D-dimer
D-dimer is a fibrin breakdown product. A negative result effectively rules out blood clots in low-risk patients; a positive result is non-specific and needs imaging to confirm.
What it measures
When the body forms a blood clot and then breaks it down, fibrin is cleaved into fragments — D-dimer is one of these. Levels rise in any state of accelerated clot formation and lysis, so the test has very high sensitivity but poor specificity. The clinical playbook is well established: in a patient with LOW pre-test probability of pulmonary embolism or deep-vein thrombosis (Wells score), a negative D-dimer reliably rules out the clot without imaging. A positive D-dimer says little on its own — the next step is CT-pulmonary-angiogram or compression ultrasound. Age-adjusted thresholds (age × 10 ng/mL above 50) reduce false positives in elderly patients.
What a high value can mean
- Venous thromboembolism — pulmonary embolism, deep-vein thrombosis; the clinical reason the test exists.
- Disseminated intravascular coagulation (DIC) — sepsis, obstetric emergencies; markedly elevated.
- Recent surgery, trauma, burns — acceptable to be high days to weeks afterwards.
- Pregnancy — rises through gestation; pregnancy-trimester thresholds exist.
- Cancer — chronically raises baseline.
- COVID-19, severe infection — non-specific elevation; not necessarily a clot.
- Acute aortic dissection — typically very high.
What a low value can mean
- Excludes acute thrombosis in a low-pretest-probability patient. A normal D-dimer is reassuring; clinical scoring (e.g. Wells) determines whether 'low' applies.
When to discuss with a doctor
D-dimer is not a screening test for healthy patients. It is ordered when there is acute suspicion of VTE — sudden breathlessness, leg swelling, pleuritic chest pain — and a structured pre-test probability score has been calculated. If your D-dimer was ordered in an emergency setting and came back positive, an imaging study (CT-PA or compression ultrasound) is the next step, not self-interpretation. Mediora.AI shows the value with reference range; clot diagnosis is an emergency-medicine call.