Coagulation laboratory tests
Factor assays

Topic Completed: 1 June 2012

Revised: 7 January 2019, last major update June 2012

Copyright: (c) 2002-2016,, Inc.

PubMed Search: Factor assays [title]

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Cite this page: Parsons J. Factor assays. website. Accessed August 7th, 2020.
Definition / general
  • PT (factors II, V, VII, X) or PTT (factors V, VIII, IX, XI, XII) based reactions, performed by mixing patient plasma with plasma that is deficient in the factor being measured
  • The rate limiting reactant is the deficient factor which must be supplied by the patient
  • PT or PTT is compared to standard curve, to determine amount of factor present in patients plasma
  • Used to determine the etiology of a prolonged PT or PTT
  • Factor levels are expressed as % of normal plasma concentration, or units per mL of normal plasma
  • Reference range is often 60-140% (should be determined based on the laboratorys patient population)
  • Perform at multiple dilutions to rule out an inhibitor - at higher dilutions, inhibitor interference should decrease due to dilution of the inhibitor (this gives a nonlinear curve)
  • Can also use chromogenic assays to quantitate
  • Levels at birth of factors other than factor VIII are 10 - 100% of adult levels, but reach adult levels at 6 months
Additional references
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