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.

Index of Tests

Activated Partial Thromboplastin Time [CO003400]

Related Information

Synonyms APTT; aPTT; Partial Thromboplastin Time; PTT

Applies to Argatroban; Common Pathway; Extrinsic Pathway; Heparin; Heparin Resistance; High Molecular Weight Kininogen; Hirudin; Intrinsic Pathway; Prekallikrein

Abstract The activated partial thromboplastin time (PTT) measures the clotting time from the activation of factor XII, through the formation of fibrin clot (see figure). This measures the integrity of the intrinsic and common pathways of coagulation, whereas the prothrombin time (PT) measures the integrity of the extrinsic and common pathways of coagulation. PTT prolongations are caused by either factor deficiencies (especially of factors VIII, IX, XI, and/or XII), or inhibitors (most commonly, lupus anticoagulants, or therapeutic anticoagulants such as heparin, hirudin, or argatroban).


Specimen Plasma

Container One blue top (citrate) tube; 3.2% citrate tubes are now recommended instead of 3.8% citrate tubes.1

Sampling Time See Additional Information for sampling times with the various anticoagulant therapies.

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, otherwise factor VIII may degrade thereby falsely raising the PTT, or falsely low values may occur in heparinized samples as platelets release platelet factor 4 (PF4) which neutralizes heparin.

Specimens drawn from a heparinized line are easily contaminated with heparin, even when the initial volume drawn is discarded. 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, preferably within 1 hour if the PTT is used to monitor heparin, otherwise, PF4 released from platelets neutralizes heparin and can falsely lower the PTT value. To minimize the amount of PF4 in specimens, laboratories should ensure that the plasma contains <10 x 109/L platelets. With or without heparin, plasma may be stored on ice for up to 4 hours, otherwise, store frozen.

Causes for Rejection Specimen received more than 4 hours after collection, tubes not filled, clotted specimens, 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 20-25 seconds; the approximate upper limit of normal is 32-39 seconds. Newborns normally have prolonged PTTs in comparison with adults. The PTT is up to 55 seconds at birth, and the PTT gradually decreases 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 >100-150 seconds (varies depending on reagent-instrument combination and laboratory policies)

Use To screen the integrity of the intrinsic pathway of coagulation (factors VIII, IX, XI, and XII) and to a lesser extent the common pathway (fibrinogen and factors II, V, and X). May detect lupus anticoagulants, but the PTT should not be used to screen for lupus anticoagulants because the PTT may or may not be prolonged (depending on the reagents). Also used to monitor therapeutic heparin, hirudin, or argatroban anticoagulation.

Limitations With single factor deficiencies, the deficient factor has to be below 15% to 45% before the PTT becomes prolonged, depending on the reagent and the deficient factor. The PTT is more sensitive to intrinsic pathway factor deficiencies than to common pathway factor deficiencies. With multiple factor deficiencies, the PTT becomes prolonged with less severe decreases in factor levels.4 Factor VIII elevations shorten the PTT. Factor VIII elevations are common because they occur during acute phase reactions.

Factors VII and XIII do not affect the PTT. The PT can screen for factor VII (and common pathway factor) deficiencies and a specific factor XIII assay can screen for factor XIII deficiencies.

Lupus anticoagulants and deficiencies of certain factors (eg, factor XII) may prolong the baseline PTT and/or accentuate the prolongation of the PTT when heparin is added. Therefore, in these situations, an alternative assay, such as the Heparin Antifactor Xa Assay, should be used rather than the PTT to monitor heparin. If the heparin anti-Xa assay demonstrates that the heparinized PTT is not affected by the lupus anticoagulant, cautious use of the PTT may be considered in that patient.

With very high doses of heparin, as used in cardiac bypass surgery, the PTT is unclottable (>150 seconds) and therefore not useful. The activated coagulation time (ACT) is typically used instead in such situations.

Methodology PTT reagent (phospholipid with an intrinsic pathway activator such as silica, celite, kaolin, ellagic acid) and calcium are added to patient plasma, and the time until clot formation is measured in seconds. Phospholipid in the PTT assay is called "partial thromboplastin" because tissue factor is not present. Tissue factor is present with phospholipid in (complete) thromboplastin reagents that are used for PT assays. Tissue factor activates the extrinsic pathway of coagulation, which is not measured in PTT assays. Phospholipid and calcium are required cofactors in the coagulation cascade. Citrate in the blue top tube prevents clotting by chelating calcium. When the PTT test is ready to be performed, excess calcium is added to overcome citrate.

