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Can a blood test detect a blood clot?

A blood clot, also known as a thrombus, is a clump of blood that has changed from a liquid to a gel-like or semi-solid state. Blood clots form in the body as part of the normal clotting process that stops bleeding after an injury. However, clots can also form inside blood vessels when there is no apparent injury, blocking normal blood flow. This is known as thrombosis. A blood clot that forms inside a blood vessel is dangerous because it can block or prevent normal blood flow. If a clot becomes lodged in an artery that supplies blood to the heart or brain it can cause a heart attack or stroke.

Detecting a blood clot as early as possible is important, as prompt treatment can help prevent these serious complications. While some blood clots cause symptoms like pain, swelling, redness or warmth in the affected limb, others do not produce any noticeable signs. This makes them difficult to diagnose. Imaging tests like ultrasound, CT scans or MRI can be used to detect clots, but these tests are not routinely performed unless a clot is already suspected based on a person’s symptoms.

This is why researchers have been investigating whether certain substances released into the bloodstream by a clot could be detected through relatively simple blood tests. If an elevated level of one of these “biomarkers” associated with clotting is found, it could indicate the presence of a clot that warrants further testing.

What substances are released when a blood clot forms?

When a blood clot starts to develop, several processes occur within the blood and the cells lining the blood vessel walls that alter the composition of the blood. These changes can be detected through blood tests that look for elevated levels of specific enzymes, cell fragments or chemicals.

Some examples of substances released during clot formation include:

– D-dimer – A protein fragment produced when a clot starts to degrade. It is a biomarker of clot formation and breakdown.

– Fibrin and fibrinogen – Fibrinogen is converted into fibrin threads that make up the structure of the clot. Raised fibrin levels indicate active clotting.

– Platelets – Platelets are blood cell fragments that clump together to initially form the clot. Raised platelet levels can signify increased clotting.

– Thrombin – An enzyme that converts fibrinogen into fibrin. Its presence indicates clotting activity.

– Prothrombin F1+2 – A protein fragment released when prothrombin is converted to thrombin during clot formation.

– Thromboxane A2 – A compound secreted by platelets that facilitates clotting. Elevated levels can indicate platelet activation and thrombosis.

– Plasmin and plasminogen – Plasminogen is converted into plasmin, which breaks down clots. Raised levels show the body’s attempt to dissolve a clot.

– Tissue factor – Released from cells to activate the clotting cascade. Heightened tissue factor indicates increased coagulation.

By testing blood samples for elevated levels of these biomarkers, doctors may be able to detect clotting activity in the body. However, while promising, more research is still needed to confirm the accuracy and usefulness of this approach.

What blood tests can detect biomarkers associated with blood clots?

Some specific blood tests that can be used to look for biomarkers of blood clot formation include:

D-dimer test

This test measures the level of D-dimer protein fragments in the blood. It is often used as an initial screening test for clots because D-dimer is elevated when a clot is present and the body is trying to break it down.

Result Interpretation
Normal D-dimer A normal result indicates that a clot is less likely, but does not completely rule it out.
Elevated D-dimer An elevated level indicates that increased clot breakdown has occurred. This raises suspicion for a clot.

However, while a high D-dimer indicates clotting, a normal level does not conclusively rule out a clot. Other tests are often needed. Conditions like pregnancy, cancer, infection and inflammation can also elevate D-dimer absent a clot.

Fibrinogen test

This measures the level of fibrinogen protein that helps form blood clots. It may be used alongside other clot markers.

Result Interpretation
Normal fibrinogen Levels in the normal range make a clot less likely.
Elevated fibrinogen High levels indicate the body may be producing more fibrin to form clots.

Prothrombin time (PT) and activated partial thromboplastin time (aPTT)

These tests measure how long it takes blood to clot. Prolonged times can suggest an increased risk of excessive clotting.

Result Interpretation
Normal PT/aPTT A normal result indicates lower odds of a clotting problem.
Prolonged PT/aPTT Longer clotting times raise suspicions of a possible clotting disorder or increased clot risk.

