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Discordant D‐dimer results of two rapid quantitative automated assays are related to age

We read with interest the recently published paper by Carrier et al. [1] on the use of D-dimer (DD) in the elderly. This study demonstrates that the combination of a low Wells pre-test probability with a negative DD value safely excludes deep vein thrombosis (DVT) in elderly patients (aged over 60 years). However, the results of this study, based on pooled results of three technically different DD assays, are not totally in agreement with three other studies [2–4], and the authors suggest that this discrepancy might be explained by differences in the specificity of the DD assays. DD assays have been extensively evaluated, and meta-analyses were recently performed to determine the best tests [5,6]. One limitation of these approaches is that the findings are mainly based on an indirect comparison of the test performance from various studies and hence, mostly from heterogeneous patient populations. In the study, we compared performances in two rapid quantitative automatedDDassays on the same population and demonstrated that age was the main clinical characteristic related to discordant results between the two tests. We previously published a management study of 1134 consecutive non-selected patients clinically suspected of pulmonary embolism (PE) [7]. In this study, DD (cut-off 0.5 mg L, quantitative ELISA test, Vidas D-Dimer; BioMerieux, Lyon, France): (V) was determined straight away for 656 patients who fulfilled the criteria of our decision-making. For 608 of these patients, citrated plasma was immediately stored at )80 C, and a second DD measurement performed retrospectively using a rapid immunoturbidimetric assay (cutoff 0.5 mg L, STA-Liatest D-DI; Stago, Paris, France): (L). Data were complete for 603 patients (five patients were lost at follow-up) who achieved a 3-month follow-up. DDwere found negative (<0.5 mg L) by both methods (L)/V)) in 259 patients (43%; mean age 50.4 years, SD 16.6). The mean DD values for the two tests were not statistically different. False negatives accounted for 1.92% (CI 95% 0.26–3.58%) and this was identical for the two methods. DD were found to be positive (‡0.5 mg L) by both methods (L+/V+) in 277 patients (mean age 64.2 years, SD 14). Results were not concordant for 67 (11%) patients, of these, 63 (94%) patients neither had PE nor develop PE during the 3-month follow-up. Among these 67 discordant results, 54 were L)/V+ and 13 L+/V). Three patients (4.5%, 95% CI 0–9%) had PE in the L)/V+ group, comparable with one (1.5%, 95% CI 0– 4.3%) (P = 0.6) in the L+/V) group. In fact, all these patients were at high clinical probability. Conversely, 51 of the 67 with discordant results (76.1%, 95%CI 65.9–86.3) were L)/ V+ and did not have any thromboembolic events, compared with 12 (17.9%, 95% CI 8–27.1%) in the L+/V) group (P < 0.0001). To explain this discrepancy in the positive rate between the two tests, we analysed the performances in each test related to the patients clinical characteristics. Both gender and ward origin of the 67 patients with discrepant results did not differ significantly from those of the 536 patients with concordant results. Interestingly, when V DD values only were positive, the patients mean age was 64.2 years (SD 14) but significantly lower, 54.3 years (SD 18), when LDD values were only positive (P = 0.03). Therefore, we tested the difference between DD using both methods according to age. For this purpose, DD obtained using the two tests were recorded for each patient, and DDD (i.e. DD Vidas minus DD Liatest) calculated and expressed for the three age groups. This analysis was performed for 420 patients out of 603, as values above 1 mg L (not given quantitatively by the ELISA method at the time of the study) were excluded. As shown in Fig. 1, there was a significant increase in the difference between the Vidas Correspondence: Gilles Pernod, Vascular Medicine Unit, PPM, CHU Grenoble, BP 217X, 38043 Grenoble cedex 09, France. Tel.: 33 476765717; fax: 33 476765524. E-mail: GPernod@chu-grenoble.fr