Reply

Dear Sir,

Thank you for asking us to respond to Professor Brian Hughes and Dr David Tuller’ comments on our paper Chronic fatigue syndrome and occupational status: a retrospective cohort study. We have carefully considered their comments and checked our analyses. Whilst we accept there is a need for some clarification on certain points, we stand by our data and the paper’s key findings.

Comment: Incorrect description of statistical findings, using ‘X% of patients who did Y at baseline’ when they should have used the construction ‘X% of all 316 patients (i.e. those who provided follow-up data)’.

Response: We made clear at the start of the results section that we only included the 316 patients in our analyses for whom we have baseline and follow-up data on our outcome of interest, namely employment status. We also stated this clearly in the methods section of the abstract.

We also reiterate in paragraph 2 of the results that ‘119/316 (38%) reported that they were on sick leave from their job at baseline’.

We therefore disagree with the criticism.

Comment: Incorrect description of statistical findings. For example, in the Abstract, the authors state that ‘53% of patients who were working [at baseline] remained in employment [at follow-up]’. This is not accurate. Their own data (Table 2) show that 185 patients (i.e. 167 + 18) were working at baseline, and that 167 patients were working at both time points. In other words, the proportion working continuously was in fact 90% (i.e. 167 out of 185). The ‘53%’ that the authors refer to is the percentage of the sample who were employed at both time points (i.e. 167 out of 316), which is an entirely different subset.

Response: We reported on the proportion of patients affected by CFS in relation to our main outcome of interest changes in work status over the course of follow-up. These proportions were calculated based on the overall sample and shown in Table 2. However, we agree that the wording used in the abstract was less precise than it could have been. The 53% was derived from the overall sample when looking across all the categories of our main outcome of interest (remained in employment when considering the total sample) but as pointed out, it would have been better to amend the wording or alternatively use all patients who were working at baseline (n = 185) as the denominator. We are happy to correct this.

Comment: The authors indicate that the patients fulfilled the NICE criteria for CFS, whereas in their previous paper, also published in Occupational Medicine, it was suggested they fulfilled the Oxford criteria. This inconsistency is non-trivial, because the differences between these two diagnostic approaches have substantive implications for how the findings should be interpreted.

Response: We apologise for this error. We should have said all patients fulfilled NICE criteria for CFS. A proportion also fulfilled Oxford criteria. We would be happy to write a correction in the manuscript.

Comment: According to Table 1, there were either no changes or no meaningful changes in average scores for fatigue, physical function and multiple other secondary outcomes between the preliminary sample of 508 and the final follow-up sample of 316. The authors themselves acknowledge that the patients who dropped out before follow-up were likely to have had poorer health than those who remained. Therefore, the fact that Table 1 presents combined averages for the entire preliminary sample—i.e. combined averages for patients who dropped out and those who did not—muddies the waters. Presenting combined baseline scores for all patients will mask any declines that occurred for these variables in the subset who were followed up.

Response: The aim of this paper was to explore changes in work status from baseline to follow-up among the 316 patients we had employment status for on both occasions. Table 1 describes the baseline outcomes for those in the entire baseline sample compared with the subgroup who were successfully followed up. The data in Table 1 shows that for all data collected at baseline, data were comparable, except for occupational status, which we discussed in the results. We also commented on this in the discussion and added a warning about the interpretation of the results. There is nothing incorrect in our presentation of results in Table 1.

Another way to present these results is to compare the baseline scores for those who had baseline only data versus those who had baseline and follow-up data (see Table 1, available as Supplementary data at Occupational Medicine Online). These findings suggest that there were no differences between these groups with regards to gender, age, marital status and only a borderline significant difference in education (P = 0.043). Most importantly, no difference was found between the two groups regarding whether participants reported their work to be physically demanding, emotionally demanding, depression/anxiety symptoms, work and social adjustment scale scores, fatigue severity or any of their responses on the cognitive and behavioural responses questionnaire, except for catastrophizing thoughts. However, patients were more likely lost to follow-up if they reported poorer physical functioning at baseline and were by affected by CFS for a longer duration (average 5.8 years in the lost to follow-up group, versus 4.8 years in the group who provided data on both occasions).

The results provided in Table 2 address the aim of the current paper and the insights derived from Table 1 (available as Supplementary data at Occupational Medicine Online) do not change the interpretation of our findings.

Comment: The authors state that ‘Studies into CFS have placed little emphasis on occupational outcomes, including return to work after illness’. However, they conspicuously fail to mention the PACE trial, a high-profile large-scale British study of interventions for CFS. The PACE trial included employment status as one of four objective outcomes, with the data showing that the interventions used—the same ones as in the Occupational Medicine study—have no effect on occupational outcomes. The omission is all the more disquieting given that the corresponding author of the paper was a lead investigator on the PACE trial itself.

Response: The aim of the current study was to explore factors associated with changes in work status over time among patients affected by CFS and did not look at the effectiveness of treatment. Treatment was not specifically targeting work outcomes and effectiveness was out of scope for the current paper. We did not reference the paper previously as we did not think it was appropriate.

We noted that the authors suggested the article should be retracted. We stand by the results described in our paper; however, we would like the opportunity to correct the issue of definition of cases and are happy to add whatever clarifications on the other points that the editors see fit.

Yours sincerely

Competing interests

T.C. has received royalties from Sheldon Press and Constable and Robinson. She has delivered workshops on persistent physical symptoms and fatigue in the context of long-term conditions. She is the author of self-help books on fatigue. N.T.F. is a specialist member of the Independent Group Advising on the Release of Data for NHS Digital. M.H. is a National Institute for Health Research (NIHR) Senior Investigator. S.A.M.S and K.M.M. do not have any competing interests to report.