Partial Thromboplastin Time

Purpose

Screening test to indentify acquired or inherited deficiencies in the activities of Factors IX, VIII, and XI.
Screening test for lupus anticoagulants
Monitor heparin anticoagulation
Screening test to assess reduction in the activity of fibrinogen, Factors V and X. However, the PT is more sensitive for these.

Method

Collect blood from the patient into a citrated tube
Take the collected blood to the laboratory
Centrifuge to isolate the plasma
Take a small sample of the plasma ~0.1 ml
Add calcium (catalyst for coagulation)
Add partial thromboplastin -phospholipid source without tissue factor
Add a negatively charged surface such as kaolin, silica, celite, or dextran sulfate to activate coagulation
The time that elapses until clot forms is the activated Partial Thromboplastin Time (aPTT).

Interpretation

aPTT shorter than the reference range is usually the result of poor sample handling.
Prolongation of the aPTT occurs when there is:

- Factor Deficiency (see below)

- Factor Inhibitor (see below)

- Anticoagulation with heparin

- Contamination of sample with heparin

Note: Once heparin is started, the laboratory work-up of an abnormal aPTT is difficult.
Low molecular weight heparin and heparinoids generally do not prolong the aPTT.

When attempting to determine the cause of a prolonged aPTT, one must first rule out the most common cause which is contamination of the blood specimen with heparin. This usually occurs when small amounts of heparin are used to keep venous or arterial catheters patent. If this is ruled out and the patient is not on intravenous heparin for anticoagulation, then one can begin to suspect either a factor deficiency or a factor inhibitor. A factor inhibitor is usually an antibody targeted against a specific coagulation factor, which results in activation of that factor. Examples of this are the lupus anticoagulant and Factor VIII inhibitors. The distinction of inhibitor versus factor deficiency can be made by performing a 1:1 mix of the patient's plasma with plasma pooled from normal individuals. (Pooled plasma is plasma taken from a large number of individuals with no known coagulation defects.) In theory, it should contain normal levels of all coagulation factors. In a patient with factor deficiency, aPTT measured after mixing the patient's and pooled plasma should be normal. In patients with factor inhibitor, mixing the pooled plasma and the patient's plasma will still yield a prolonged aPTT because the inhibitor in the patient's plasma will also inactivate the specific factor in the pooled plasma.

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