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We thank Dr Mertz and Arnaud1 for their comments on our recent article in the Annals of the Rheumatic Diseases entitled ‘Risk of systemic lupus erythematosus in patients with idiopathic thrombocytopenic purpura: a population-based cohort study’.2 They raised the question about selection of control group and suggest a control group consisting of patients with other autoimmune diseases, such as autoimmune haemolytic anaemia (AIHA), Evans syndrome and thyroiditis, instead of using a standard control group.
We agree that many immune-mediated diseases or chronic infectious diseases might also be attributed to systemic lupus erythematosus (SLE).3 4 As Mertz and Arnaud1 mentioned, previous studies had reported that SLE is associated with AIHA5 and autoimmune thyroid diseases.6 Sometimes, AIHA and idiopathic thrombocytopenic purpura (ITP) can coexist in patients with SLE.7 Therefore, we did not mean to compare the difference between ITP and other immune diseases, but simply ask the question: ‘Is risk of SLE increased in patients with ITP, compared to non-ITP controls?’
To answer this question, which we think is more clinically relevant, we thus select the non-ITP general population as control. With regard to the control group selection, we have two strategies—negative or positive control. To compare with the normal or non-exposure group, a healthy population is the best ‘negative exposure’ control. In some cases, especially for drug comparative effectiveness study, an active comparator group is an example of a ‘positive control’. This kind of control selection had been published in many previous studies with similar design.8 9
We also agree that it will be interesting to compare different immune-mediated diseases on the risk of incidental SLE. Thus, we did additional analysis to respond to this comment (table 1). In the Taiwan National Insurance Database with data on one million individuals, we retrieved data on newly diagnosed Hashimoto’s disease, Graves’ disease, AIHA, ITP and a general population control. The outcome is the subsequent incidence of SLE. Briefly, we found that age-adjusted and sex-adjusted HR was 25 for ITP, 19 for AIHA, 7.3 for Hashimoto’s thyroiditis and 1.6 for Graves’ disease, compared with the general population.
In conclusion, patients with ITP, AIHA, Hashimoto’s thyroiditis or Graves’ diseases are all at a higher risk for subsequent incidental SLE.
Patient consent for publication
Handling editor Josef S Smolen
Funding The present study was supported by the Programme of Scientific and Technology Project (Guilin Science Research and Technology Development; grant no 2016012706–2).
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Provenance and peer review Commissioned; internally peer reviewed.
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