Chromogenix Coamatic® Factor VIII

von Willebrand factor (vWf) is a glue-like adhesive protein that is responsible for the adhesion of platelets to damaged vascular endothelium. It also carries and protects factor VIII. von Willebrand disease is a hereditary bleeding disorder caused by moderate-to-severe factor VIII deficiency and low-levels of factor VIII-related antigen (substances necessary for blood clotting). Additionally, there is insufficient von Willebrand factor which also helps blood clot. The von Willebrand factor helps platelets to stick to the blood vessel wall and to each other, which is necessary for normal blood clotting.

There is evidence that a high level of factor VIII is a risk factor for venous thrombosis. There are two articles in Thrombosis and Haemostasis 2000; volume 83:

1.Kraaijenhagen et al. High plasma concentration of factor VIII:c is a major risk factor for venous thromboembolism (p. 5-9)
2.O’Donnell et al. Elevation of FVIII:c in venous thromboembolism is persistent and independent of the acute phase response (p. 10-13)
Both of these recent articles discuss the issue of elevated factor VIII levels as a risk factor for venous thrombosis.

An adaptation of Chromogenix Coamatic® FVIII has been developed to allow accurate determination of elevated factor VIII levels. The method is for research use only.

FVIII inhibitors appear in about 25% of severe hemophilia A patients. They are most common in patients with inversions, large deletions and nonsense mutations. Inhibitor titer varies 1000-fold; high titer seriously impairs treatment efficacy. Inhibitors can be transient or persistent. and appear irrespective of which FVIII concentrate is selected. High titer patients are treated with high doses of human or porcine concentrate. View the FVIII product pages for kits and assays for FVIII Inhibitor measurements.

Thrombin, 1 NIH unit, is included in the factor reagent. It has been shown that thrombin can activate FX, and over time you will see an increase of up to 5% activity. If you are doing numerous samples at once on an automated instrument, you might want to consider Coatest® FVIII.

No, glass surfaces may interfere with the generation of FXa. Use plastic tubes for the manual method.

No, hydrolysis of S-2765™ by thrombin is prevented by the addition of the synthetic thrombin inhibitor I-2581.

Clinicians or researchers interested in determining FVIII levels in patients with hemophilia A or with elevated factor VIII levels, which may be associated with thrombotic risk. Also, manufacturers interested in determining factor VIII potencies. The chromogenic FVIII activity method has been selected as the reference method by the EP for potency estimation of VIII concentrates, and it is a valuable tool for diagnosis an monitoring of hemophilia and for thrombophilia screening.

While clotting assays are cheap, rapid, and simple to perform, they are sensitive to pre-activation, show interferences in the assays, and can give overestimation of FVIII concentrates. They also require a considerable amount of FVIII deficient plasma.

Coamatic® FVIII is also rapid and simple, and has additional benefits. It is suited for a wide range of automated instrument applications, there is no heparin influence, and it is highly sensitive, precise and accurate. The low measurement range provides a reliable tool for the classification of bleeders.

All of these kits measure the same thing — factor VIII activity in plasma and potency estimation of FVIII concentrates. Coamatic® FVIII includes the FXa substrate S-2765™, and the factor reagent FIXa + FX and thrombin colyophilized with CaCl2 and phospholipid. The Coatest® SP FVIII includes the FXa substrate S-2765™, and the FIXa + FX reagent, CaCl2, and phospholipid are all separate components in the kit. Incubation times are a little longer, and the reagent prep is less simple. Coatest® SP4 FVIII is exactly the same as Coatest® SP FVIII, but the FIXa + FX reagent is divided into 4 vials of 1.8 U (reconstituted with 3 ml) instead of 1 vial of 6 U (reconstituted with 10 ml).

DiaPharma has calibrators, normal and abnormal controls available. Clinicians can also use their own pooled normal plasma; however, it must be calibrated against an international standard to be sure what concentration the plasma has. If the normal plasma does not contain exactly 1 IU/ml ( which corresponds to 100%) factor VIII, the values of the standards must be recalculated in order to obtain a correct factor potency assignment. Reference plasma is available from the National Institute for Biological Standards and Controls (NBSB). The calibration should be performed by running complete standard curves on two or more independent occasions. Industrial manufacturers, in contrast, use for their standard Mega-2, which is a compilation of factor VIII concentrate.

First, factor VIII circulates in the plasma bound to von Willebrand factor (vWf). Thrombin cleaves and activates factor VIII and releases vWf. The vWf is then free to bind to ruptured endothelial cell surfaces where it activates platelet aggregation. The released FVIIIa acts as a cofactor of factor IXa to generate factor Xa. In the presence of Ca2+ and phospholipids, FX is activated to FXa by FIXa (look at the coagulation cascade for a better understanding). Since FVIIIa is a cofactor to FIXa, it greatly stimulates the reaction. By using optimal amounts of Ca2+, phospholipid, and FIXa, and an excess of FX, the rate of activation of FX is linearly related to the amount of FVIII. FXa hydrolyses the chromogenic substrate S-2765™ which releases the chromophoric group pNA. The color is read at 405 nm, and generated FXa and thus the intensity of color, is proportional to the FVIII activity in the sample.

1. A concentrate standard should be used in lieu of plasma standards.
2. Sample and standard concentrates should be pre-diluted in Factor VIII deficient plasma
3. All assay buffers should contain 1% (10 mg/ml) of high purity albumin, either bovine or human.
4. The chromogenic method is the reference method.

Thrombin activates FX, and this happens when the reagent is reconstituted. An incubation step is required in the Chromogenix Coatest® FVIII kit.