Hemostasis laboratory directors are aware of the problems created by the variety of D-dimer result expressions. D-dimer reagent kit manufacturers have failed to harmonize results, in part because their reagent monoclonal antibodies possess variant specificities, and in part because some report results in "D-dimer units" [DDUs] and some in "fibrinogen equivalent units" [FEUs]. FEU values are slightly over double DDU values. When DDUs are reported in nanograms [ng]/mL, a typical normal manufacturer's limit is 240 ng/mL , FEUs reports often use 500 ng/mL as their limit. However, D-dimers are variably reported in micrograms [µg]/mL, or milligrams [mg]/L, in which case the limits are expressed as the unweildy 0.24 µg/mL [or mg/L] in DDUs or 0.5 µg/mL [or mg/L] in FEUs. Compounding the problem is that many authors [and many laboratories] report D-dimer values with no units at all, leaving the reader to guess their clinical signicance. For addition information, see Favaloro EJ, Thachil J, Reporting of D-dimer data in COVID-19: some confusion and potential for misinformation. Clin Chem Lab Med 2020;58:1191–9. A further confounding factor is age-related adjustment to D-dimer cutoffs, for example, employing the formula age X 10 ng/mL for any age over 50, thus at 60 the cutoff is 600 ng/mL , at 70, 700 ng/mL , and so forth. See Lippi G, Favaloro EJ, Cervellin G. A review of the value of D-dimer testing for prediction of recurrent venous thromboembolism with increasing age. Semin Thromb Hemost. 2014;40:634–9.
The harmonization issue has become amplified by D-dimer's application to SARS-CoV2 infection as a means to monitor disease progression and predict the necessity for antithrombotic intervention. Not only are D-dimer testing volumes increasing because of the need to follow Covid patients, it seems, anecdotally, that increased awareness of the assay had inspired primary care and emergency department physicians to order the D-dimer as part of an initial workup, even in patients who lack Covid indications. Owing to D-dimer's sensitivity to mild inflammation, many intelligent lay people are alarmed when they see mildly elevated D-dimer laboratory results, especially when they arrive in their portals with no limits, units, or narratives. Please comment here if this issue has affected patient management in your institution.