Factor Assays - Introduction
Factor assays are commonly undertaken when there is prolongation of the APTT or PT suggesting a deficiency of one or more clotting proteins. However, assays of other clotting factors e.g. FXIII or of the natural anticoagulants e.g. Protein C, Protein S and Antithrombin also constitute factor assays. Thrombophilia testing is covered in a separate section.
Factor assays can be broadly divided into:
- 1-Stage PT-Based assays
- 1-Stage APTT-Based assays
- 2-Stage APTT/Chromogenic assays
- 1-Stage RVV Factor X assay
- Factor XIII Assays
- Inhibitor assays
Assays of Von Willebrand Factor are also included in this section although a functional assay for Von Willebrand factor that uses platelets is covered in the section on platelets.
The general principles of all functional clotting factor assays are the same and involve plotting clotting time (from either a PT or an APTT depending upon which factor is being assayed) against sample dilution. The degree of correction of the clotting time when the plasma is added to a clotting system specifically deficient in the clotting factor to be measured allows the level of that clotting factor to be determined e.g. A factor VIII deficient plasma is used to assay the level of FVIII.
In each case a reference plasma with a known level of a specific clotting factor is required. historically this was a pool of normal plasma and was assumed to have a value of 100% (100 IU/dL, 1.00 IU/ml). This has now been replaced by commercial plasma standards and the concentrations of the various factors in this reference material are provided by the manufacturer.
The methods for the various assays are covered throughout P-H.com
An assay must be both specific [i.e. it measures only the factor of interest and under investigation] and it must be sensitive i.e. it can detect low levels of the clotting factor under investigation.
This is a 'hot' topic in haemostasis. This will be covered in more detail in the section of quality control in haemostasis.
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