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• Could assay choice and sample storage explain the poor D-dimer sensitivity found by the DiPEP study?
  Danny McLernon-Billows, Lucy Ellis and Natalie Whitton
  Published on: 5 May 2021

• Published on: 5 May 2021

  Could assay choice and sample storage explain the poor D-dimer sensitivity found by the DiPEP study?

  • Danny McLernon-Billows, Emergency Medicine Registrar Emergency Department, Great Western Hospital, Swindon, UK
  • Other Contributors:
    • Lucy Ellis, Emergency Medicine Registrar
    • Natalie Whitton, Emergency Medicine Consultant

  Having recently updated our Emergency Department guidelines for suspected PE in pregnancy, we read the secondary analysis of the DiPEP study with great interest.1 However, we were quite surprised at the poor overall D-dimer sensitivity. Only 66% (8/12) of PEs would have been identified based on the recommended positivity threshold of 400ng/ml. This is considerably lower than the pooled estimate of 97% (95% CI 96-98%) found by a recent meta-analysis evaluating D-dimer for PE, and largely explains the poor performance of the YEARS and Geneva algorithms in the DiPEP cohort.2

  This result does not seem to fit with the known physiology of pregnancy. We know that D-dimer levels increase throughout pregnancy, which should improve sensitivity and worsen specificity.3 To our knowledge there are no other studies demonstrating impaired sensitivity of D-dimer in pregnant vs. non-pregnant populations.

  The DiPEP authors note that most of the study participants had received anticoagulation before blood samples were taken, which can decrease D-dimer levels by up to 25% in the first 24 hours.1 They also note however, that this would be insufficient to explain all their false negative D-dimer results. Aside from random error, we wondered if anything else could explain the poor sensitivity.

  One feature of the DiPEP study that stood out to us was the D-dimer assay used. As a microplate ELISA assay, the Zymutest D-dimer should be very sensitive but we could not find any st...

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  Having recently updated our Emergency Department guidelines for suspected PE in pregnancy, we read the secondary analysis of the DiPEP study with great interest.1 However, we were quite surprised at the poor overall D-dimer sensitivity. Only 66% (8/12) of PEs would have been identified based on the recommended positivity threshold of 400ng/ml. This is considerably lower than the pooled estimate of 97% (95% CI 96-98%) found by a recent meta-analysis evaluating D-dimer for PE, and largely explains the poor performance of the YEARS and Geneva algorithms in the DiPEP cohort.2

  This result does not seem to fit with the known physiology of pregnancy. We know that D-dimer levels increase throughout pregnancy, which should improve sensitivity and worsen specificity.3 To our knowledge there are no other studies demonstrating impaired sensitivity of D-dimer in pregnant vs. non-pregnant populations.

  The DiPEP authors note that most of the study participants had received anticoagulation before blood samples were taken, which can decrease D-dimer levels by up to 25% in the first 24 hours.1 They also note however, that this would be insufficient to explain all their false negative D-dimer results. Aside from random error, we wondered if anything else could explain the poor sensitivity.

  One feature of the DiPEP study that stood out to us was the D-dimer assay used. As a microplate ELISA assay, the Zymutest D-dimer should be very sensitive but we could not find any studies demonstrating its test characteristics. Product information available online contained no references and states it is “for research use only and should not be used for patient diagnosis or treatment”.4 This is perhaps a concern when considering whether we can generalise these results to clinical practice. In contrast, the Artemis5 and CT-PE-Pregnancy6 studies used assays that are well validated, highly sensitive, and widely used in clinical practice:7
  - Quantitative latex agglutination assays: TinaQuant (Roche); Liatest (Stago); Innovance (Siemens); HemosIL (Instrumentation Laboratory).
  - Rapid modified ELISA assays (ELFA): VIDAS (Biomerieux).

  A second feature of the DiPEP study that stood out was the method in which samples were stored and analysed. After collection and initial processing, samples were stored in freezers for “the duration of the study until all samples were transported for analysis to Guy’s St Thomas Trust”. This presumably could be up to 18 months as recruitment occurred between February 2015 and August 2016. The Zymutest D-dimer product information however, states that samples should only be frozen for up to 6 months.4 A 2015 review by Kline and Kabrhel noted that false negative D-dimer results can be caused by “proteolysis that can occur with prolonged time from sample draw to analysis”.3

  Could this extended storage of blood samples possibly explain the low D-dimer sensitivity and therefore poor performance of the revised YEARS and Geneva algorithms in this cohort? Given these limitations, the iatrogenic risks of imaging, and the imperfect sensitivity and specificity of CTPA (94% vs. 98%) and V/Q scans (96% vs. 95%),2 it is not clear to us that a “scan-all strategy” for PE in pregnancy appropriately balances the competing risks of under-diagnosis and over-treatment.

  References
  1. Goodacre S, Nelson-Piercy C, Hunt B, & Fuller G. Accuracy of PE rule-out strategies in pregnancy: secondary analysis of the DiPEP study prospective cohort. Emerg Med J. 2020;37:423-28.
  2. Patel P, Patel P, Bhatt M, Braun C, Begum H, Wiercioch W et al. Systematic review and meta-analysis of test accuracy for the diagnosis of suspected pulmonary embolism. Blood Adv. 2020;4(18):4296-311.
  3. Kline J, & Kabrhel C. Emergency evaluation for pulmonary embolism, part 2: Diagnostic approach. J Emerg Med. 2015;49(1):104-17.
  4. https://www.aniara.com/mm5/PDFs/IFU/IFU_ARK023A.pdf
  5. Van der Pol L, Tromeur C, Bistervels I, ni Ainle F, van Bemmel T, Bertoletti L et al. Pregnancy-adapted YEARS algorithm for diagnosis of suspected pulmonary embolism. N Engl J Med. 2019;380:1139-49.
  6. Righini M, Robert-Ebadi H, Elias A, Sanchez O, Le Moigne E, Schmidt J et al. Diagnosis of pulmonary embolism during pregnancy. Ann Intern Med. 2018;169(11)766-773.
  7. Righini M, Perrier A, de Moerloose P, & Bounameaux H. D-dimer for venous thromboembolism diagnosis: 20 years later. J Thromb Haemost. 2008;6:1059-71.

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  Conflict of Interest:
  None declared.

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