Which factors may interfere with an activated partial thromboplastin time (aPTT) test?

Updated: Jul 02, 2021
  • Author: Muhammad Bader Hammami, MD; Chief Editor: Eric B Staros, MD  more...
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Many drugs can change the results of the activated partial thromboplastin time (aPTT), including nonprescription drugs. [1]

No specific preparation is required. However, since lipemia may interfere with photoelectric measurements of clot formation, specimens should not be obtained after a meal. [5]

If the patient is receiving heparin by intermittent injection, the sample should be drawn 30-60 minutes before the next dose, while, if the patient is receiving a continuous heparin infusion, the sample can be drawn at any time. [1]

Factors that interfere with the aPTT test are include the following [1, 2] :

  • Drugs that may prolong the test values, including antihistamines, ascorbic acid, chlorpromazine, heparin, and salicylates

  • Incorrect blood-to-citrate ratio

  • Hematocrit that is highly increased or decreased

  • Blood samples drawn from heparin lock or a heparinized catheter

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