Chromogenix S-2765™ is a chromogenic substrate for determination of Factor Xa activity. It is also very sensitive to trypsin.

S-2765™ is suitable for measuring FXa inhibition in heparin anti-Xa assays and antithrombin anti-Xa assays.

Each vial contains the chromogenic substrate S-2765™, 25 mg and mannitol 60 mg as a bulking agent.

Stability: Lyophilized substance: stable at 25°C until expiry date printed on the label. The substance is hygroscopic and should be stored in a dry place.

Solution: 2 mmol/L in H2O is stable for six months at 2 to 8°C. Suitable stock solution: 1-2 mmol/L in H2O.

Chemical name: N-a-Benzyloxycarbonyl-Darginyl-L-glycyl-L-arginine-pnitroaniline-dihydrochloride

Formula: N-a-Z-D-Arg-Gly-Arg-pNA·2HCl

Mol. wt.: 714.6

chromogenix chromogenic substrate assay test kit

Factor Xa, which has a molecular weight of 44 KDa, is the activated form of Factor X (MW: 59 KDa).

The International Units of Factor X correspond to the amount of Factor X contained in 1 ml of normal plasma. This is about 8 mg/l or 0.13 µmol/l.

Since there is no WHO standard for FXa, one would assume that if all the Factor X in normal plasma was converted to the activated form, the Factor Xa concentration would be approximately 5.7 mg/l.

The activity of human Factor Xa as calculated from the kinetic tables is 1.5 nkat/µg with the chromogenic substrate S-2222™, and 4.4 µkat/µg with the chromogenic substrate S-2765™.

The activity of 1 µg of Factor Xa as determined by Frieberger (1) is 1.9 nkat chromogenic substrate S-2222™.

Thus, 1 plasma equivalent unit of Factor X would correspond to 15.2 nkat chromogenic substrate S-2222™.

Friberger P et al.
Synthetic peptide substrate assays and fibrinolysis and their application on automates. In: Seminars in Thrombosis and Haemostasis, Vol. 9, 281-300 (1983).

Determination of factor X in plasma with Chromogenic Substrate S-2765™

Measurement Principle

The method is based on a two-stage principle. In stage one, Factor X is activated in the presence of calcium to Factor Xa (FXa) using the activator Russell’s Viper venom (RVV). In stage two, the generated FXa hydrolyses the chromogenic substrate Z-D-Arg-Gly-Arg-pNA (S-2765™), thus liberating the chromophoric group pNA (p-nitroaniline). The color is then read photometrically at 405 nm. The generated FXa (and thus the intensity of color) is proportional to the FX activity of the sample.

Factor XRVV
Factor Xa
Z-D-Arg-Gly-Arg-pNA + H2OFXa
Z-D-Arg-Gly-Arg-OH + pNA


  1. S-2765™, 25 mg Art. No. S821413
    Reconstitute the substrate S-2765™ (MW: 714.6) with 20 ml sterile water.
  2. Russell’s Viper Venom (RVV)
    Prepare a solution of Russell’s Viper Venom at a concentration of 0.087 mg/ml.
  3. CaCl2
    0.1 mol/l calcium chloride solution.
  4. Tris EDTA Buffer
    Dilute the buffer 1:10 with distilled water according to the insert sheet instructions.
  5. Normal Plasma
    Calibrated, lyophilized or fresh frozen human plasma is used for the standardization of the assay. A pooled normal plasma can be prepared by taking samples from 20 healthy donors. 10-30 ml citrate blood (9 vol blood and 1 vol 0.1 mol/l sodium citrate) from each donor is centrifuged at 2000 x g for 20 minutes at 15-25°C. The plasma is pooled and subsequently dispensed in small volumes, which are frozen rapidly at -20°C or below. To avoid low temperature activation of prekallikrein the plasma is kept at 15-25°C before use or freezing. Thawing of plasma should be performed at 37°C and then kept at 15-25°C until used.
  6. RVV + CaCl2
    Before use, mix 1 volume of RVV with 1 volume of CaCl2. The mixture is stable for 48 hours at 2-8°C.
  7. Acetic acid 20%
    Acetic acid is used as a stopper solution in the end-point method.

Specimen collection

Blood (9 vol) is mixed with 0.1 mol/l sodium citrate (1 vol) and centrifuged at 2000 x g for 20 minutes at 20-25°C. Storage: 1 week at 2-8°C or 3-4 months at -20°C.

Standard curve


Final Dilution

FX%Normal Plasma mlBuffer mlPredil plasma mlBuffer ml


Sample Dilution
Test plasma or standard50

Acid Stopped MethodAB
Diluted Sample200 ml50 ml
Incubate at 37°C3-4 min3-4 min
Substrate (37°C)200 ml50 ml
Mix and add within 30 sec
RVV + CaL2200 ml50 ml
Mix and incubate at 37°C3 min3 min
Acetic acid 20%200 ml50 ml

A= test tube method
B= microplate method

Sample blank activities should be determined and subtracted when analyzing strongly colored plasma, e.g. lipemic and hemolytic. The sample blanks are prepared by mixing the diluted sample, acetic acid 20% and water instead of the reagents (400 µl for test tubes and 100 µl for microplates). Read the absorbance of the samples and blanks at 405 nm. The color is stable for at least four hours. When possible, use a dual wavelength mode with 490 nm as the reference wavelength.

Initial rate method

When performing the initial rate method, transfer the microplate to a microplate reader immediately after the addition of RVV+CaCl2 and read the change in A/min. The microplate reader must be pre-incubated at 37°C.


Plot A or ΔA/min for the standards against their concentration of Factor X. Read the Factor X value for the corresponding A or ΔA/min of the unknown test sample from the standard curve.


  1. Kiesel W et al. Factor X activating enzyme from Russell’s Viper Venom; isolation and characterisation. Biochemistry 15, 4901-4905 (1976).
  2. Lindhout MJ et al. Activation of decarboxyfactor X by a protein from Russell’s Viper Venom. Purification and partial characterisation of activated decarboxyfactor X. Biochem Biophys Acta 533, 327-341, (1978).
  3. Bergström K and Egberg N. Determination of vitamin K sensitive coagulation factors in plasma. Studies on three methods using synthetic chromogenic substrates. Thromb Res 12, 531-547 (1978).
  4. Van Wijk EM et al. A rapid manual chromogenic factor X assay. Thromb Res 22, 681-686 (1981). Egberg N and Heedman PA. Simplified performance of amidolitic factor X assay. Thromb Res 25, 437-440 (1982).
  5. Chabbat J et al. Aprotinin is a competitive inhibitor of the factor VIIa-tissue factor complex. Thromb Res 71, 205-215 (1993).
  6. Mielicki WP and Gordon SG. Three stage chromogenic assay for the analysis of activation properties of factor X by cancer procoagulant. Blood Coagul Fibrinol 4, 441-446 (1993).
  7. Koppaka V et al. Soluble phospholipids enhance factor Xa-catalyzed prothrombin activation in solution. Biochemistry 35, 7482-7491 (1996).
  8. Riesbeck K et al. Human tissue factor pathway inhibitor fused to CD4 binds both FXa and TF/FVIIa at the cell surface. Thromb Haemost 78, 1488-1494 (1997).
  9. Romisch J et al. Comparative in vitro investigation of prothrombin complex concentrates. Semin Thromb Hemost 24, 175-181 (1998)
  10. Faria F, et al. A new factor Xa inhibitor (lefaxin) from the Haementeria depressa leec.Thromb Haemost 82, 1469-73 (1999).