Further commentary on the 6MWD issue:
White et al 2013 said:
Second, and importantly, there were practical limitations to our conduct of the walking test. Due to concerns about patients with CFS coping with physical exertion, no encouragement was given to participants as they performed the test, by contrast to the way this test is usually applied (Guyatt et al. 1984; American Thoracic Society, 2002). Rather than encouragement, we told participants, 'You should walk continuously if possible, but can slow down or stop if you need to.' Furthermore we had only 10 metres of walking corridor space available, rather than the 30-50 metres of space used in other studies; this meant that participants had to stop and turn around more frequently (Guyatt et al. 1984; Troosters et al. 1999; American Thoracic Society, 2002), slowing them down and thereby vitiating comparison with other studies. Finally, we had follow-up data on 72% of participants for this test, which was less than for the self- report measures (White et al. 2011).
American Thoracic Society (2002). ATS Statement: Guidelines for the six-minute walk test. American Journal of Respiratory and Critical Care Medicine 166, 111-117
http://www.thoracic.org/statements/resources/pfet/sixminute.pdf
American Thoracic Society said:
Encouragement
Only the standardized phrases for encouragement (as specified previously here*) must be used during the test.
Rationale: Encouragement significantly increases the distance walked (42). Reproducibility for tests with and without encouragement is similar. Some studies have used encouragement every 30 seconds, every minute, or every 2 minutes. We have chosen every minute and standard phrases. Some studies (53) have instructed patients to walk as fast as possible. Although larger mean 6MWDs may be obtained thereby, we recommend that such phrases not be used, as they emphasize initial speed at the expense of earlier fatigue and possible excessive cardiac stress in some patients with heart disease.
* These are "you are doing well" and "keep up the good work", but it also states earlier, "Do not use other words of encouragement (or body language to speed up)."
42. Guyatt GH, Pugsley SO, Sullivan MJ, Thompson PJ, Berman LB, Jones NL, Fallen EL, Taylor DW. Effect of encouragement on walking test performance. Thorax 1984;39:818–822
53. Troosters T, Gosselink R, Decramer M. Six minute walking distance in healthy elderly subjects. Eur Respir J 1999;14:270–274.
American Thoracic Society said:
A low 6MWD is nonspecific and nondiagnostic. When the 6MWD is reduced, a thorough search for the cause of the impairment is warranted. The following tests may then be helpful: pulmonary function, cardiac function, ankle–arm index, muscle strength, nutritional status, orthopedic function, and cognitive function.
PACE have not demonstrated any concern about the cause of the impairment i.e. low 6MWD, probably dismissing it as fear-avoidance and deconditioning while simultaneously denying that the scores are much lower than average for healthy people because encouragement was lacking and short walking spaces were used? As I mentioned before, I doubted lack of encouragement cited above can explain the low scores in the PACE Trial participants, and in the sources White et al use, encouragement only accounted for about 30m on average (not 250-300m), as explored below:
White et al and the American Thoracic Society (2002) both cite Guyatt et al 1984, so I had a closer look at this small study of 43 patients, but I wonder if there are more recent and better conducted research on encouragement:
Guyatt GH, Pugsley SO, Sullivan MJ, Thompson PJ, Berman L, Jones NL, Fallen EL, Taylor DW. Effect of encouragement on walking test performance. Thorax. 1984 Nov;39(11):818-22. PMID: 6505988
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC459930/pdf/thorax00227-0020.pdf
Figure 1 shows that encouragement and repeated testing makes a difference (but not big enough to explain the similar PACE scores). Also, I wonder if patients may need more encouragement than healthy controls anyway.
Interestingly, Guyatt et al wrote that
"The difference between corrected scores in the E+ and E- groups constituted the encouragement effect and averaged 30.5m." In other words, coincidently, the average effect of encouragement on 6MWD scores was about the same as the GET advantage after 52 weeks.
According to page 53 of the PACE Trial complete protocol, the 6MWT was conducted at baseline, 24 weeks (end of therapy), 52 weeks (trial end), and also in the situation of "treatment discontinuation". GET specifically encourages increased activity, and walking was the most commonly chosen activity, so others on this forum have stated previously that the minor improvement in 6MWD for the GET group could have merely been a learning effect.
The 6MWT is far from perfect, I have come across research before which suggests it is often not a particularly good indicator, and whether or not it correlates with other domains may be illness specific. But the reason it has become a prominent issue with the PACE Trial is because it was the closest to an objective measurement of physical fitness used.
From the abridged PACE Trial protocol, White et al even assured us that, "
The six-minute walking test will give an objective outcome measure of physical capacity." Yet they did not conduct the test properly and continue to show disregard for the importance and implications of objective measurements in an illness which is known to have objectively measured impairments. At the same time, White et al want to use the minor improvement in the GET group to support the effectiveness of GET. Not very impressive for a large "definitive" trial costing 5 million pounds that supposedly, according to its supporters, settled the ongoing CBT/GET controversy about effectiveness and safety.
Proponents are still downplaying objective measures while still promoting CBT/GET as evidence-based therapies which significantly improve symptoms and function, despite no objective evidence demonstrating clinically significant improvements in function, and the evidence available actually suggesting no such improvement. At the same time, potential sources of bias present when measuring patients' perceptions about illness after researchers have actively attempted to alter these same perceptions, are also being conveniently ignored.