Spot the clot – smell the rat

One study published in the UK medical journal The Lancet in May 2001 found that the risk of developing blood clots, or deep-vein thrombosis (DVT), during long journeys is up to 40 times higher than had previously been thought (1).

The study also claimed that wearing elastic stockings during the flight significantly reduced (or even eliminated) the risk of blood clots. Of the study group who went on a flight but did not wear the stockings, 12 out of 116 developed symptomless DVT in the calf. Of the study group who went on the flight and wore the stockings, not one developed blood clots.

Can you smell the rat?

Firstly, these results are anomalous – they do not fit into the pattern established by previous studies. Anomalous results are normally treated with suspicion; if there are mistakes, these are more likely to be found in the exception, than they are in the general trend.

Secondly, as some scientists pointed out, even if elastic compression stockings had an effect, it is unlikely that they would reduce the numbers of cases to zero.

In a commentary in the same issue of The Lancet, Jack Hirsh and Martin J O’Donnell explored the holes in this study (2). Given that the small blood clots in question are quite difficult to see, Hirsh and O’Donnell proposed that the technicians looking for clots ‘may have been inadvertently unmasked by communicating with the volunteers during the examination’. That is, if a volunteer told the technician that they had not worn the stockings, the technicians would have been more likely to spot clots.

This interpretation is supported by the fact that the blood clot diagnoses could not be confirmed by subsequent tests, meaning that ‘the thrombi were either very small or were false positive findings’.

Yet with headlines like ‘Flight blood clots “hit one in ten”‘ and ‘Blood clots may affect one in 10 air travellers’ (3), it seems that Hirsh and O’Donnell’s criticism did not substantially affect interpretation of the study.

(1) Frequency and prevention of symptomless deep-vein thrombosis in long-haul flights: a randomised trial, by John H Scurr et al, The Lancet, 12 May 2001. To access this article, first register at http://www.thelancet.com

(2) Venous thromboembolism after long flights: are airlines to blame?, by Jack Hirsh and Martin J O’Donnell, The Lancet, 12 May 2001. To access this article, first register at http://www.thelancet.com

(3) Headlines from BBC News Online and the Daily Telegraph