D-dimer is a blood test, performed on the same tube as a coagulation profile or INR Test. It requires the tube to be filled to the correct level. A few millilitres of blood is taken from a vein.
A D-Dimer is most commonly requested when the possibility of a Deep Venous Thrombosis (DVT: a blood clot in the leg) or Pulmonary Embolism (PE: a blood clot in the lung) is being considered. The test is used as risk-stratifying test, because it may make further testing unecessary in the correctly-selected patient. For example, a patient with a low risk of DVT or PE, with a negative (normal) D-Dimer test, will often not need to undergo further testing such as a Doppler Ultrasound of the legs or a CT Pulmonary Angiogram (CTPA). The test is less useful at excluding clotting conditions in the body, when the clinical risk is moderate or high. The clinical probability, known as the pre-test probability, is based on a composite impression from the doctor, often incorporating specific items, which may have a points system assigned as a guide.
D-dimer is one of the products of clot breakdown that can be measured in the bloodstream. Normally, this protein is present in small amounts because there is a constant turnover of tiny clots forming and being broken down by the body’s natural processes.
D-dimer may be NEGATIVE (normal): Below a set cut-off value, determined by the laboratory and the brand of test, a small quantity of D-dimer in the blood is very reassuring, in the low-risk patient. It suggests that there is not much clot turnover in the body.
If the D-dimer is POSITIVE: This may be due to non-specific inflammation, bedrest, after injury or surgery, and is more likely with increasing age. The majority of people with a positive d-dimer do not have a DVT or a PE; it simply means that these conditions cannot be excluded to a high degree of certainty.
The real value of a D-dimer test is in the low-risk patient, when the result is negative. Unfortunately it will more often be positive, and further testing will then usually be required to exclude a DVT or PE.
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