We read with interest the article published by the Rheumatology Department of the José María Cullen Hospital in Santa Fe, Argentina.1

It seems relevant to compare the evolution of systemic lupus erythematosus (SLE) in a private and a public center, since the outcome of this disease could be affected by various factors involved in being at any of these centers, such as pharmacological treatment provided and the demographic characteristics of the patient attending a public or a private center. These factors influence the quality of life and the degree of disease activity in a patient.2

However, the study has some limitations that put into question the reported results. First, the time at which the authors started monitoring the patients is not defined. This could bias the study because the survival prognosis of SLE varies according to several factors, including the degree of disease activity of the patient, the age and comorbidities at the time of diagnosis, among others.3

Second, the study does not determine whether patients are exclusive users of the health system (public or private), if they have access to both health systems or if, during the years of follow up, they moved from one system to another. This would lead to patients receiving more than one treatment simultaneously or different treatments in each center in different periods.

Finally, the use of informed consent is important in a study, as it ensures that the patient is aware and accepts to participate.4 It is unusual to require informed consent in a retrospective study, as the methods section says that it had not been sought, but later mentions that it was indeed obtained, which can cause confusion about the ethics of the study.

Finally, we believe that some data could have been better explained, both in tables and in text, as it can confuse readers causing misinterpretation of the data. For example, in the results section, the authors explain that, given two groups of people, both black, one with health insurance and the other without health insurance increased mortality was observed in the group without health insurance, which results in ethnicity not playing a decisive role in the survival of patients, which may be debated, since in this case, the ethnicity was the same in all patients studied, and not a variable.5