What Does an Elevated D-Dimer Level Mean When It Increases by Several Times?

Dimer is a substance present in the blood. When we conduct blood tests and find an elevation in dimer levels, it is necessary to make judgments based on specific conditions, as the impacts of an increased dimer can vary, such as hemorrhage and thrombosis, which both can result in changes in dimer values. To understand the specific reasons for the elevation of dimer levels, it is also necessary to evaluate the clinical manifestations exhibited by the patient. Chemically, a dimer refers to a new substance formed by the combination of two molecules, whether through physical interactions or chemical changes. Common examples include dicopper chloride, dialuminum chloride, diketene, gaseous dicarboxylic acid, dicyclopentadiene, and dicyclobutadiene. Dimers can be a special case within polymers. For instance, sucrose, composed of glucose and fructose units, is still classified as a dimer despite being a single molecule. The dimers discussed in biochemistry and molecular biology are usually observable macromolecules such as proteins and nucleic acids. During their formation, if the two subunits are identical, they are referred to as homodimers; whereas, if they are composed of incompletely identical monomers, they are heterodimers. When dimers in biochemistry are linked by covalent bonds, they often use disulfide bonds to connect the monomers. However, the majority of dimers are not formed through covalent bonds, such as ribosomes. Another example is the pyrimidine dimer, where adjacent pyrimidines on DNA or RNA covalently bind to each other through ultraviolet radiation, primarily due to the biological effects of ultraviolet light. This dimer can be restored to monomers through the catalytic action of photoreactivation enzymes after absorbing light. Clinically, elevated or positive dimer levels are observed in secondary fibrinolytic hyperfunction, such as hypercoagulability, disseminated intravascular coagulation, kidney diseases, organ transplant rejection reactions, thrombolytic therapy, and other conditions. Whenever there is activated thrombus formation and fibrinolytic activity within the blood vessels of the body, dimer levels will increase. Conditions such as myocardial infarction, cerebral infarction, pulmonary embolism, venous thrombosis, surgery, cancer, disseminated intravascular coagulation, infection, and tissue necrosis can all lead to elevated dimer levels. Especially in elderly individuals and hospitalized patients, conditions like bacteremia can easily lead to coagulation abnormalities and subsequent elevations in dimer levels. During the coagulation process, thrombin hydrolyzes fibrinogen, releasing fibrin FPA and FPB, which then forms fibrin monomers (SFM). Cross-linking occurs between the SFMY chains through ε(—γ-glutamine)—lysine bonds, ultimately forming fibrin. This covalent cross-linking between γ-chains serves as the structural basis for the formation of DD. During the dissolution of cross-linked fibrin, fragments such as X’, Y’, D’, E’ are released, forming complexes like DD, DD/E, YD/YD, YY/DD. These fragments further degrade into the smallest fragments, DD and DD/E complexes. DD has a molecular weight of approximately 62000D and a half-life in the body of greater than 3 hours, primarily excreted through the kidneys. Therefore, it can serve as a marker for fibrinolysis involving cross-linked fibrin, while single-chain D can originate from fibrinogen and fibrin without cross-links.