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• W1964844549 abstract "John Scurr and colleagues (May 12, p 1485)1Scurr JH Machin SJ Bailey-King S Mackie IJ McDonald S Coleridge Smith PD Frequency and prevention of symptomless deep-vein thrombosis in long-haul flights: a randomised trial.Lancet. 2001; 357: 1485-1489Summary Full Text Full Text PDF PubMed Scopus (394) Google Scholar presented some of the data for their study of frequency of deep-vein thrombosis (DVT) during long-haul flights at the 3rd Pacific Vascular Symposium in Hawaii in November, 1999, and reported a 20% prevalence of DVT. No mention was made of a randomised trial at that stage.2Scurr J Coleridge Smith PD Machin S Deep vein thrombosis in airline passengers: the incidence of deep vein thrombosis and the efficacy of elastic stockings.Cardiovasc Surg. 2001; 9: 159-160Crossref PubMed Scopus (14) Google ScholarAn exclusion rate of 248 participants seems excessive in a study population of 419. In the report, we can find reasons given for only 55 exclusions. This high rate raises a concern about bias within the study. Perhaps they did not intend originally to do a randomised trial? Was it simply that some patients enrolled at the beginning of the study did not receive stockings?We have done a power calculation based on an incidence of 1% of DVT per year in the general population,3Browse NL Burnand KG Thomas ML Deep vein thrombosis: pathology.in: Diseases of the veins: pathology, diagnosis and treatment. Arnold, London1988: 443-474Google Scholar and have extrapolated this to a 24 h flight. 2500 airline passengers would have to be studied to confirm a significant increase in the prevalence of DVT compared with controls. Only the surprisingly high prevalence of DVT in Scurr and colleagues' study prevents a type II error. Was a power calculation made at the outset and what data did they base it on?Scurr and colleagues used duplex scanning to assess the presence or absence of DVT. Despite their citing five studies in support of the accuracy of this method, a meta-analysis has shown that it is only 70% accurate in the calf segment in patients with symptomatic DVTs, and even less so in those without symptoms.4Simons GR Skibo LK Polak JF Creager MA Klapec-Fay JM Goldhaber SZ Utility of leg ultrasonography in suspected symptomatic isolated calf DVT.Am J Med. 1995; 99: 43-47Summary Full Text PDF PubMed Scopus (75) Google Scholar Duplex is highly operator-dependent. What external audit was applied to the sonographers who were obviously not masked to the exposure to flight? For instance, are the scans available to a third party for review? Concern about the accuracy of the duplex scan is borne out by the lack of correlation between raised D-dimer concentrations and positive scans. Incidentally, D-dimer concentrations in participants who were diagnosed by duplex scanning as having a DVT were never higher than the upper limit of normal.Our final concern is the enormously high numbers (one in ten) of passengers who developed DVT during travel. This rate does not correspond with any previously published study; prevalence is normally estimated to be 1% or less.5Eklof B Kistner RL Masuda EM et al.Venous thromboembolism in association with prolonged air travel.Dermatol Surg. 1996; 22: 637-641PubMed Google Scholar Scurr and colleagues accept that their method of recruitment was not ideal, and passengers who on previous flights had leg swelling might have been more enthusiastic to volunteer for the study than those who had not.We are pleased that the researchers conclude that further truly prospective and randomised studies are required, since we do not agree with their statement that the frequency of symptomatic DVT recorded is reliable. If their 10% prevalence of DVT was extrapolated to one long-haul flight between Australia and the UK, at least 40 passengers would develop DVT. John Scurr and colleagues (May 12, p 1485)1Scurr JH Machin SJ Bailey-King S Mackie IJ McDonald S Coleridge Smith PD Frequency and prevention of symptomless deep-vein thrombosis in long-haul flights: a randomised trial.Lancet. 2001; 357: 1485-1489Summary Full Text Full Text PDF PubMed Scopus (394) Google Scholar presented some of the data for their study of frequency of deep-vein thrombosis (DVT) during long-haul flights at the 3rd Pacific Vascular Symposium in Hawaii in November, 1999, and reported a 20% prevalence of DVT. No mention was made of a randomised trial at that stage.2Scurr J Coleridge Smith PD Machin S Deep vein thrombosis in airline passengers: the incidence of deep vein thrombosis and the efficacy of elastic stockings.Cardiovasc Surg. 2001; 9: 159-160Crossref PubMed Scopus (14) Google Scholar An exclusion rate of 248 participants seems excessive in a study population of 419. In the report, we can find reasons given for only 55 exclusions. This high rate raises a concern about bias within the study. Perhaps they did not intend originally to do a randomised trial? Was it simply that some patients enrolled at the beginning of the study did not receive stockings? We have done a power calculation based on an incidence of 1% of DVT per year in the general population,3Browse NL Burnand KG Thomas ML Deep vein thrombosis: pathology.in: Diseases of the veins: pathology, diagnosis and treatment. Arnold, London1988: 443-474Google Scholar and have extrapolated this to a 24 h flight. 2500 airline passengers would have to be studied to confirm a significant increase in the prevalence of DVT compared with controls. Only the surprisingly high prevalence of DVT in Scurr and colleagues' study prevents a type II error. Was a power calculation made at the outset and what data did they base it on? Scurr and colleagues used duplex scanning to assess the presence or absence of DVT. Despite their citing five studies in support of the accuracy of this method, a meta-analysis has shown that it is only 70% accurate in the calf segment in patients with symptomatic DVTs, and even less so in those without symptoms.4Simons GR Skibo LK Polak JF Creager MA Klapec-Fay JM Goldhaber SZ Utility of leg ultrasonography in suspected symptomatic isolated calf DVT.Am J Med. 1995; 99: 43-47Summary Full Text PDF PubMed Scopus (75) Google Scholar Duplex is highly operator-dependent. What external audit was applied to the sonographers who were obviously not masked to the exposure to flight? For instance, are the scans available to a third party for review? Concern about the accuracy of the duplex scan is borne out by the lack of correlation between raised D-dimer concentrations and positive scans. Incidentally, D-dimer concentrations in participants who were diagnosed by duplex scanning as having a DVT were never higher than the upper limit of normal. Our final concern is the enormously high numbers (one in ten) of passengers who developed DVT during travel. This rate does not correspond with any previously published study; prevalence is normally estimated to be 1% or less.5Eklof B Kistner RL Masuda EM et al.Venous thromboembolism in association with prolonged air travel.Dermatol Surg. 1996; 22: 637-641PubMed Google Scholar Scurr and colleagues accept that their method of recruitment was not ideal, and passengers who on previous flights had leg swelling might have been more enthusiastic to volunteer for the study than those who had not. We are pleased that the researchers conclude that further truly prospective and randomised studies are required, since we do not agree with their statement that the frequency of symptomatic DVT recorded is reliable. If their 10% prevalence of DVT was extrapolated to one long-haul flight between Australia and the UK, at least 40 passengers would develop DVT. Deep-vein thrombosis in long-haul flightsAuthors' reply Full-Text PDF" @default.
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• W1964844549 title "Deep-vein thrombosis in long-haul flights" @default.
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