR Evolution instrument changed from 5
to 9 seconds and the upper limit changed from 240 to 160 seconds.
- The new PT reference range of 9.5-13.8 seconds was determined from a Mayo Clinic reference range study.
- The new INR reference range of 0.8-1.2 was calculated using the new PT reference range, an ISI of 1.05, and a geometric mean of the reference range of 11.9.
Method Description Describes how the test is performed and provides a method-specific reference
The STA-R Evolution is a fully automated coagulation instrument, which uses an electromechanical viscosity detection system. The oscillation of a steel ball within the cuvette with the thromboplastin and plasma is monitored by the instrument. At constant viscosity, pendular swings of the ball are obtained within an electromagnetic field. As the plasma clots, the viscosity increases, and the oscillation amplitude of the ball swing decreases. An algorithm uses these variations in oscillation amplitude to determine the clotting time.(RecombiPlasTin 2G Package Insert, 2008; STA-R Evolution Operators Manual)
Day(s) and Time(s) Test Performed Outlines the days and times the test is performed. This field reflects the day and time the sample must be in the testing laboratory to begin the testing process and includes any specimen preparation and processing time required before the test is performed. Some tests are listed as continuously performed, which means assays are performed several times during the day.
Daily; Continuously
Analytic Time Defines the amount of time it takes the laboratory to setup and perform the test. This is defined in number of days. The shortest interval of time expressed is "same day/1 day," which means the results may be available the same day that the sample is received in the testing laboratory. One day means results are available 1 day after the sample is received in the laboratory.
Maximum Laboratory Time Defines the maximum time from specimen receipt at Mayo Medical Laboratories until the release of the test result
Specimen Retention Time Outlines the length of time after testing that a specimen is kept in the laboratory before it is discarded
Performing Laboratory Location The location of the laboratory that performs the test
Test Classification Provides information regarding the medical device classification for laboratory test kits and reagents. Tests may be classified as cleared or approved by the US Food and Drug Administration (FDA) and used per manufacturer's instructions, or as products that do not undergo full FDA review and approval, and are then labeled as an Analyte Specific Reagent (ASR), Investigation Use Only (IUO) product, or a Research Use Only (RUO) product.
CPT Code Information Provides guidance in determining the appropriate Current Procedural Terminology (CPT) code(s) information for each test or profile. The listed CPT codes reflect Mayo Medical Laboratories interpretation of CPT coding requirements. It is the responsibility of each laboratory to determine correct CPT codes to use for billing.
85610
LOINC® Code Information Provides guidance in determining the Logical Observation Identifiers Names and Codes (LOINC) values for the result codes returned for this test or profile.
| Result ID | Reporting Name | LOINC Code |
|---|---|---|
| PTTIC | Prothrombin Time, P | 5902-2 |
| INRC | INR | 6301-6 |
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