Partial thromboplastin time
Common notation (Fishbone Diagram) of coagulation times in medical records
Other names	Activated partial thromboplastin time; Activated partial prothrombin time; Activated partial thrombin time
MeSH	D010314
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Thepartial thromboplastin time (PTT), also known as theactivated partial thromboplastin time (aPTT orAPTT), is ablood test that characterizescoagulation of theblood. A historical name for this measure is theKaolin-cephalin clotting time (KCCT),^[1] reflectingkaolin andcephalin as materials historically used in the test. Apart from detecting abnormalities in blood clotting,^[2] partial thromboplastin time is also used to monitor the treatment effect ofheparin, a widely prescribeddrug that reduces blood's tendency to clot.

The PTT measures the overall speed at which blood clots form by means of two consecutive series of biochemical reactions known as theintrinsic pathway andcommon pathway ofcoagulation. The PTT indirectly measures action of the followingcoagulation factors:I (fibrinogen),II (prothrombin),V (proaccelerin),VIII (anti-hemophilic factor),X (Stuart–Prower factor),XI (plasma thromboplastin antecedent), andXII (Hageman factor). The PTT is often used in conjunction with another measure of how quickly blood clotting takes place called theprothrombin time (PT). The PT measures the speed of clotting by means of theextrinsic pathway andcommon pathway.

Activated partial thromboplastin time (APTT) is typically analyzed by a medical technologist or laboratory technician, either manually or using an automated instrument at 37°C, which approximates normal human body temperature.Prothrombin time utilizes completethromboplastin, a combination oftissue factor andphospholipids. In contrast, APTT employs partial thromboplastin, containing only phospholipids and no tissue factor—hence the term "partial thromboplastin time." An activator is used in the APTT test to initiate the intrinsic pathway of blood coagulation. Common activators include kaolin, silica, celite, and ellagic acid.^[3]

• Blood is drawn into atest tube containingoxalate orcitrate, molecules which act as ananticoagulant by binding the calcium in a sample. The blood is mixed, then centrifuged to separate blood cells from plasma (as partial thromboplastin time is most commonly measured usingblood plasma).
• A sample of the plasma is extracted from the test tube and placed into a measuring test tube.
• Next, an excess ofcalcium (in aphospholipid suspension) is mixed into theplasma sample (to reverse the anticoagulant effect of the citrate enabling the blood to clot again).
• Finally, in order to activate theintrinsic pathway of coagulation, an activator is added, and the time the sample takes to clot is measured optically. Some laboratories use a mechanical measurement, which eliminates interferences from lipemic andicteric samples.

The typicalreference range is between 25seconds and 33 s (depending on laboratory). Longer times of up to 50 s do apply to infants. Shortening of the PTT is considered to have little clinical relevance, but some research indicates that it might increase risk ofthromboembolism.^[4] Normal PTT requires the presence of the following coagulation factors: I, II, V, VIII, IX, X, XI and XII. Notably, deficiencies in factors VII or XIII will not be detected with the PTT test.^{[citation needed]}

Prolonged aPTT may indicate:^[5]

• Use ofheparin orwarfarin (or contamination of the sample)
• Vitamin K deficiency
• Antiphospholipid antibody (especiallylupus anticoagulant, which paradoxically increases propensity tothrombosis)
• Coagulation factor deficiency (e.g.,haemophilia,cirrhosis,von Willebrand disease)
• Coagulation factor consumption (e.g.,sepsis,disseminated intravascular coagulation)
• Presence ofantibodies against coagulation factors (factor inhibitors)
• Von Willebrand disease

To distinguish the above causes,mixing tests are performed, in which the patient's plasma is mixed (initially at a 50:50 dilution) with normal plasma. If the abnormality does not disappear, the sample is said to contain an "inhibitor" (either heparin, antiphospholipid antibodies or coagulation factor specific inhibitors), while if it does disappear a factor deficiency is more likely. Deficiencies offactors VIII,IX,XI andXII and rarelyvon Willebrand factor (if causing a low factor VIII level) may lead to a prolonged aPTT correcting on mixing studies.^{[citation needed]}

The aPTT is usually normal inpregnancy but tends to slightly decrease in late pregnancy.^[6]

The aPTT-basedactivated protein C (APC) resistance test is used in the diagnosis ofAPC resistance (APCR).^[7] It involves a modified aPTT test performed in the presence and absence of APC.^[7][8] The ratio of these aPTT values is calculated and is called the APC sensitivity ratio (APCsr) or simply APC ratio (APCr).^[7][8] This ratio is inversely related to the degree of APC resistance.^[9] The aPTT-based APC resistance test was developed in 1993.^[8]

The partial thromboplastin time was first described in 1953 by researchers at theUniversity of North Carolina at Chapel Hill.^[10] The initial exogenous phospholipid used in PTT testing wasCephalin.^[11]

• Thromboplastin

• Blood tests
• Medical signs
• Hematology

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