On 2014 Feb 28, Tom Kindlon commented:
Using two MFI scales ("General Fatigue" or "Reduced Activity") to ensure patients satisfying the definition have "severe fatigue"
(This was originally posted here http://www.biomedcentral.com/1741-7015/3/19/comments#304567 but the formatting has gone from that page)
Initially when I read this paper, where it says "we defined severe fatigue as >= medians of the MFI general fatigue (>=13) or reduced activity (>=10) scales", I thought this referred to medians of the general population.Hearing other people commenting on it, that's how some other people have been interpreting it also. It is probably somewhat natural to do this as the sentence before reads: "We defined substantial reduction in occupational, educational, social, or recreational activities as scores lower than the 25th percentile of published US population [11] on the physical function (<=70), or role physical (<=50), or social function (<=75), or role emotional (<=66.7) subscales of the SF-36."
However from looking at the scores for controls in other papers, these MFI scores do not look like medians for the whole US population but in fact are medians for this particular group of patients. This seems a strange way to set cut-off points for a CFS definition that is used for numerous studies into the illness, given the cohort that is being used as a basis: "This population-based case control study enrolled 227 adults identified from the population of Wichita with: (1) CFS (n = 58); (2) non-fatigued controls matched to CFS on sex, race, age and body mass index (n = 55); (3) persons with medically unexplained fatigue not CFS, which we term ISF (n = 59); (4) CFS accompanied by melancholic depression (n = 27); and (5) ISF plus melancholic depression (n= 28)." i.e. this is not a random sample of the US population but a group of people selected for a specific purpose (or purposes) (not necessarily to design a definition, but as a follow-up study of people previously diagnosed with CFS or given some other label).
Some of the groups are of different sizes - if the relative size of these groups had been changed, with relatively more people taken from some classification groups and less people taken from other groups, the median scores would likely have been different.
It should also be remembered that in this context the categories listed in the last paragraph refer to their classification when they evaluated years before (from 1997 to 2000), and not necessarily at the time when they were evaluated in this study (December 2002 to July 2003) (as is clear from the tables in this paper).
I thought it would be interesting to look at MFI scores in some other papers on CFS that did not use the "empirical definition".
I don't claim this is a definitive list but, at the same time, mean MFI scores with standard deviations only seem to be listed in a small percentage of papers. The papers use cohorts from a variety of locations: England [3], The Netherlands [4], Germany [5] and the USA (New Jersey) [6]. I did not see any ranges given which would be useful given the task at hand (selecting cut-off points for a definition).Unfortunately not all of the papers I found used the Fukuda [1] definition for CFS; some also used the Sharpe [2] definition for CFS. I indicate which definition is used in each case.
MFI: General Fatigue
(i) Sample/(ii) Sample Size/(iii) Mean/(iv) SD/ (v) (Mean - 13)/SD/ (vi) Definition
Weatherley-Jones [3] 53 18.4 1.7 3.176470588 Sharpe (1991)
Vermeulen (Group 1) [4] 30 18.6 1.9 2.947368421 Fukuda (1994)
Vermeulen (Group 2) [4] 30 18.4 1.8 3 Fukuda (1994)
Vermeulen (Group 3) [4] 30 19.1 1.4 4.357142857 Fukuda (1994)
Gaab [5] 21 17.7 0.5 9.4 Sharpe (1991) and Fukuda (1994)
Brimacombe [6] 65 18.41 2.02 2.678217822 Fukuda (1994)Combining these give a sample of 229 patients with a mean "General Fatigue" score of 18.45655022.
This data suggests that a threshold of >=13 will have a very very high sensitivity. This would suggest that another measure would not be necessary (unless it was being used as an extra criterion to increase the specificity, which isn't done with this definition).
However for completeness, I'm including the "Reduced Activity" data from the same papers:Reduced activity (MFI)
(i) Sample / (ii) Sample Size / (iii) Mean Score / (iv) SE (v) (Mean-10)/SD (vi) Definition
Weatherley-Jones [3] 53 16.1 3.1 1.967741935 Sharpe(1991)
Gaab [5] 21 15 0.7 8.714285714 Sharpe (1991) and Fukuda(1994)
Brimacombe [6] 65 15.93 4.55 1.340659341 Fukuda 1994
Combining these give a sample of 139 patients with a mean Reduced Activity score of 15.85431655.
Note: the Vermeulen paper[4] did not collect the MFI scores for Reduced Activity, just "the fatigue axes of the Multidimensional Fatigue Inventory" (which they defined as the MFI scores for General fatigue, Physical fatigue, Mental fatigue).
It seems strange in the definition of Chronic Fatigue Syndrome defined in this paper (i.e. Reeves et al) that the "severe fatigue" criterion can be satisfied by a patient having a low score on a subscale of the MFI testing activity levels (as opposed to one of the 3 subscales measuring fatigue), especially when the function of the SF-36 is to "measure functional impairment". Just because someone is inactive doesn't mean they have severe fatigue. Allowing patients to be included if they simply have a "Reduced Activity" score of 10 or more (without necessarily having a low score on one of the fatigue axes of the MFI) risks reducing the specificity of the definition.
References:
[1] Fukuda K, Straus SE, Hickie I, Sharpe MC, Dobbins JG, Komaroff A. The chronic fatigue syndrome; a comprehensive approach to its definition and study. Ann Int Med 1994, 121:953-959.
[2] Sharpe MC, Archard LC, Banatvala JE, Borysiewicz LK, Clare AW, David A, Edwards RH, Hawton KE, Lambert HP, Lane RJ, et al. A report--chronic fatigue syndrome: guidelines for research. J R Soc Med. 1991 Feb;84(2):118-21.
[3] Weatherley-Jones, E., Nicholl, JP., Thomas, KJ., Parry, GJ., McKendrick, MW., Green, ST., Stanley, PJ and Lynch, SPJ. A randomised, controlled, triple-blind trial of the efficacy of homeopathic treatment for chronic fatigue syndrome. Journal of Psychosomatic Research, 2004, 56, 2, 189-197.
[4] Vermeulen, RCW and Scholte, HR. Exploratory open label, randomized study of acetyl- and propionylcarnitine in chronic fatigue syndrome. Psychosomatic Medicine, 2004, 66, 276-282.
[5] Gaab J, Hüster D, Peisen R, Engert V, Heitz V, Schad T, Schürmeyer TH, Ehlert U. Hypothalamic-pituitary-adrenal axis reactivity in chronic fatigue syndrome and health under psychological, physiological, and pharmacological stimulation.Psychosom Med. 2002 Nov-Dec;64(6):951-62.
[6] Brimacombe, Michael; Lange, Gudrun; Bisuchio, Kim; Ciccone, Donald S.; Natelson, Benjamin. Cognitive Function Index for Patients with Chronic Fatigue Syndrome Journal of Chronic Fatigue Syndrome, 2004, vol 12; number 4, pages 3-24
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