More recently point-of-care PTT test methods have become available which use a single drop of whole blood, and these methods are undergoing evaluation.5

Additional Information To determine the etiology of an unexplained PTT prolongation, the first step is usually to determine if heparin contamination is the cause (see Heparin Neutralization). If this demonstrates that heparin is not present, a mixing study is usually the next step (see Mixing Studies). Mixing studies can predict whether the cause of the PTT prolongation is a factor deficiency or an inhibitor.

Factor deficiencies that prolong PTT: The PTT is more sensitive to deficiencies of the intrinsic pathway (factors VIII, IX, XI, XII, prekallikrein, HMWK) than it is to deficiencies of the common pathway (fibrinogen, and factors II, V, and X). If a mixing study suggests a factor deficiency, assays for factors VIII, IX, XI, and XII can be performed. If the PT is also prolonged, assays for fibrinogen and factors II, V, VII, and X can also be performed. Prekallikrein and HMWK are often not assayed because deficiencies of these two factors are rare and do not cause bleeding, despite causing a prolonged PTT. Factor XII deficiencies also do not cause bleeding, but factor XII deficiencies are relatively common. If the factor assays are all normal, lupus anticoagulant tests can be considered, because occasionally the mixing study will not detect the presence of a lupus anticoagulant.

The effects of hereditary or acquired factor deficiencies on PT and PTT are shown in Tables 1 and 2 in Coagulation Factor Assays.

Inhibitors that prolong PTT: Inhibitors are usually antibodies (lupus anticoagulants or specific factor inhibitors) or anticoagulants such as heparin, hirudin, or argatroban. Lupus anticoagulants bind to phospholipid and interfere with phospholipid's role as an essential cofactor in the coagulation cascade, thereby prolonging various clotting times such as the PTT. Despite the PTT prolongation, lupus anticoagulants are associated with thrombosis rather than bleeding. Specific factor inhibitors are antibodies directed against a specific coagulation factor, such as a factor VIII inhibitor. PTT mixing studies have a characteristic pattern when a factor VIII inhibitor is present. In such cases, factor VIII inhibitor tests should be performed (see Mixing Studies and Factor Inhibitors). When a PTT mixing study suggests the presence of an inhibitor other than a factor VIII inhibitor, lupus anticoagulant tests may be performed, as lupus anticoagulants are by far the most common inhibitor. If the lupus anticoagulant tests are negative, factor assays may be performed as described above. If one factor is significantly decreased, specific factor inhibitor assays may be performed to determine if there is an inhibitor against that factor (see Factor Inhibitors). Specific factor inhibitors are rare.

Acquired causes of PTT prolongations are much more common than hereditary causes, especially among inpatients (see following list). 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 impairs 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.6 Heparin inhibits activated factors II, X, IX, XI, XII, and kallikrein by enhancing antithrombin activity. Hirudin and argatroban inhibit only activated factor II (thrombin).

Causes of PTT Prolongations


* Deficiency of factor VIII, IX, XI, XII, prekallikrein, or HMWK (PT is normal)

* Deficiency of fibrinogen or factor II, V, or X (PT is also prolonged)


* Lupus anticoagulants (PT usually normal)

* Heparin (PT less affected than PTT, PT may be normal)

* Hirudin or argatroban (PT usually also prolonged)

* Liver dysfunction (PT affected earlier and more than PTT)

* Vitamin K deficiency (PT affected earlier and more than PTT)

* Coumadin®(PT affected earlier and more than PTT)

* Disseminated intravascular coagulation (DIC) (PT affected earlier and more than PTT)

* Specific factor inhibitors (PT normal except in the rare cases of an inhibitor against fibrinogen, factor II, V, or X)

