"I did approach my new rheumatologist (first appt with her) about checking for antibodies to Actemra…. She was not sure how available that testing was, and it was not pursued."
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I only know it was something my rheumatologist was worried about when I first started Actemra.

"Moreover in patients with RA, methotrexate comedication was associated with fewer antidrug antibodies."
https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2807157
I have been on Actemra for almost 6 years and it still seems to work for me. I'm not taking MTX and I haven't been tested for anti-drug antibodies either. I think my rheumatologist wanted to start MTX when he started Actemra. However, I said my liver didn't handle MTX very well when I took MTX in the past so he didn't pursue it.
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Was the relapse you had while on Actemra a PMR/GCA relapse?

When Actemra enabled me to taper off Prednisone the first time, I had a relapse too. It wasn't a PMR relapse though. I had a uveitis relapse that was associated with reactive arthritis. