From: Elizabeth M. Van Cott, M.D., and Michael Laposata, M.D., Ph.D., “Coagulation.” In: Jacobs DS et al, ed. The Laboratory Test Handbook, 5th Edition. Lexi-Comp, Cleveland, 2001; 327-358.
Related Information
Activated Partial Thromboplastin Time
Coagulation Factor Assays
Disseminated Intravascular Coagulation Screen
Factor Inhibitors
Fibrinogen
Mixing Studies
Protein C
Protein S
Reptilase® Time
Thrombin Time
Synonyms Protime; PT
Applies to Argatroban; Common Pathway; Coumadin®; Extrinsic Pathway; Heparin; Hirudin; INR; International Normalized Ratio; Intrinsic Pathway; Thromboplastin; Vitamin K
Abstract The prothrombin time (PT) measures the clotting time from the activation of factor VII, through the formation of fibrin clot (see figure). This test measures the integrity of the extrinsic and common pathways of coagulation, whereas the activated partial thromboplastin time (PTT) measures the integrity of the intrinsic and common pathways of coagulation. PT prolongations are most commonly caused by factor deficiencies involving fibrinogen or factors II, V, VII, or X. Less commonly, PT prolongations are due to an inhibitor, such as therapeutic anticoagulants including heparin, hirudin, or argatroban. Rarely, PT prolongations are caused by lupus anticoagulants or by specific factor inhibitors against fibrinogen or factor II, V, VII, or X. (SEE GRAPHIC IN BOOK)
Specimen Plasma
Container Blue top (sodium citrate) tube; 3.2% citrate tubes are now recommended instead of 3.8% citrate tubes.1
Collection Routine venipuncture. If multiple tests are being drawn, draw blue top tubes after any red top tubes but before any lavender top (EDTA), green top (heparin), or gray top (oxalate/fluoride) tubes. Recent data suggest that an initial discard tube is not necessary.2 Immediately invert tube gently at least 4 times to mix. Tubes must be appropriately filled. Deliver tubes immediately to the laboratory.
Specimens drawn from a heparinized line are easily contaminated with heparin, even when the initial volume drawn is discarded. Although heparin prolongs the PTT, it can also prolong the PT to a lesser extent. Hirudin and argatroban prolong the PT and PTT. Therefore, coagulation tests are best drawn directly from a peripheral vein, avoiding the arm in which heparin, hirudin or argatroban is being infused (if relevant).
Storage Instructions Separate plasma from cells as soon as possible. Plasma (or uncentrifuged specimen) may be stored at room temperature or on ice for up to 24 hours, otherwise store frozen.1
Causes for Rejection Specimen received more than 24 hours after collection, tube not filled, clotted specimen, visible hemolysis
Turnaround Time Less than 1 day; often less than 1 hour if requested stat. The PT and PTT are the most readily available coagulation tests.
Reference Interval Varies significantly among different reagent-instrument combinations. The approximate lower limit of normal is 10-12 seconds; the approximate upper limit of normal is 12-14 seconds. Newborns normally have prolonged PTs in comparison with adults. The PT is up to approximately 16 seconds at birth, and the PT gradually shortens into the adult normal range by the age of 6 months.3 However, newborns and infants do not normally experience bleeding, because a balance between procoagulants and natural anticoagulants is maintained.
Critical Values Longer than 30 seconds is the most commonly used PT panic value in specialized coagulation laboratories according to the College of American Pathologists 1999 Survey CG2-C, but the value varies depending on the reagent-instrument combination and individual laboratory policies.
Use Screen the integrity of the extrinsic (factor VII) and common (fibrinogen and factors II, V, and X) pathways of coagulation; monitor warfarin (Coumadin®) anticoagulation
Limitations With single factor deficiencies, the deficient factor has to be below 15% to 45% before the PT becomes prolonged, depending on the reagent. With multiple factor deficiencies, the PT becomes prolonged with less severe decreases in factor levels.4
Deficiencies of factors VIII, IX XI, XII, prekallikrein, or high-molecular weight kininogen do not affect the PT, but do affect the PTT. Factor XIII does not affect the PT nor PTT. A specific factor XIII assay can screen for factor XIII deficiencies.
Heparin can prolong the PT, depending on the reagent. Some reagents contain a heparin neutralizer to reduce or eliminate heparin interference.
Lupus anticoagulants uncommonly prolong the baseline PT. Most PT reagents contain excess phospholipid such that lupus anticoagulants (which are antiphospholipid antibodies) do not prolong the PT. However, with some PT reagents, lupus anticoagulants can accentuate the prolongation of the PT when patients are on warfarin.5 In these situations, an alternative assay such as a chromogenic factor X assay can be used rather than (or in addition to) the PT to monitor warfarin (see Additional Information).
