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We agree with the view of Walsh et al that it is not acceptable to miss a deep vein thrombosis (DVT) in any patient,1 and the negative predictive value of any test (or combination of tests) must approach 100%. The authors mentioned impedance plethysmography, but did not elaborate the utility of such an investigation, yet importantly they discussed the cost issues at the end of their discussion section for a planned management of DVT at a district general hospital.
We propose that the impedance plethysmography together with pre-test probability scoring and a modern D-dimer test would be an ideal choice for a district general hospital, in order to reduce the cost of investigations for DVT, without jeopardising the safety of the patients.
Flanagan et al recently reported that computed strain gauge plethysmography can be used as a satisfactory first line investigation for the diagnosis of DVT, with a negative predictive value of 97%.2 In a recent study a cheaper version known as digital photoplethysmography (D-PPG) has also shown a negative predictive value of 100% when used on its own.3 We believe that these techniques, together with pre-test probability scoring and a modern D-dimer test, would approach a negative predictive value of 100%, fulfilling the authors’ assumption of good practice.
We carried out a trial using combined D- PPG and D-dimer testing to investigate DVT in the outpatient department and compared the results against “gold standard” ultrasonography. In a six month trial (from September 2001 to February 2002) of 134 patients we were able to pick up 18 cases of positive DVT (all Doppler positive), 77 cases of negative DVT (all Doppler negative), and 39 cases with equivocal/unable results (all Doppler negative). This confirmed that 77 cases would not have required a Doppler study, 18 cases would definitely have required a Doppler study, and 39 cases would have required further examinations. A summary of the results are shown in the table 1.
All the recent studies1,4 have major implications for practice because they show that the combination of a low pre-test probability score, derived from a formal scoring system, and a negative D-dimer test safely excluded DVT in outpatients, obviating further investigation in 40% of patients.4 The very low incidence of DVT (0.6%) during follow up among those with a low pre-test probability score and negative D-dimer test is reassuring4 when compared with an incidence of about 1% at three months among untreated patients with suspected DVT and a negative venogram.5 However, the occurrence of DVT in up to 20% of patients with a high pre-test probability score and negative D-dimer test emphasises the point that the D-dimer test cannot be used in isolation.4
Therefore, it is possible that an objective of combining a formally derived pre-test probability score, the modern D-dimer test, and either computed strain gauge plethysmography or D-PPG can safely exclude DVT in outpatients and will be the most cost effective approach at a district general hospital and should now be the initial diagnostic step.5
Following the decision of the General Medical Council�s Fitness to Practise Panel,  the publisher is issuing a correction that affects the authorship of a letter published in the Postgraduate Medical Journal. 
Following the Panel�s decision, this paper should now always be cited as:
Bird, AD. Cost effective strategy for a safe diagnosis of deep vein thrombosis at a district general hospital. Postgrad Med J 2003;19:363.
Ms Bird acknowledges the help of Dr JIG Strang, Dr R Winter and Mr R Hibberd.
2. Sinharay R, Strang G, Bird D, Cost effective strategy for a safe diagnosis of DVT at a district general hospital. Postgrad Med J 2003;19:363.