The activated partial thromboplastin time (aPTT) is the clotting time in seconds after a sample of citrated plasma is mixed with phospholipid, a contact activator (such as celite, Kaolin or ellagic acid) and calcium
ions. The aPTT is a simple coagulation screening test that assesses the intrinsic (factors VIII, IX, XI, XII, prekallikrein and high molecular weight kininogen) and common (prothrombin, fibrinogen) pathways of the coagulation cascade. Generally, it is used to determine the cause of bleeding or to disclose any coagulation defects in patients about to undergo an invasive procedure. This article briefly describes the steps to perform a aPTT time in a test tube
1
Prepare the phospholipid according to the manufacturer’s package insert, i.e. reconstitute if freeze-dried, and mix 1:1 in a test tube with the negatively charged contact activator reagent and incubate the mixture in the water bath for 15 minutes. Incubate the plasma simultaneously in the water bath for 15 minutes.
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Step 2
Mix 0.2 mL of the phospholipid/activator mixture and 0.1 mL of the pre-warmed plasma into a test tube and incubate it in the 37ºC for precisely 5 minutes.
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Step 3
Add 0.1 mL of the calcium chloride to the test tube containing the phospholipid/activator and plasma and start the timer.
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Step 4
Invert the tube gently ever other second stopping the timer when the mixture is no longer fluid (indicative of clot formation) and record the time result in seconds. Dispose of the test tube as biohazardous waste.
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Step 5
Repeat the test with the patient sample to obtain duplicate results, repeating if the coefficient of variance between the duplicates is greater than 5%.
