Near patient D-dimer test for DVT

Dr Stephen Bright discusses a DVT test available for use in GP practices which saves time and money.

Deep vein thrombosisDeep vein thrombosis (DVT) and pulmonary embolism (PE) are a major concern for patients and healthcare staff.

Most cases of DVT resolve without sequelae but a proportion do lead to chronic venous problems and some to fatal PE.

Recently, the chief medical officer advised the profession of the scale of venous thromboembolism (VTE) in the UK, where more than 25,000 deaths are attributed to VTE per year.1,2

Diagnosing DVT
It is not uncommon for GPs to see patients who are concerned about an aching or swollen leg, or they could just be worried about the possibility of DVT following a long flight.

The clinical diagnosis of VTE in general practice is difficult and often inaccurate, even when probability scoring methods such as the Wells clinical probability tool are used (see below left).3

Only 15-20 per cent of patients referred from primary to secondary care with possible VTE will later prove to actually have DVT or PE when confirmed by ultrasound or laboratory techniques.4-6

The consequences of a missed DVT or PE can be grave, making a swift and conclusive diagnosis to be of essential importance.

Sending blood samples to a laboratory for processing is not usually an option for GPs as it often takes more than 24 hours for results to arrive.

Patients are either referred to hospital as inpatients or, in some areas, to specific same-day outpatient clinics. Definitive diagnosis in secondary care typically involves Doppler scanning or venography.

A blood test for D-dimer is also commonly used as an initial filter to exclude patients who do not have VTE.

Referral costs
There are disadvantages to referring patients to hospital.

The cost to PCTs is a minimum of £268 if the case is done as an outpatient assessment and found to be negative or £1,160 if carried out as an inpatient assessment (figures for the Oxford Radcliffe Trust based on national tariffs).

Near patient D-dimer test
A new option is available in the form of a near patient D-dimer test, based on the same technology used in hospital laboratories, that can be carried out within GP practices.

This monoclonal antibody-based test (Clearview Simplify D-dimer) is 100 per cent sensitive and so has a 100 per cent negative predictor value, meaning that a negative D-dimer result is sufficient to safely rule out the condition.

Conditions such as infection, recent surgery and malignancy can give a raised D-dimer so a positive test does not imply a VTE but a negative D-dimer test can rule out a VTE. Warfarin does not effect the validity of the test. Independent trials have confirmed that negative D-dimer tests can eliminate the possibility of DVT in around 40 per cent of suspected cases.6-8

The test can is easy to do, provides results in 10 minutes and currently costs approximately £10 per test (test cartridges have a typical shelf life of 18 months).

By using the D-dimer tests, GPs can cut the volume of suspect DVT and VTE cases referred to secondary care by about 40 per cent, saving patients who might otherwise have required referral time and expense, as well as quickly and safely assuring the patient that they do not have a VTE.

Cost savings
A rough estimate is that implementing this test in Oxfordshire alone could save £350,000 for the local PCT.

Eastbourne PCT trialled the use of this test in general practice and saved approximately £400,000 over just one year (their previous system involved inpatient assessments).

The Windrush Health Centre in Witney, Oxfordshire has also conducted a trial of this test in 22 patients.

Clinicians feel it has improved patient care, while the patients themselves have reported that they were pleased with the diagnostic pathway they were given using the D-dimer test at the Windrush Health Centre (this was supported by a patient participation group).

The local practice-based commissioners are now working with the PCT to evolve the management pathway and funding stream for VTE in primary care in support of the new diagnostic algorithm and the use of D-dimer testing in primary care.

Dr Bright is a GP in Witney, Oxfordshire

Wells probability tools
Wells clinical probability tool for DVT
Active cancer +1
Paralysis, paresis or recent plaster, or immobilisation of
lower limb
+1
Recently bedridden for more than three days or major surgery
within the last four weeks
+1
Localised tenderness +1
Entire leg swollen +1
Calf swelling >3cm compared with asymptomatic leg +1
Pitting oedema +1
Collateral superficial veins +1
Alternative diagnosis as likely or greater than deep vein
thrombosis
-2
Total
0 < points: low probability; 1-2 points: moderate probability;
3 > points: high probability

 

Wells clinical probability tool for PE
Clinical signs and symptoms of a DVT (minimum of leg swelling
and pain with palpation of the deep veins)
+3
An alternative diagnosis less likely than PE +3
Heart rate >100 +1.5
Immobilisation or surgery in the previous 4 weeks +1.5
Previous DVT/PE
+1
Malignancy (at treatment, treated in the past six months
or palliative
+1
Total
< 2 points: low probability; 2-6 points: moderate probability; >6 points: High probability

References

1. CMO letter 19th April 2007. www.dh.gov.cmo

2. VTE working Group www.dh.gov.uk/vte

3. Personal communication with Oxford DVT service. In financial year 2005, 1,272 patients seen, 13.75% confirmed to have DVT.

4. Wells P, Hirsh J, Anderson D et al. A simple clinical model for the diagnosis of deep-vein thrombosis combined with impedance plethysmography: potential for an improvement in the diagnostic process. J Int Med 1998; 243: 15-23.

5. Subramaniam R, Snyder B, Heath R, Tawse F, Sleigh J. Diagnosis of lower limb deep venous thrombosis in emergency department patients: performance of Hamilton and modified Wells scores. Ann Emerg Med 2006; 48: 678-85.

6. Kearon C, Ginsberg J, Douketis J et al. An evaluation of D-dimer in the diagnosis of pulmonary embolism: a randomized trial. Ann Int Med 2006; 144: 812-21.

7. Fancher T, White R, Kravitz R. Combined use of rapid D-dimer testing and estimation of clinical probability in the diagnosis of deep vein thrombosis: systematic review. BMJ 2004; 329: 821.

8. Bockenstedt P. D-dimer in venous thromboembolism. N Engl J Med 2003; 349: 1,203-4.

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