Plasma activated partial thromboplastin time (APTT) measurement is used to monitor heparin therapy (non-fractionated heparin), as a complementary test for the control of certain coagulation deficiencies as well as for the detection of certain inhibitors, such as the wolf anticoagulant.
The hemostatic process involves many steps and the proper functioning of a wide variety of coagulation factors and other substances. Partial thromboplastin time (PTT) and activated partial thromboplastin time (APTT) are used to evaluate how well the coagulation process works. This test is useful in detecting blood clotting disorders caused by either deficiency or defective function of the coagulation factors that make up the endogenous system. These agents include I, II, V, VIII, IX, X, XI, and XII. Normal APTT values may reflect a normal blood coagulation function, but moderate deficiencies of a single coagulation factor may exist. The reduction of a single coagulation factor is not reflected in the APTT value only when the reduction reaches 30-40% of its normal activity.
PTT (partial thromboplastin time) is also used to monitor heparin therapy. Heparin inactivates prothrombin and prevents the formation of thromboplastin. Thus, in conditions where prevention of thrombus formation is necessary, heparin is usually given in the form of continuous intravenous infusion. It is important that the patient's response to this anticoagulant treatment is adequate, that is, sufficient to prevent thrombus formation, but not large enough to cause automatic bleeding. This fine balance can be monitored through the use of PTT.
PTT involves measuring the time it takes for a clot to form in a plasma sample to which calcium and partial thromboplastin are added. If several chemicals are added to standardize and accelerate the assay, the result is referred to as activated partial thromboplastin time, or APTT. If APTT is greater than 100 seconds, the patient is considered to be high risk for bleeding.
In case of overdose of heparin with subsequent bleeding, the antidote is protamine sulfate.
Possible Interpretations of Pathological Values
- Increase: The main causes are the genetic or acquired deficiency of blood coagulation factors IX, X, XI, XII, or even factor V or II. These deficiencies usually have to be below 30% - 40% of the normal levels of coagulation factors to produce increased APTT and bleeding tendencies, such as in haemophilia A. Increased times are associated with deficiencies in high molecular weight (HMW) quinogen and the Fletcher factor (procalcillin). Longer periods also occur when placental abruption, aninodogenemia, cardiac surgery, hypothermia, cirrhosis, diffuse intravascular coagulation, dysinodogenemia, fibrinolysis, Fitzgerald's factor deficiency, hemorrhagic neovascularization, severe hemorrhage, hemorrhage, lupus anticoagulant, Willebrand disease, patients on dialysis. Medications: Alcohol, anisetraplase (thrombolytic agent), hydroxycoumarin (excess), chlorpromazine, codeine, ectifibatide, heparin, methotrexate, phenothiazines, salicylic acid, warfarin
- Decrease: Shorter times occur in Fletcher abnormalities, which are not associated with bleeding and which thromboembolisms can occur. A smaller APTT in patients with chest pain is associated with an increased risk of acute myocardial infarction
Laboratory test results are the most important parameter for the diagnosis and monitoring of all pathological conditions. 70%-80% of diagnostic decisions are based on laboratory tests. Correct interpretation of laboratory results allows a doctor to distinguish "healthy" from "diseased".
Laboratory test results should not be interpreted from the numerical result of a single analysis. Test results should be interpreted in relation to each individual case and family history, clinical findings and the results of other laboratory tests and information. Your personal physician should explain the importance of your test results.
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