Testing for suspected venous thromboemblism

(Aotea News, May 2010)

By Chani Tromop Van-Dalen
University of Otago, trainee intern

Venous thrombi are composed of red blood cells and fibrin, and form at areas of slow blood flow with low shearing forces. 1

D-dimers are the breakdown products of the fibrin clot, and a negative D-dimer assay (<500ng/mL) can be used to rule out the presence of venous thromboembolism (VTE).

The test can only be used to rule out the diagnosis as there are many other conditions that may activate the coagulation system, raising the level of D-dimers in the blood. These include common conditions such as trauma, cancer, pregnancy, diabetes, older age and the post-surgical state. 2

Just as there are conditions that can give false positive D-dimer assay results, there are those that can give false negative results. Prior Low Molecular Weight Heparin treatment and thromboembolism present for longer than 10 days or less than 24 hours may give a lower than expected D-dimer result. 3

For these reasons it is important that doctors clinically assess a pre-test probability before requesting D-dimer tests.

Use the Wells score to assess the pre-test probability of VTE

The Wells score is one of several scoring systems used to assess the pre-test probability of VTE. It has been shown to be more useful than empirical judgement as it is a standardised and reproducible assessment of pre-test probability of VTE. 4

A high Wells score markedly increases the probability of deep venous thrombosis (DVT), indicating the need for further definitive testing: leg ultrasound for suspected DVT, or CT pulmonary angiogram (CTPA) for pulmonary embolism (PE).

A low Wells score markedly reduces the probability of DVT, indicating the need for a simple, non-invasive test such as a D-dimer blood test to further exclude the diagnosis. 5

The Wells score should be completed before D-dimer testing so that clinicians are not influenced by a normal D-dimer result when evaluating clinical probability. 6

In practice, if VTE is clinically suspected, a Wells score should be determined.

  • If the Wells score is high (PE score > 2, DVT score > 1) then the patient should be further investigated with ultrasound scan or CTPA in the emergency department.
  • If the Wells score is low (PE score <2, DVT score <1) and the D-dimer is negative, the clinician can be relatively reassured as this has been shown to be a safe way of excluding the diagnosis of DVT and PE. 2, 3, 8-10

In the scenario of a low Wells score and a high D-dimer, the clinician will need to “make an assessment as to the significance of this result, taking into account the factors that may be present that may have falsely elevated the D-dimer.

Any uncertainty around the result warrants further discussion.

If you have any clinical enquiries about the interpretation of D-Dimers, contact Dr Ken Romeril (04 381 5900, kromeril@apath.co.nz).

Chani Tromop-van-Dalen is a trainee intern (6th year student) at the Univeristy of Otago, Wellington School of Medicine and Health sciences. In the 2008/09 university holidays, Chani completed a University of Otago Summer Studentship Project entitled “Do emergency doctors calculate a risk score before requesting D-dimer assays in patients with possible thromboembolic disease”.

Chani worked at Aotea Pathology as a phlebotomist from the end of 2007 until the start of 2009. “Interacting with patients and learning to take blood as a phlebotomist have been invaluable skills that I have taken with me into the hospital.”

Footnotes

1. Ageno W, Dentali F. Venous thromboembolism and arterial thromboembolism. Many similarities, far
beyond thrombosis per se. Thrombosis and Haemostasis 2008 August;100(2):181—3.
2. Marlar RA. D-dimer: establishing a laboratory assay for ruling out venous thrombosis. Mlo: Medical Laboratory Observer 2002 Nov;34(11):28—32.
3. Kraaijenhagen RA, Wallis J, Koopman MM, de Groot MR, Piovella F, Prandoni P, et al. Can causes of false-normal D-dimer test [SimpliRED] results be identified? Thrombosis Research 2003;111(3):155—8.
4. Ten Cate-Hoek AJ, Prins MH. Management studies using a combination of D-dimer test result and clinical probability to rule out venous thromboembolism: a systematic review. Journal of Thrombosis & Haemostasis 2005 Nov;3(11):2465—70.
5. Goodacre S, Sutton AJ, Sampson FC. Metaanalysis: The value of clinical assessment in
the diagnosis of deep venous thrombosis. Annals of Internal Medicine 2005 Jul 19;143(2):129—39.
6. Gibson, N. S., Sohne, M., Gerdes, V. E. Nijkeuter, M. & Buller, H. R. (2008). The Importance of Clinical Probability Assessment in Interpreting a Normal d-Dimer in Patients With Suspected Pulmonary Embolism. Chest, 134, 789—793.
7. Wellington Public Hospital Emergency Department Suspected Venous Thromboembolism Guidelines. Issued 01/03/2007.
8. Campbell A, Fennerty A, Miller AC. British Thoracic Society guidelines for the management of suspected acute pulmonary embolism. Thorax 2003;58:470—84.
9. Bates SM, Kearon C, Crowther M, Linkins L, O’Donnell M, Douketis J, et al. A diagnostic strategy involving a quantitative latex D-dimer assay reliably excludes deep venous thrombosis. Annals of Internal Medicine 2003 May 20;138(10):787—94.
10. Kruip MJHA, Slob MJ, Schijen JHEM, Van der Heul C, Buller HR. Use of a clinical decision rule in combination with D-dimer concentration in diagnostic workup of patients with suspected pulmonary embolism: A prospective management study. Archives of Internal Medicine 2002 22;162(14):1631—5.