Age-Adjusted D-Dimer: Why the Cut-Off Rises With Age

D-dimer is a fragment produced when a clot is broken down. Lab Tests Online UK explains that it is normally undetectable and is produced only after a clot has formed and is being broken down, which is why the test is useful for ruling out venous clots as a cause of symptoms in the legs or lungs. A negative D-dimer makes a significant venous clot extremely unlikely.

The difficulty is that many other things raise D-dimer. Lab Tests Online UK lists recent surgery, trauma, infection, liver or kidney disease, cancers and pregnancy, and also notes that D-dimer concentrations may rise in the elderly. Because levels climb with age, a test that would reassure a 35-year-old is positive in a large share of people in their seventies and eighties, who then need imaging to look for a clot that is not there.

A review in Research and Practice in Thrombosis and Haemostasis puts it plainly: D-dimer levels naturally increase with age, which complicates interpretation in older patients, and it is this that the age-adjusted threshold sets out to address.

The rule most often described is simple. For people aged over 50, the threshold is the person's age multiplied by 10 micrograms per litre, expressed in fibrinogen equivalent units, with the conventional fixed threshold of 500 micrograms per litre kept for younger adults. The same review gives the example of 730 micrograms per litre for a 73-year-old.

There is one practical wrinkle. Some assays report results in D-dimer units rather than fibrinogen equivalent units. The review explains that for one widely used assay reporting in D-dimer units the conventional threshold is 250 micrograms per litre, equivalent to 500 in fibrinogen equivalent units, so the age is multiplied by 5 instead of 10. The two versions express the same cut-off on different scales, which is why a laboratory's report should always be read against its own stated threshold.

NICE guideline NG158, on venous thromboembolic diseases, recommends in recommendation 1.1.14 that when using a point-of-care or laboratory D-dimer test, an age-adjusted D-dimer test threshold should be considered for people aged over 50. It also advises that if a point-of-care test is used it should be fully quantitative.

In the rationale, the committee notes that evidence in people over 50 was limited, with only limited prospective evidence for deep vein thrombosis and only retrospective evidence for pulmonary embolism. That evidence suggested that adjusting thresholds for age improves the usefulness of the test for ruling out venous thromboembolism and does not reduce its accuracy in identifying it. The committee also said adjusting could reduce anxiety and unnecessary imaging.

The wording is therefore "consider", not "must". Local pathways in hospitals and urgent treatment services decide how the threshold is applied and with which assay.

The narrative review reports that applying the age-adjusted threshold to people over 50 increased the proportion of negative D-dimer results by 11 to 18 percent depending on the cohort, with a false-negative rate of under 1 percent across groups. In one cohort of people over 75, negative tests rose from 6.4 percent to 29.7 percent, which reduced CT imaging by about one third without raising false negatives.

The review is equally clear about caveats. Real-world diagnostic strategy studies remain limited, particularly for deep vein thrombosis, retrospective studies dominate, and more than 30 commercial assays are available, not all of which have been validated for age adjustment. It also reports that the threshold is intended for people with a non-high clinical probability, which is the group in whom D-dimer is used at all.

NICE sets out that, for suspected deep vein thrombosis or pulmonary embolism, a clinician first estimates the probability using the Wells score. If a clot is unlikely, a D-dimer test is offered with a result available within 4 hours if possible, and if the result cannot be obtained in that time interim anticoagulation may be offered while waiting. If the D-dimer is negative, a clot is considered unlikely and alternative diagnoses are explored. If it is positive, imaging follows.

If a clot is considered likely, D-dimer is not the deciding test, and imaging is offered directly. That is why a person who has an obviously swollen painful leg may be sent for an ultrasound scan without a D-dimer, and why a normal D-dimer cannot be taken as reassurance when the probability is high.

Anticoagulant therapy can cause a false-negative D-dimer, according to Lab Tests Online UK, and false positives may be seen with high rheumatoid factor, lipaemia, raised bilirubin and haemolysis, which is another reason it is read in context.