Vitamin B12: a Biological Marker of Systemic Disease or Infection Flare-up in Patients Treated with Tocilizumab?

"There will be no specific visit to the study. Follow-up in consultation will be done by your referring doctor. A determination of the serum level of vitamin B12 will be carried out at inclusion, during a consultation where the pathology is in remission (during the classic assessment of follow-up under tocilizumab, left to the discretion of the patient's referring doctor. ). It will also be necessary to carry out, in addition to the classic assessment left to the discretion of the patient's doctor, a determination of the serum level of vitamin B12 in the event of clinical suspicion of infection or an outbreak of MAI/I.~The determination of serum vitamin B12 requires the collection of an additional tube of 300 μl of venous blood but no additional venipuncture"