How to Perform a PTT

Doctors order the partial thromboplastin time test--known as "PTT" test--to help investigate unexplained bleeding or clotting, monitor intravenous heparin therapy and as a pre-surgical screen. Activators added to the PTT test increase the speed of the test. The PTT test with activators is known as the activated partial thromoplastin time test, or "aPTT". Laboratories often use the terms PTT and aPTT interchangeably.



A prolonged aPTT indicates bleeding tendencies caused by a problem in the intrinsic or common clotting pathway. Other tests evaluate the extrinsic clotting pathway.



To determine aPTT, technicians mix and incubate citrated plasma with a contact activator of the intrinsic pathway and a phospholipid, incubate at 37 degrees Celsius, then mix with calcium chloride. The phospholipid replaces natural platelet-derived phospholipid in the test. The mixture forms a fibrin clot in a time recorded in seconds as the PTT or aPTT.



Advanced clinics use aPTT test machines. The procedure for these machines requires sample introduction but further steps are automated.

Things You'll Need

• Fresh whole blood sample
• Sodium citrate blood tube
• Centrifuge
• Incubator
• Freezer
• aPTT test reagent
• Calcium chloride
• Coagulometer
• Timer
• Automated coagulation profile machine

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Instructions

• 1

  Collect blood. Thoroughly mix the blood using a ratio of nine parts blood to one part sodium citrate; some point-of-care machines can analyze whole blood instead instead of citrated blood. Unless using whole citrated blood for a point of care machine, separate the plasma within 30 minutes in a centrifuge. If the sample is not tested immediately, store the sample at room temperature for no more than four hours, refrigerate at four degrees Celsius for no more than 48 hours, or rapidly freeze for later testing.

• 2

  Pre-incubate the plasma for one to two minutes if it was cooled. Mix reagent including phospholipid and either particulate coagulation activator or ellagic acid with an equal volume of plasma sample. Incubate for three minutes.

• 3

  Add a volume of pre-incubated calcium chloride equal to the plasma volume to the mix. Start a timer or start a coagulometer (an automated clot detector).

• 4

  Detect fibrin clot formation. A coagulometer is usually used for consistency of results. Read the timer when clotting occurs. Record the time as the aPTT.