Checklist Individual Strength

The Checklist Individual Strength (CIS) is a 20-item fatigue questionnaire developed by the Dutch research team of Vercoulen et al. in 1994. The questionnaire has been translated into multiple languages and is used in various illnesses, including myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). An 8-item subscale of CIS that measures subjective fatigue regularly acts as a primary outcome in randomized trials on the effects cognitive behavior therapy in ME/CFS. The use of this subscale in a ME/CFS patient population however has been criticized for displaying ceiling effects.

Origin
The Checklist Individual Strength (Checklist individuele Spankracht) was developed by the Dutch research team of Vercoulen et al. in 1994 at the universities of Amsterdam and Rotterdam. CIS was intended to be a questionnaire to test the “behavioural, emotional, social, and cognitive aspects” of ME/CFS and to identify the multiple dimensions of ME/CFS patients’ disability.

20 questions, 4 subscales:
Initially the questionnaire consisted of 24 items but after testing in 298 patients who suffered from unexplained chronic fatigue for more than a year, 4 items were removed. The factor analysis indicated 4 components in the remaining 20 questions. These were easy to interpret and were called:
 * 1) Subjective fatigue (8 items)
 * 2) Concentration (5 items)
 * 3) Motivation (4 items)
 * 4) Physical activity (3 items)

1-7 Likert score:
The CIS consists of 20 statements on fatigue-related problems respondents might have experienced in the past 2 weeks. A Likert scoring scheme is used. With each statement respondents have to indicate a score from 1 to 7, indicating either “yes, that is true” or “no, that is not true.”  Examples of statements are:  “I feel tired”, “I have trouble concentrating” or “, I don’t do much during the day” etc. Almost half of the questions are inverted, meaning the statements indicate fitness instead of fatigue and the scoring system is reversed. “Yes, that is true” would then indicate a score of 1 instead of 7. Examples of such statements are: “. I feel fit, “I feel rested” or “I am full of plans”.

Internal consistency
CIS has shown good internal consistency, with a Cronbach alpha of 0.90 and a Gutman split-half reliability coefficient of 0.92.

Convergent validity
Research has shown that the results of the CIS  are comparable to those other measure of fatigue, such as fatigue measured on a unidimensional five point Likert scale or the scale exhaustion of the Maslach burnout inventory—general survey (MBI-GS). Using a large sample of 351 persons of the working population, De Vries et al. showed that the CIS correlated with other fatigue questionnaires such as the Chalder Fatigue Scale. In other studies, the concentration subscale of the CIS correlated with the subscale concentration problems of the Sickness Impact Profile (SIP), while the subscale fatigue severity correlated with activity levels measured by an Actometer.

Discriminative validity
Discriminative validity of the CIS was tested by Beurskens et al. in five sets of employees with expected differences in fatigue. The CIS was able to differentiate between healthy employees and those with a somatic or mental reason for fatigue. Vercoulen et al. showed that the CIS discriminates between ME/CFS patients from healthy controls and patients with functional bowel syndrome or multiple sclerosis.

Multiple languages and diseases
The CIS has been translated into different languages, including English, Portuguese, Polish, Japanese and Turkish. The questionnaire has been used in various illnesses including fibromyalgia, rheumatoid arthritis, multiple sclerosis, cancer, asthma, amyotrophic lateral sclerosis, sarcoidosis, and mitochondrial disorders.

Average scores
Indicative scores and standard deviations for the CIS and the subjective fatigue subscale are given below. In a study on work absence due to fatigue, a cut-off score of 76 on the CIS indicates a risk for subsequent sick leave or work disability. On the subjective fatigue subscale of CIS, a score of 35 is seen as indicative of severe fatigue.

Ceiling effects
The subjective fatigue (also called fatigue severity) subscale of the CIS has been the most widely used in the field of ME/CFS. Because this subscale asks general questions about fatigue such as “I “feel tired” or “I feel weak”, ME/CFS often score close to the maximum score. The fatigue severity subscale asks 8 questions which gives a score from 8 to 56. While healthy persons score on average 17.3 and patients with other chronic conditions usually score below 40, ME/CFS patients easily reach a score above 50. For example, in a study of homebound ME/CFS patients (which included some of the authors of the CIS), it was noted that “The CIS-fatigue score involves an overall rating and in CFS samples easily reaches the extreme end of its scale.”

As a result ME/CFS patients can no longer indicate a worsening of their fatigue, a phenomenon that is called the ceiling effect. This can influence the findings of randomized trials. For example: If ME/CFS patients record the maximum score on the CIS and half of them improves while the other half deteriorates with the same amount during follow-up, then only the improvement will become visible on the questionnaire. If such a result were used as a primary outcome in randomized trial, it might overestimate improvements in fatigue and underestimate deterioration or harms by the intervention. Bart Stouten calculated the lower bounds for the number of items with the maximum score on the CIS fatigue severity subscale. In some studies this was as high as 46%.