A comment on using the SF-36 PF subscale in CBT and GET trials for CFS (from 2007, but timeless)

(I originally posted this on Co-Cure back in 2007 but thought I'd post it somewhere else now. The same points still apply

I thought people might find it interesting to look at the SF-36 physical functioning sub-scale as it is one of the two primary outcome measures in the PACE trial and in other trials.

A lot of people use the whole SF-36 for their research but they've decided to use this part (one of 8 measures) as one of the two outcome measures (the other being the 11-item Chalder Fatigue Scale) to see whether people have improved or not.

[My opinion: This may be because people have tended to get bigger increases on this after GET or CBT than some other measures E.g. in Fulcher and White (1997), Ellen Goudsmit pointed out: http://listserv.nodak.edu/cgi-bin/wa.exe?A2=ind0702C&L=CO-CURE&P=R2886&I=-3 d (the effect size) calculated from the data for the MOS-SF physical functioning subscale was .69, but d for the MOS-SF as a whole was .499. One is higher than that for fatigue, the other is lower].

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SF-36 physical functioning sub-scale

2. The following items are about activities you might do during a typical day. Does your health now limit you in these activities? If so, how much?

Yes, Limited A Lot | Yes, Limited A Little | No, Not Limited At All

a. Vigorous activities such as running, lifting heavy objects, participating in strenuous sports

b. Moderate activities, such as moving a table, pushing a vacuum cleaner, bowling, or playing golf

c. Lifting or carrying groceries

d. Climbing several flights of stairs

e. Climbing one flight of stairs

f. Bending, kneeling, or stooping

g. Walking more than one mile

h. Walking several hundred yards (some of them have "more half a mile") (in the US it's "walking several blocks")

I. Walking one hundred yards (in the US, it's "walking several blocks")

j. Bathing or dressing yourself

Scoring: 5 for each "Yes, Limited A Little" 10 for each "No, Not Limited At All"

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It is interesting because one of the aims of CBT (O'Dowd, 2006) is "Increased confidence in exercise and physical activity"

One part of their programme (and probably other CBT and GET programmes) is:
"Introduction to graded exercise as a means of regaining fitness and confidence in movement."

If one gets somebody to go for one or two walks a day or aim for this, their confidence in their ability to do it may increase. Across the course of a day, they may be doing no more or even less than a person pacing themselves (but not doing one or two "big" walks), and mentally they might be more brain-fogged but if they say start going for a walk, they are probably much less likely to tick "Yes, Limited A Lot" or "Yes, Limited A Little" (and more likely to tick "No, Not Limited At All") than somebody who paces themselves and listens to their body for the questions about walking ability (for example).

Just because they answer the questionnaire in a certain way doesn't mean that they're actually healthier. It very interesting how, although there was some change in subjective measures in the Belgian clinics, on the exercise testing before and after (and follow-up) there was basically no change (tableaux 66 & 67) (This is mentioned (but not necessarily in the same way) in the text:

"Il ne semble pas y avoir de lien entre l'évolution de la qualité de vie perçue (subjectivement) et l'évolution de la capacité physique (constatée objectivement). Entre l’évolution (entre la mesure baseline dans le cadre du programme de rééducation de bilan et les mesures d’effet à la fin de la rééducation) sur l’échelle ‘perception générale de la santé’ du SF-36 et l’évolution de la prise d’oxygène maximale des patients qui ont effectué l’épreuve d’effort à chaque fois au maximum, il existe en effet une corrélation de -0,04 (ce qui indique qu'il n'y a pas de lien entre les deux scores). Subjectivement, les patients se sentent donc mieux sans que cette amélioration ne se perçoive dans les résultats de l’épreuve d’effort.").

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In the PACE trial, regarding outcome measures, they say:

"The SF-36 physical function sub-scale [29] measures physical function, and has often been used as a primary outcome measure in trials of CBT and GET. We will count a score of 75 (out of a maximum of 100) or more, or a 50 % increase from baseline in SF-36 subscale score as a positive outcome. A score of 70 is about one standard deviation below the mean score (about 85, depending on the study) for the UK adult population [51, 52]."

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[Aside: The other primary outcome measure is:

"The 11 item Chalder Fatigue Questionnaire measures the severity of symptomatic fatigue [27], and has been the most frequently used measure of fatigue in most previous trials of these interventions. We will use the 0,0,1,1 item scores to allow a possible score of between 0 and 11. A positive outcome will be a 50 % reduction in fatigue score, or a score of 3 or less, this threshold having been previously shown to indicate normal fatigue [27]."

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- because they're using the 0,0,1,1 item scores, I think there's going to be a ceiling effect and less people will be seen to be getting worse (even if they are)]

[Aside: Inclusion criteria for the trial use the SF-36 PF subscale:
1. Both participant and clinician agree that randomisation is acceptable.
2. The participant has given written informed consent.
3. The participant meets operationalised Oxford research diagnostic criteria for CFS [2].
4. The participant’s Chalder Fatigue Questionnaire score is 6 or more [27].
5. The participant’s SF-36 physical function sub-scale score [28] is 65 or less.
6. The participant is aged at least 18 years old].