The use of an age-adjusted D-dimer cutoff in ruling out venous thromboembolism for patients over the age of 50 is now largely accepted. Most commonly, the age-adjusted dimer cutoff is calculated based on the patient’s age x 10 (e.g. the age-adjusted upper limit of normal for a 70-year-old patient is 700). This makes sense when the standard reference range is 0-0.5 or 0-500, but what about when the reference range for the test is 0-230?
It turns out that, even though all quantitative D-dimer assays seem to use similar methodology (“latex enhanced immunoturbidimetric immunoassays), some assays report in units of FEU (fibrinogen equivalent units), while others report in DDU (D-dimer units). The reason for this is a difference in machine calibration.Those that report in FEU will have cutoffs of 0-0.5 ug/L or 0-500 ug/mL or some equivalent conversion. Assays that report using DDU typically have cutoffs of 0-230 or 0-250 ng/mL.
2 DDU = 1 FEU
Therefore, that 70-year-old patient has an age-adjusted d-dimer of 700ug/mL FEU or 350 ng/mL DDU. To simplify, instead of multiplying by 10, you can just multiply by 5.
And why should you even use an age-adjusted D-dimer? One meta-analysis demonstrated that, among older patients determined to be low risk for DVTor PE by Wells’ score, using an age-adjusted D-dimer improved specificity by an absolute range of 5-30%, while maintaining >97% sensitivity at all age groups. This allows you to practice good resource management by minimizing unnecessary testing without compromising safety.
References:
- Widely used types and clinical applications of D-dimer assays. Laboratory Medicine, May 2016
- http://unitslab.com/index.php/node/83
- Diagnostic accuracy of conventional or age adjusted D-dimer cut-off values in older patients with suspected venous thromboembolism: systematic review and meta-analysis. BMJ 2013
