What is D-Dimer
When a blood vessels or tissues are injured in our body, bleeding occurs. Body has a mechanism to stop the bleeding called homeostasis by which blood clot develops initially and block the leaked or injured area. Later on, injured area repaired permanently. When full repairing is done, blood clot is no longer needed and begins to break down.
Blood clot consists mainly of a protein polymer called fibrin. Fibrin is a linear protein and it polymerize in a zig-zag way forming a protein fiber net. The fibrin thread cross link with each other and form a net. Platelet participates in this process and acts as a sand bag and form a guard wall in the injured vessel site. This net, together with platelet prevent blood leak forming blood clot.
After healing the injured site, clot is no longer needed and become a hazard to the body- need immediate removal, otherwise it will dislodge and block other vessels causing tissue injury.
One enzyme, called Plasmin break the fibrin into small pieces so that it can be removed. These small pieces of fibrin are called Fibrin Degradation Product (FDP). One of the final fibrin degradation products produced is D-dimer.
D-dimer is normally undetectable or detectable at a very low level.
The level of D-dimer in the blood can significantly rise when there is significant formation and breakdown of fibrin clots in the body.
So, presence of D-dimer in blood determines the like-hood of a clot’s presence. A positive D-dimer result may indicate the presence of an abnormally high level of fibrin degradation products. It indicates that there may be significant blood clot (thrombus) formation and breakdown in the body, but it does not tell the location or cause.
However, an elevated D-dimer does not always indicate the presence of a clot, elevated levels may be seen in conditions in which fibrin is formed and then broken down, such as recent surgery, trauma, infection, and some cancers or conditions in which fibrin is not cleared normally, such as liver disease
False positive test:
If high level of rheumatoid factor is present, as in rheumatoid arthritis.
A negative D-dimer test indicates that it is highly unlikely that a thrombus is present.
Positive test needs further investigations (USG, CT, Angiography) for further evaluation.
A normal or “negative” D-dimer result means that it is most likely that the person tested does not have a disease, causing abnormal clot formation and breakdown. The test is used to help rule out clotting as the cause of symptoms.
Cause of Inappropriate blood clot
- Most common cause is Deep vein Thrombosis (DVT) – thrombosis or clot formation in deep veins inside body, most frequently in lower legs. From there clot (embolus) may obstruct pulmonary arteries, causing pulmonary embolism.
- In Myocardial infractions, clot develops in coronary arteries causing heart attack.
- Similarly, clot may develop in Brain, causing cerebral infraction, In kidneys- causing renal necrosis, in all the veins throughout the body- a condition called Disseminated Intravascular Coagulation (DIC). Numerous clots are formed inside body. Other condition may be
- Poisonous snake bite
- Liver disease
- After child birth
Whatever may be the condition if there is a clot formation and breakdown, The D-dimer level will typically be very elevated. In DIC, it is highly elevated.
Some other points:
Negative D-dimer rules out DVT, Pulmonary embolism and stroke due to cerebral embolism.
Positive D-dimer test help to diagnose DIC and to monitor effectiveness of DIC treatment.
Covid-19 and D-dimer
Recent studies have focused on the role of serum inflammatory markers that predict Covid-19, such as
- lymphocyte counts and
- C-reactive protein (CRP),
- homocysteine and
- D-dimer levels.
- The levels of ferritin, a crucial immune response mediator, increase in severe Covid-19 cases.
Increased D-dimer levels have been reported in 3.75–68.0 per cent of patients with Covid-19; D-dimer levels above 1 μg/ml are accepted as a risk factor for increased mortality in adults. (Link) Moreover, Yao et al. reported an association between D-dimer levels and disease severity.