Methodology PT reagent is called thromboplastin (phospholipid with tissue factor and calcium). It is added to patient plasma, and the time until clot formation is measured in seconds. Tissue factor activates the extrinsic pathway of coagulation. Phospholipid and calcium are required cofactors in the coagulation cascade. Citrate in the blue top tube prevents clotting by chelating calcium. PT reagents contain excess calcium to overcome the citrate. More recently, point-of-care PT test methods have become available which use a single drop of whole blood, and these methods are undergoing evaluation.6
Additional Information If indicated, a vitamin K trial may be performed in a patient with an unexplained PT prolongation. If the PT prolongation is due to vitamin K deficiency, the PT becomes normal or significantly shorter within 12-24 hours after vitamin K administration.
To determine the etiology of an unexplained PT prolongation, a mixing study is usually the first step (if the PTT is also prolonged, the presence of heparin or related anticoagulants must first be excluded – see Mixing Studies). Mixing studies can predict whether the cause of the PT prolongation is a factor deficiency or an inhibitor. The majority of PT prolongations are due to factor deficiencies. If the PT mixing study suggests a factor deficiency, assays for fibrinogen and factors II, V, VII, and X can be performed to identify the deficient factor(s). Inhibitors that prolong the PT are rare. Factor VII inhibitors prolong the PT but not the PTT. Factor II, V, or X inhibitors typically prolong the PTT as well as the PT (see Coagulation Factor Assays for more information). As mentioned above, lupus anticoagulants are inhibitors that commonly prolong the PTT, but uncommonly prolong the PT.
In patients with both a lupus anticoagulant and a prolonged PT, a factor II assay could be considered, because occasionally lupus anticoagulants cause decreased factor II due to increased clearance.
Acquired causes of PT prolongations are much more common than hereditary causes, especially among inpatients (see list below). The liver synthesizes all of the coagulation factors. Therefore, with liver disease, multiple factor deficiencies can develop which prolong the PT earlier and more than the PTT. Coumadin® or vitamin K deficiency impair the function of factors II, VII, IX, and X, leading to PT and eventually PTT prolongations. In disseminated intravascular coagulation (DIC), multiple factor deficiencies may arise due to activation and consumption of factors, prolonging the PT more often than the PTT.7 Heparin inhibits activated factors II, X, IX, XI, XII, and kallikrein by enhancing antithrombin activity, prolonging the PTT more than the PT. Hirudin and argatroban inhibit only activated factor II (thrombin), prolonging the PT and PTT.
CAUSES OF PT PROLONGATIONS:
Hereditary:
* Deficiency of factor VII (PTT is normal)
* Deficiency of fibrinogen or factors II, V, or X (PTT may also be prolonged)
Acquired:
* Liver dysfunction (PT affected earlier and more than PTT)
* Vitamin K deficiency (PT affected earlier and more than PTT)
* Warfarin (PT affected earlier and more than PTT)
* Disseminated intravascular coagulation (DIC) (PT affected earlier and more than PTT)
* Lupus anticoagulants (may or may not prolong the PTT; PT is rarely prolonged)
* Heparin (PT less affected than PTT, PT may be normal)
* Hirudin or argatroban (PTT also prolonged)
* Specific factor inhibitors (PTT also prolonged except in the rare case of an inhibitor against factor VII)
The effects of hereditary or acquired factor deficiencies on PT and PTT are shown in Tables 1 and 2 in Coagulation Factor Assays. Factor half-lives are summarized in Table 3 in that listing.
Monitoring warfarin: Warfarin is monitored by the international normalized ratio (INR). The usual therapeutic goal is an INR of 2-3. The INR is calculated from the PT and is intended to allow valid comparisons of results regardless of the type of PT reagent used among different laboratories:
INR = [patient PT / mean normal PT]ISI
The international sensitivity index (ISI) is a measure of the sensitivity of a particular PT reagent. Different PT reagents have different sensitivities to factor deficiencies. For example, with an insensitive reagent, the PT will not become prolonged until the factor levels are very decreased, whereas with a sensitive reagent, the PT will become prolonged with milder factor deficiencies. Insensitive reagents have higher ISI values, up to about 3.0. Sensitive reagents have lower ISI values, down to about 1.0. The ISI for each reagent is determined by the manufacturer.
During warfarin initiation, the PT/INR is typically checked daily or at least 4-5 times per week until the dose and INR are therapeutic and stable.8 The interval between PT/INR tests can then be gradually decreased to as infrequently as every 4 weeks, depending on the stability of the dose and the PT/INR result.8,9 It takes 4-5 days for warfarin’s antithrombotic action to take effect, because the half-lives of factors II and X are relatively long. For this reason, patients who need immediate anticoagulation are treated with an immediate-acting anticoagulant (eg, heparin) while waiting for warfarin to become therapeutic. Heparin is typically continued until the INR is in the desired range for two consecutive days.9
To treat warfarin overdose (bleeding), vitamin K or fresh frozen plasma can be administered.9 If the INR is >5 without bleeding, vitamin K administration can be considered. If large doses of vitamin K are administered, patients can become temporarily warfarin resistant.
