This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
The role of D-dimer
D-dimer is a fragment released when a blood clot is broken down. A normal result makes a recent clot unlikely, so in patients with a low or intermediate clinical probability of pulmonary embolism (PE), a negative D-dimer test rules it out without imaging. The problem is that D-dimer rises with age, infection, cancer and pregnancy. With the usual cut-off of 500 ng/mL, fewer than 10% of patients over 80 have a negative test, so most go on to have a CT scan.
The age-adjusted cut-off
- Up to 50 years: 500 ng/mL (µg/L) FEU
- Over 50 years: age × 10 ng/mL FEU, for example 720 ng/mL at 72
The ADJUST-PE study, published in JAMA in 2014 by Righini and colleagues, managed 3,346 patients this way. Among those whose D-dimer was between 500 and their age-adjusted cut-off and who had no imaging, only 1 of 331 had a clot over the next three months, 0.3%. In patients aged 75 and over, the proportion in whom PE could be ruled out without imaging rose from 6.4% to 29.7%.
How to use it
- Assess the clinical probability with the Wells score or the revised Geneva score.
- If the probability is high, or PE is likely, go straight to imaging: D-dimer cannot rule it out.
- If it is low or intermediate, measure D-dimer and compare it with the age-adjusted cut-off.
- Below the cut-off, PE is ruled out. At or above it, imaging is needed.
Example
A 72-year-old patient with a low clinical probability and a D-dimer of 650 ng/mL:
- Age-adjusted cut-off: 72 × 10 = 720 ng/mL
- Result: below the cut-off, so PE can be ruled out without a CT scan
- With the conventional 500 cut-off, the same patient would have been scanned
Units matter
The rule was validated with sensitive quantitative assays reported in fibrinogen equivalent units (FEU). Some laboratories report D-dimer units (DDU), about half the size: in that case the conventional cut-off is about 250 ng/mL DDU, and the age adjustment should follow your laboratory’s guidance. Results in mg/L or µg/mL FEU must be multiplied by 1,000.
Other strategies
The YEARS algorithm uses a cut-off of 1,000 ng/mL when none of three items is present, and the PEGeD strategy does the same with a low Wells score. The PERC rule can avoid testing D-dimer altogether in very low-risk patients. The age-adjusted cut-off is also used, with less evidence, for deep vein thrombosis.