Monitoring heparin: Low-dose, subcutaneous, prophylactic unfractionated heparin (eg, 5000 units two or three times daily) is typically not monitored with coagulation tests. Platelet counts should be followed to ensure that if heparin-induced thrombocytopenia develops, the diagnosis will be made promptly. These low levels of unfractionated heparin usually do not affect the PTT. Full-dose, therapeutic levels of unfractionated heparin should be monitored, and the platelet count also followed. The PTT is the most commonly used assay for unfractionated heparin monitoring because it is inexpensive, automated, and usually available 24 hours a day. The therapeutic range is the PTT range that corresponds to an antifactor Xa level of 0.3-0.7 units/mL. Each laboratory determines its own therapeutic range, but it is often a PTT range that is about 1.5-2.5 times the mean of normal PTT. Therapeutic levels of heparin are most often administered as an initial intravenous bolus followed by a continuous intravenous infusion. The PTT is measured every 6 hours during the first day of unfractionated heparin therapy and 6 hours after any dosage change. If the PTT is therapeutic, it can be checked once daily while patients are on heparin. A less common approach is to administer therapeutic unfractionated heparin doses subcutaneously twice daily, drawing the PTT 6 hours after injection.7,8 Peak levels are reached 2-4 hours after subcutaneous injection, although this is variable. If patients on unfractionated heparin are started on Coumadin,® heparin is continued until the INR is therapeutic for 2 days. With some PT reagents, heparin can prolong the PT (and therefore the INR) to some extent. Conversely, the PTT can be prolonged somewhat by Coumadin.® Low-molecular weight heparin (LMWH) usually does not significantly prolong the PTT, therefore, the PTT is not used to monitor LMWH. Antifactor Xa assays can be used to monitor LMWH, when indicated.

Heparin resistance is a condition in which the PTT does not prolong as much as expected despite high doses of heparin. This is commonly due to an acute phase reaction, because many acute phase reactant proteins bind and neutralize heparin. Additionally, factor VIII becomes elevated during acute phase reactions, which shorten the PTT. Rarely, heparin resistance is due to antithrombin deficiency. Mild decreases of antithrombin commonly occur as a result of heparin therapy, but mild decreases do not cause significant heparin resistance. Thus, if a patient has heparin resistance, indices of an acute phase reaction may be ordered (eg, fibrinogen, factor VIII), and a heparin assay (antifactor Xa assay) may be helpful.

Monitoring hirudin (lepirudin, RefludanTM): Hirudin is a direct thrombin inhibitor that is commonly used as an anticoagulant for the treatment of thrombosis in patients with heparin-induced thrombocytopenia. Hirudin treatment should be monitored with the PTT. The usual therapeutic dose of hirudin in patients with normal kidney function is 0.4 mg/kg intravenous bolus followed by 0.15 mg/kg/hour continuous intravenous infusion. The dose has to be significantly reduced when the creatinine is >1.6 mg/dL. The PTT is performed 4 hours after starting hirudin and 4 hours after any dosage change. If the PTT is in the desired therapeutic range (1.5-2.5 times mean of normal PTT), the PTT can be checked once daily while on hirudin. Note: See Coagulation Factor Assays for the use of chromogenic factor X assays to monitor Coumadin® in patients receiving hirudin or argatroban.


1. National Committee for Clinical Laboratory Standards (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. Solomon HM, Mullins RE, Lyden P, et al, "The Diagnostic Accuracy of Bedside and Laboratory Coagulation: Procedures Used to Monitor the Anticoagulation Status of Patients Treated With Heparin,"Am J Clin Pathol, 1998, 109(4):371-8.

6. Spero JA, Lewis JH, and Hasiba U, "Disseminated Intravascular Coagulation: Findings in 346 Patients,"Thromb Haemost, 1980, 43(1):28-33.

7. Prandoni P, Bagatella P, Bernardi E, et al, "Use of an Algorithm for Administering Subcutaneous Heparin in the Treatment of Deep Venous Thrombosis,"Ann Intern Med, 1998, 129(4):299-302.

8. Kearon C, Harrison L, Crowther M, et al, "Optimal Dosing of Subcutaneous Unfractionated Heparin for the Treatment of Deep Vein Thrombosis,"Thromb Res, 2000, 97(6):395-403.


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 Suppl):561S-78S.

Laposata M, Green D, Van Cott EM et al, "College of American Pathologists Conference XXXI on Laboratory Monitoring of Anticoagulant Therapy. The Clinical Use and Laboratory Monitoring of Low-Molecular Weight Heparin, Danaparoid, Hirudin and Related Compounds, and Argatroban,"Arch Pathol Lab Med, 1998, 122(9):799-807.

Olson JD, Arkin CF, Brandt JT, et al, "College of American Pathologists Conference XXXI on Laboratory Monitoring of Anticoagulant Therapy: Laboratory Monitoring of Unfractionated Heparin Therapy,"Arch Pathol Lab Med, 1998, 122(9):782-98.


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