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Compression stockings


Another example of using the NNT worksheet in issue 59 is shown here. Consecutive patients with a first episode of venogram-proven deep vein thrombosis were randomised to use of a made-to-measure graduated compression stocking or to no stocking in preventing post-thrombotic syndrome [1], and followed up for a minimum of 60 months.

Outcome


Independent nurses examined patients at three-month intervals for compliance and for recurrent symptoms of venous thromboembolism. Post-thrombotic syndrome was measured on a number of pre-determined subjective criteria (pain in calf, for instance, or leg oedema) and on objective criteria (for example calf circumference, venous ulcer). A mild-to-moderate disease was defined as a score of 3 or more plus one objective symptom, with severe disease defined as a score of four or more; to qualify these had to occur on two consecutive three-month follow up visits.

Results


Patients were mostly men (56%) with an average age of 60 years. Ninety-six had stockings and 98 no stockings. Of those in the stockings group, compliance checks showed that all but seven wore their stockings most of the time.

Over a minimum period of 60 months, 19 patients with stockings had mild-to-moderate post-thrombotic syndrome, of whom six went on to have severe disease. Another five developed severe post-thrombotic syndrome without mild-to-moderate disease first. Overall, 72 of 96 patients (75%) were free of post-thrombotic syndrome at 60 months.

Over the same period, 46 patients without stockings had mild-to-moderate post-thrombotic syndrome, of whom 23 went on to have severe disease. Another 13 developed severe post-thrombotic syndrome without mild-to-moderate disease first. Overall, 39 of 98 patients (40%) were free of post-thrombotic syndrome at 60 months.

For every three patients with proximal deep vein thrombosis treated for five years with made-to-measure graduated compression stockings, one will not develop at least mild post-thrombotic syndrome - NNT 2.9 ( 2.1 to 4.5).

Bandolier's NNT worksheet

A number needed to treat (NNT) is defined by a number of characteristics. This worksheet is designed as an aide memoir for working out NNTs from papers and systematic reviews. First fill in the answers to the questions, where appropriate, graph the data on the L'Abbé plot, and finally do the NNT calculation.

Now graph the percentages for the trial on the graph from the percentages from F and J. This can be done for different outcomes of a trial, or individual trials in a systematic review or meta-analysis.


Now calculate the NNT using the proportions from F and J.


Comment


This is one trial, albeit with a large effect and reasonable numbers. The NNT is calculated from all patients randomised, and about 20 died or were lost to follow up in each group. The NNT worksheet, once done, could be added to others to create a firm or practice list of easily-available evidence, just as on the evidence cart .

Reference:

  1. DP Brandjes et al. Randomised trial of effect of compression stockings in patients with symptomatic proximal-vein thrombosis. Lancet 1997 349: 759-62.



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