Partial thromboplastin time
Encyclopedia
The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT or APTT) is a performance indicator measuring the efficacy of both the "intrinsic" (now referred to as the contact activation pathway) and the common coagulation pathways. Apart from detecting abnormalities in blood clotting, it is also used to monitor the treatment effects with heparin
, a major anticoagulant
. It is used in conjunction with the prothrombin time
(PT) which measures the extrinsic pathway. Kaolin cephalin clotting time (KccT) is a historic name for the activated partial thromboplastin time.
or citrate
to arrest coagulation by binding calcium.
The specimen is then delivered to the laboratory. In order to activate the intrinsic pathway, phospholipid
, an activator (such as silica,
celite, kaolin, ellagic acid
), and calcium
(to reverse the anticoagulant effect of the oxalate) are mixed into the plasma
sample . The time is measured until a thrombus
(clot) forms. This testing is performed by a medical technologist
.
The test is termed "partial" due to the absence of tissue factor
from the reaction mixture.
s and 39 s (depending on laboratory). Shortening of the PTT has little clinical relevance. Normal PTT times require the presence of the following coagulation factors: I, II, V, VIII, IX, X, XI, & XII. Notably, deficiencies in factors VII or XIII will not be detected with the PTT test. Prolonged APTT may indicate:
To distinguish the above causes, mixing test
s 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 correct a factor deficiency is more likely. Deficiencies of factors VIII
, IX
, XI
and XII
and rarely von Willebrand factor
(if causing a low factor VIII level) may lead to a prolonged aPTT correcting on mixing studies.
Heparin
Heparin , also known as unfractionated heparin, a highly sulfated glycosaminoglycan, is widely used as an injectable anticoagulant, and has the highest negative charge density of any known biological molecule...
, a major anticoagulant
Anticoagulant
An anticoagulant is a substance that prevents coagulation of blood. A group of pharmaceuticals called anticoagulants can be used in vivo as a medication for thrombotic disorders. Some anticoagulants are used in medical equipment, such as test tubes, blood transfusion bags, and renal dialysis...
. It is used in conjunction with the prothrombin time
Prothrombin time
The prothrombin time and its derived measures of prothrombin ratio and international normalized ratio are measures of the extrinsic pathway of coagulation. This test is also called "ProTime INR" and "INR PT". They are used to determine the clotting tendency of blood, in the measure of warfarin...
(PT) which measures the extrinsic pathway. Kaolin cephalin clotting time (KccT) is a historic name for the activated partial thromboplastin time.
Method
Blood samples are collected in tubes with oxalateOxalate
Oxalate , is the dianion with formula C2O42− also written 22−. Either name is often used for derivatives, such as disodium oxalate, 2C2O42−, or an ester of oxalic acid Oxalate (IUPAC: ethanedioate), is the dianion with formula C2O42− also written (COO)22−. Either...
or citrate
Citrate
A citrate can refer either to the conjugate base of citric acid, , or to the esters of citric acid. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate.-Other citric acid ions:...
to arrest coagulation by binding calcium.
The specimen is then delivered to the laboratory. In order to activate the intrinsic pathway, phospholipid
Phospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
, an activator (such as silica,
celite, kaolin, ellagic acid
Ellagic acid
Ellagic acid is a natural phenol antioxidant found in numerous fruits and vegetables including blackberries, raspberries, strawberries, cranberries, walnuts, pecans, pomegranates, wolfberry and other plant foods...
), and calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
(to reverse the anticoagulant effect of the oxalate) are mixed into the plasma
Blood plasma
Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
sample . The time is measured until a thrombus
Thrombus
A thrombus , or blood clot, is the final product of the blood coagulation step in hemostasis. It is achieved via the aggregation of platelets that form a platelet plug, and the activation of the humoral coagulation system...
(clot) forms. This testing is performed by a medical technologist
Medical technologist
A Medical Laboratory Scientist is a healthcare professional who performs chemical, hematological, immunologic, microscopic, and bacteriological diagnostic analyses on body fluids such as blood, urine, sputum, stool, cerebrospinal fluid , peritoneal fluid, pericardial fluid, and synovial...
.
The test is termed "partial" due to the absence of tissue factor
Tissue factor
Tissue factor, also called platelet tissue factor, factor III, thrombokinase, or CD142 is a protein present in subendothelial tissue, platelets, and leukocytes necessary for the initiation of thrombin formation from the zymogen prothrombin. An incorrect synonym is thromboplastin...
from the reaction mixture.
Interpretation
The typical reference range is between 25 secondSecond
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s and 39 s (depending on laboratory). Shortening of the PTT has little clinical relevance. Normal PTT times require the presence of the following coagulation factors: I, II, V, VIII, IX, X, XI, & XII. Notably, deficiencies in factors VII or XIII will not be detected with the PTT test. Prolonged APTT may indicate:
- use of heparinHeparinHeparin , also known as unfractionated heparin, a highly sulfated glycosaminoglycan, is widely used as an injectable anticoagulant, and has the highest negative charge density of any known biological molecule...
(or contamination of the sample) - antiphospholipid antibody (especially lupus anticoagulantLupus anticoagulantLupus anticoagulant is an immunoglobulin that binds to phospholipids and proteins associated with the cell membrane. It is important to note that this disease name is actually a misnomer...
, which paradoxically increases propensity to thrombosisThrombosisThrombosis is the formation of a blood clot inside a blood vessel, obstructing the flow of blood through the circulatory system. When a blood vessel is injured, the body uses platelets and fibrin to form a blood clot to prevent blood loss...
) - coagulation factor deficiency (e.g. hemophilia)
To distinguish the above causes, mixing test
Mixing test
Mixing studies are tests performed on blood plasma used to distinguish factor deficiencies from factor inhibitors, such as lupus anticoagulant, or specific factor inhibitors, such as antibodies directed against factor VIII...
s 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 correct a factor deficiency is more likely. Deficiencies of factors VIII
Factor VIII
Factor VIII is an essential blood clotting factor also known as anti-hemophilic factor . In humans, Factor VIII is encoded by the F8 gene...
, IX
Factor IX
Factor IX is one of the serine proteases of the coagulation system; it belongs to peptidase family S1. Deficiency of this protein causes hemophilia B. It was discovered in 1952 after a young boy named Stephen Christmas was found to be lacking this exact factor, leading to...
, XI
Factor XI
Factor XI or plasma thromboplastin antecedent is the zymogen form of factor XIa, one of the enzymes of the coagulation cascade. Like many other coagulation factors, it is a serine protease. In humans, Factor XI is encoded by the F11 gene....
and XII
Factor XII
Coagulation factor XII also known as Hageman factor is a plasma protein. It is the zymogen form of factor XIIa, an enzyme of the serine protease class. In humans, factor XII is encoded by the F12 gene.- Function :...
and rarely von Willebrand factor
Von Willebrand factor
von Willebrand factor is a blood glycoprotein involved in hemostasis. It is deficient or defective in von Willebrand disease and is involved in a large number of other diseases, including thrombotic thrombocytopenic purpura, Heyde's syndrome, and possibly hemolytic-uremic syndrome...
(if causing a low factor VIII level) may lead to a prolonged aPTT correcting on mixing studies.