How do you know if CFS patients have been able to increase their activity levels as the result of therapy? Put more bluntly, how do you know if a therapy really works? The usual way, used in the PACE trial, is asking patients about their activity or capabilities using a structured questionnaire. However, this leads to concerns over self-report bias: are patients subconsciously over-reporting their activity to please therapists, for instance? There are other reasons to doubt the accuracy of questionaires. First, different types of self-report (daily diary, different activity questionnaires) don't correlate very strongly with each other, when they really should stronglycorrelate. Secondly, and crucially, there is no good evidence that any questionnaire can accurately measure the relatively modest gains found in most clinical trials. No one ever does the experiment to find out. The alternative is to use some kind of motion sensing device, called an actometer, that straps to the body and measures acceleration in different directions. The device 'counts' the amount of movement and the higher the count, the more active the wearer has been. These devices are impressively accurate when measuring subjects on a treadmill in a lab, compared with gold-standard measures of exertion eg amount of oxygen consumed.The manufcturers of these devices trumpet these results and researchers tend to blandly cite the manufacturers' assurances. And actometers clearly can't suffer from self-report bias. However, no one mentions that if the lab treadmill has a gradient (as in running up hill, or downhill) the actometers can't tell the difference.Or that real life doesn't involve a lot of treadmill running. Where the accuracy of actometers has been measured in 'free-living' humans, the studies have usually been small and the results generally unimpressive. The Dutch/Belgian CFS studies used a home-grown actomter without even the usual lab-based validation. One study, though, does show impressive accuracy for an actometer measuring activity levels (or more strictly, energy expenditure) using the gold-standard methog of Double-labelled water (read the paper if you are interested). There was a robust sample of 42 subjects measured over 10 days. The correlation between actometer and energy-expenditure was 0.81 (zero is no correlation, 1.0 is total correlation). By comparison, most activity questionnaires correlate with each other by around 0.3 to 0.4, sometimes less. Given this, without any good evidence that activity (or disability)questionnairesdo reliably and accurately what patients really do, it would surely make sense to have improvement in actomter-measured activity as a primary outcome in CFS treatment trials. Here's that study: Evaluation of a portable device to measure daily energy expenditure in free-living adults. St-onge 2006 (free full text) ABSTRACT Background:Increasing daily energy expenditure (EE) plays an important role in the prevention or treatment of several lifestyle-related diseases; however, its measurement remains problematic. Objective:The objective was to evaluate a portable armband device for measuring daily and physical activity EE compared with doubly labeled water (DLW) in free-living individuals. Design:Daily EE and physical activity EE were measured in 45 subjects over a 10-d period simultaneously with 2 techniques: a portable armband and DLW. Resting metabolic rate was measured by indirect calorimetry, and the thermic effect of a meal was estimated (10% of daily EE). Physical activity EE was obtained by subtracting the values for resting metabolic rate and thermic effect of a meal measured with DLW from those measured with the armband. Body composition was measured with dual-energy X-ray absorptiometry. Concordance between measures was evaluated by intraclass correlation, SEE, regression analysis, and Bland-Altman plots. Results:Mean estimated daily EE measured with the armband was 117 kcal/d lower (2375 ± 366 kcal/d) than that measured with DLW (2492 ± 444 kcal/d; P < 0.01). Despite this group difference, individual comparisons between the armband and DLW were close, as evidenced by an intraclass correlation of 0.81 (P < 0.01). Conclusions:The portable armband shows reasonable concordance with DLW for measuring daily EE in free-living adults. The armband may therefore be useful to estimate daily EE. Declaration of interest: I've spent quite a bit of time casting doubt on the accuracy of actometers (as well as questionnaires); not sure why I didn't pay more attention to this study since I did mention it briefly on the PACE thread last year! Though as I mention, plenty of other actometer studies in free-living adults are rather less impressive.