The activated partial
thromboplastin time (aPTT) test was discovered in 1953 by UNC Chapel Hill
researchers who were working to isolate plasma clotting factors (Tripodi, 2006).
Since then, it has become widely used in medical practice to evaluate bleeds
with unknown cause, diagnose clotting disorders, and monitor the therapeutic
range for anticoagulated patients, particularly those on heparin (Hammami,
2015).

An aPTT is collected in a light
blue cap vacutainer. With these tubes it is imperative that the sample
completely fills the vacutainer, as the ratio of whole blood to sodium citrate,
an anticoagulant, must be 9:1 (Pagana, 2015). To avoid skewed results, the
sample must be kept cool at 4 degrees Celsius and analyzed by a hematology lab within
4 hours of the draw (Hammami, 2015). Analysis after this period may yield
falsely high coagulation times (Zehnder, 2017).

To run the aPTT analysis, a mixture
of calcium and phospholipid are added to the plasma and lab technicians time
how long it takes for a clot to form (Zehnder 2017). The aPTT is tested in
conjunction with a control sample to ensure that environmental conditions or
slight variations in the reagents used do not impact test interpretation. An aPTT
measures the functionality and presence of clotting factors in both the
intrinsic and common clotting cascades (Hammami, 2015). It provides the same
information that PTT does but is more sensitive because it utilizes a clot
activator, thus yielding a narrower normal range (Zehnder, 2017).

A normal aPTT is 30-40 seconds, but
if a patient is anticoagulated the desired range is the normal range multiplied
by 1.5-2.5. A critical aPTT is anything greater than 70 seconds patients who are
not anticoagulated, and greater than 100 seconds in patients who are (Pagana,
2015). An extended clotting time in patients who aren’t anticoagulated can indicate
clotting factor deficiencies, Von Wildebrand disease, disseminated
intravascular coagulation (DIC), liver disease or vitamin K deficiency, the
latter two of which are critical to clotting factor synthesis (Zendher, 2017). All
patients with elevated aPTT are at risk for bleeds. Conversely, if the clotting
time is short, it is typically a result of a poorly drawn sample and a redraw
is necessary. If upon proper redraw the aPTT is still shortened, the patient is
at a higher risk for thromboses (Zendher, 2017).

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Heparin is a widely used
anticoagulant because of its potent effect in a short period of time. It can
both prevent thromboembolitic events and be used as a first line treatment for
them because it directly inhibits prothrombin, thus stopping clot formation
(Pagana, 2015). The initial dose of heparin is determined by its indication for
use and a baseline aPTT value. It is effective for 4-6 hours after it is
administered, and subsequent dosing is determined by recurrent aPTT checks
(Zendher, 2017). To gain the most accurate aPTT values, venipuncture should be
done roughly 30 minutes before the next heparin dose is due, as this is when
the previous dose of heparin’s effects will be minimal (Pagana, 2015). 

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