D-dimer is a degradation product of cross-linked fibrin in the body, and normal human plasma is negative for D-dimer. By measuring the D-dimer, if it is found to be positive, it indicates that there are degradation products of cross-linked fibrin in the body, which means that there is a thrombotic condition in the body. In clinical practice, it is commonly used in the diagnosis of various thrombotic diseases, such as for pulmonary embolism and deep vein thrombosis, a positive D-dimer is one of the requirements to confirm the diagnosis. In addition, many pathological conditions, such as diffuse intravascular coagulation, infection, pathological obstetrics, and trauma, can also present with a marked increase in D-dimer, and a higher D-dimer concentration often indicates a serious condition with a poor prognosis.