Warfarin (Coumadin®) and vitamin K deficiency share the same molecular basis for their effects. Warfarin is used as a therapeutic anticoagulant because it impairs the regeneration of active vitamin K, thereby decreasing the amount of active vitamin K. Vitamin K in its active form is a cofactor in a reaction which carboxylates glutamic acid residues to form gamma carboxyglutamic acid residues on factors II, VII, IX, and X as well as protein C and protein S. This carboxylation step is necessary for normal activity of these proteins. As a result, vitamin K deficiency or warfarin therapy decreases the activity of these proteins and prolongs the PT. Patients with mild vitamin K deficiency or low levels of warfarin anticoagulation can have a normal PTT.
In certain situations (eg, lupus anticoagulants or the concomitant use of hirudin or argatroban with warfarin) alternative assays may be used to monitor warfarin because the PT/INR will be elevated by hirudin, argatroban, and occasionally by lupus anticoagulants.5 Alternative assays (eg, chromogenic factor X assays) are not affected by these interferences. However, alternative assays have not yet been well studied in these settings. An INR of 2-3 corresponds approximately to a chromogenic factor X of 20% to 40%.
Changes in dietary vitamin K (see website reference below) and many medications (many of which are listed in reference 9) can alter the warfarin dose requirement. Hyperthyroidism, liver failure, cancer, fever, or vitamin K deficiency (from malabsorption, steatorrhea, poor nutrition, certain antibiotics etc) tend to decrease the dose required to increase the PT. Hypothyroidism or certain genetic polymorphisms tend to increase the dose requirement.10 Some patients have hereditary warfarin resistance, an uncommon condition in which very high doses of warfarin are needed to maintain a therapeutic INR.
Warfarin should not be used alone in the acute setting of heparin-induced thrombocytopenia, because paradoxical thrombosis can occur. If warfarin is used in this setting, a rapid-acting anticoagulant (eg, hirudin, danaparoid, or argatroban) must also be used until the INR is therapeutic.11 A similar approach is used for patients with hereditary protein C or protein S deficiency, to prevent Coumadin®-induced skin necrosis.
Footnotes
1. NCCLS, “Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays: Approved Guideline 3rd edition,” NCCLS Document H21-A3, NCCLS, 940 West Valley Road, Wayne, Pennsylvania 19087, USA 1998.
2. 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.
3. Andrew M, Paes B, and Johnston M, “Development of the Hemostatic System in the Neonate and Young Infant,”Am J Pediatr Hematol Oncol, 1990, 12(1):95-104.
4. Burns ER, Goldberg SN, and Wenz B, “Paradoxic Effect of Multiple Mild Coagulation Factor Deficiencies on the Prothrombin Time and Activated Partial Thromboplastin Time,”Am J Clin Pathol, 1993, 100(2):94-8.
5. Moll S and Ortel TL, “Monitoring Warfarin Therapy in Patients With Lupus Anticoagulants,”Ann Intern Med, 1997, 127(3):177-85.
6. Sawicki PT, Working Group for the Study of Patient Self-Management of Oral Anticoagulation, “A Structured Teaching and Self-Management Program for Patients Receiving Oral Anticoagulation,”J Am Med Assoc, 1999, 281(2):145-50.
7. Spero JA, Lewis JH, and Hasiba U, “Disseminated Intravascular Coagulation: Findings in 346 Patients,”Thromb Haemost, 1980, 43(1):28-33.
8. Fairweather RB, Ansell J, van den Besselaar AM, et al, “College of American Pathologists Conference XXXI on Laboratory Monitoring of Anticoagulant Therapy. Laboratory Monitoring of Oral Anticoagulant Therapy,”Arch Pathol Lab Med, 1998, 122(9):768-81.
9. Hirsh J, Dalen JE, Anderson DR, et al, “Oral Anticoagulants. Mechanism of Action, Clinical Effectiveness, and Optimal Therapeutic Range,”Chest, 1998, 114(5):445-69S.
10. Taube J, Halsall D, and Baglin T, “Influence of Cytochrome P-450 CYP2C9 Polymorphisms on Warfarin Sensitivity and Risk of Over-Anticoagulation in Patients on Long-Term Treatment,”Blood, 2000, 96(5):1816-9.
11. Warkentin TE, “Heparin-Induced Thrombocytopenia: IgG-Mediated Platelet Activation, Platelet Microparticle Generation, and Altered Procoagulant/Anticoagulant Balance in the Pathogenesis of Thrombosis and Venous Limb Gangrene Complicating Heparin-Induced Thrombocytopenia,”Transfus Med Rev, 1996, X:249-58.
References
Bajaj SP and Joist JH, “New Insights Into How Blood Clots: Implications for the Use of APTT and PT as Coagulation Screening Tests and in Monitoring of Anticoagulant Therapy,”Semin Thromb Hemost, 1999, 25(4):407-18.
Hyers TM, Agnelli G, Hull RD, et al, “Antithrombotic Therapy for Venous Thrombotic Disease,”Chest, 1998, 114(5):561-578S.