Thrombin generation test

This test assesses the body’s total thrombin-generating capacity. High levels indicate increased clot formation activity.

Platelet activity testing

Platelet function tests assess platelet reactivity. High platelet activity suggests platelets may be overactive, increasing clot risk.

Genetic testing

Variations in genes like factor V Leiden or prothrombin can make clotting more likely and be detected on genetic tests.

How accurate are these blood tests at detecting blood clots?

The accuracy of blood tests for diagnosing clots varies depending on the specific test:

– D-dimer – This test is very sensitive, meaning if it is normal it is good at ruling out a clot. But it has low specificity, so if elevated further testing is needed to confirm a clot.

– Prothrombin time – This has sensitivity and specificity of about 60-70% for detecting a clot. Not accurate enough on its own.

– Fibrinogen – On its own not very accurate at diagnosing clotting with low sensitivity of approximately 60%.

– Thrombin generation testing – Estimated to have a sensitivity of 85% and specificity of 89% for clots when thresholds optimized. More accurate than many standard coagulation tests.

– Genetic testing – If factor V Leiden or prothrombin gene mutation detected, high lifetime risks of 70-80% and 10-15% of clotting respectively. But not all with clots have these genetic markers.

No single blood test can definitively diagnose a blood clot on its own. However, combining multiple biomarker tests may provide better accuracy. For example, a model using both D-dimer and thrombin generation testing can have a sensitivity of 94% and specificity of 83% for clots.

What are the potential applications of blood clot biomarker testing?

Some possible uses of blood testing for biomarkers associated with clotting include:

– Initial screening when a clot is suspected – Blood tests like D-dimer can be performed as the first step before ordering imaging.

– Testing in high-risk groups – Those with clotting risk factors may benefit from regular biomarker screening.

– Monitoring treatment response – Testing clot biomarkers before and after treatment could assess if anticoagulants are working.

– Recurrence monitoring – Testing for markers periodically after a clot could detect early recurrences.

– Risk stratification models – Combinations of multiple biomarkers could be incorporated into models to predict clot likelihood.

– Population screening – In certain scenarios, screening average risk individuals to find asymptomatic clots may become feasible.

– Research applications – Clot biomarkers could be used as surrogate endpoints in clinical trials testing new antithrombotic therapies.

What are the current limitations and challenges?

While blood biomarker testing holds promise, there are limitations:

– No single ideal biomarker – Each test has deficiencies in sensitivity and/or specificity. Combinations of biomarkers appear more effective.

– Results affected by other factors – Biomarker levels can be elevated due to inflammation, pregnancy, cancer, medications. Confounding is possible.

– Lack of standardization – Assays, cut-offs and reference ranges differ between labs, hampering consistency.

– Unknown optimal testing approach – More research is still needed to clarify the best biomarkers to test, combinations and testing intervals.

– Unclear real-world clinical impact – Despite accuracy demonstrated in studies, advantages over current diagnostic approaches remains unproven.

– Cost-effectiveness uncertain – The high costs of some biomarker assays may outweigh benefits they provide versus current standard imaging tests.

Further research through well-designed clinical trials is required to realize the potential of blood testing for clot biomarkers. But in the future, these tests could become a useful complement to current imaging-based clot detection.


Blood testing to look for biomarkers associated with thrombosis is an emerging approach that aims to allow earlier and easier detection of dangerous blood clots. While promising, current research shows these biomarker tests alone lack the accuracy needed for definitive clot diagnosis. However, when performed in combination and incorporated into clinical assessment, elevated clot biomarkers could indicate increased clot risk and the need for further testing.

With standardized assays and more clinical data, blood biomarker screening could become a viable component of clot risk prediction and diagnosis. But for now, imaging tests like ultrasound, CT and MRI remain the mainstay for direct clot detection in suspected cases. Ongoing research seeks to refine and improve blood testing approaches to realize their utility in clinical practice. For high-risk patients or where clots are strongly suspected, blood biomarker tests may already provide useful supporting information to guide diagnosis and management.