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D-Dimer Test: What It Is and What a Positive Result Means

D-dimer is a clot breakdown product measured to assess the likelihood of dangerous blood clots. It's an excellent 'rule out' test: but a positive result needs further investigation.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: · How we check our content

Units
<500 µg/L (typical threshold)
Sensitivity for PE
~97–99%
Specificity
Low: many causes of elevation
Use
Rule OUT, not rule IN, clots

How D-Dimer Works

Rule Out, Not Rule In

D-dimer has high sensitivity but LOW specificity. A NEGATIVE D-dimer in a low-probability patient effectively rules out DVT/PE. But a POSITIVE result needs imaging: many non-clot conditions also elevate it.

Causes of Elevated D-Dimer (Not a Clot)

CauseClinical Context
Infection / sepsisInflammatory activation of coagulation
PregnancyPhysiologically elevated throughout pregnancy
CancerTumour-related procoagulant state
Surgery / traumaPost-operative state
Heart failure / atrial fibrillationChronic low-level clot activation
Liver diseaseImpaired fibrinogen clearance
Age >80 yearsAge-adjusted threshold used (age × 10 µg/L)
  • Age-adjusted threshold: for patients over 50, use age × 10 µg/L as the cut-off
  • Wells Score: calculate pre-test probability before ordering D-dimer
  • If Wells Score high: go directly to imaging without D-dimer
  • Positive D-dimer + high clinical probability → CT pulmonary angiogram (CTPA) for PE / Doppler USS for DVT
Pre-Test Probability FirstD-dimer is only useful in LOW pre-test probability patients. Always use the Wells PE or DVT score first. In high-probability cases, go straight to imaging: a normal D-dimer doesn't rule out PE in high-risk patients.
What does a positive D-dimer mean?
It means clot-related fibrin fragments are elevated in the blood. This can be from a DVT or PE, but also from many other conditions. Further imaging is needed to confirm or exclude a clot.
Can a normal D-dimer rule out a pulmonary embolism?
Yes. In patients with low-to-moderate pre-test probability (Wells Score <5), a D-dimer below the threshold effectively rules out PE with >99% sensitivity.
Is D-dimer elevated in COVID-19?
Yes. COVID-19 activates coagulation significantly, and very high D-dimer (>1500 µg/L) was associated with worse outcomes in early COVID-19 studies.
Does D-dimer stay high after a blood clot?
Yes. It can remain elevated for weeks during treatment of an established clot. It is not useful for monitoring response to anticoagulation.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

  1. D-Dimer Test. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK431064

Related reading

Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.