Diagnostic in-vitro method to determine qualitative and/or quantitative defects of von Willebrand factor characterised by, a) using a sample obtained from a human individual or an animal comprising a body fluid which comprises platelets and hemostasis factors, b) incubating at least one part of the sample with an activator of platelet aggregation, wherein the activator of platelet aggregation is a compound which is able to induce VWF dependent platelet activation, aggregation and agglutination in a fluid derived from the human body which contains platelets and VWF by causing binding of VWF to a VWF-receptor on platelets, c) inducing coagulation in the part of the sample which was incubated with an activator of platelet aggregation, d) measuring the viscoelastic change occurring after induction of coagulation in the part of the sample which was incubated with an activator of platelet aggregation in a thromboelastograph, and e) comparing the amount of the viscoelastic change determined in step (d) with a reference value, wherein the reference value is obtained: i) by splitting the original donor sample obtained in step (a) into at least two samples or obtaining in step (a) at least two samples from a donor wherein in at least one sample only step (a) and steps (c) to (e) are performed and step (b) is omitted and wherein the amount of viscoelastic change measured in the sample in which step (b) was omitted is the reference value; or ii) by applying a method comprising step (a) and steps (c) to (e) to a sample from a healthy donor or to several samples from healthy donors without increased bleeding risk wherein the amount of viscoelastic change measured in the sample or in the samples in which step (b) was omitted is the reference value; or iii) by applying a method comprising steps (a) to (e) to a sample from a healthy donor or to several samples from healthy donors without increased bleeding risk wherein the amount of viscoelastic change measured in the sample or in the samples is the reference value. a) using a sample obtained from a human individual or an animal comprising a body fluid which comprises platelets and hemostasis factors, b) incubating at least one part of the sample with an activator of platelet aggregation, wherein the activator of platelet aggregation is a compound which is able to induce VWF dependent platelet activation, aggregation and agglutination in a fluid derived from the human body which contains platelets and VWF by causing binding of VWF to a VWF-receptor on platelets, c) inducing coagulation in the part of the sample which was incubated with an activator of platelet aggregation, d) measuring the viscoelastic change occurring after induction of coagulation in the part of the sample which was incubated with an activator of platelet aggregation in a thromboelastograph, and e) comparing the amount of the viscoelastic change determined in step (d) with a reference value, wherein the reference value is obtained: i) by splitting the original donor sample obtained in step (a) into at least two samples or obtaining in step (a) at least two samples from a donor wherein in at least one sample only step (a) and steps (c) to (e) are performed and step (b) is omitted and wherein the amount of viscoelastic change measured in the sample in which step (b) was omitted is the reference value; or ii) by applying a method comprising step (a) and steps (c) to (e) to a sample from a healthy donor or to several samples from healthy donors without increased bleeding risk wherein the amount of viscoelastic change measured in the sample or in the samples in which step (b) was omitted is the reference value; or iii) by applying a method comprising steps (a) to (e) to a sample from a healthy donor or to several samples from healthy donors without increased bleeding risk wherein the amount of viscoelastic change measured in the sample or in the samples is the reference value. i) by splitting the original donor sample obtained in step (a) into at least two samples or obtaining in step (a) at least two samples from a donor wherein in at least one sample only step (a) and steps (c) to (e) are performed and step (b) is omitted and wherein the amount of viscoelastic change measured in the sample in which step (b) was omitted is the reference value; or ii) by applying a method comprising step (a) and steps (c) to (e) to a sample from a healthy donor or to several samples from healthy donors without increased bleeding risk wherein the amount of viscoelastic change measured in the sample or in the samples in which step (b) was omitted is the reference value; or iii) by applying a method comprising steps (a) to (e) to a sample from a healthy donor or to several samples from healthy donors without increased bleeding risk wherein the amount of viscoelastic change measured in the sample or in the samples is the reference value.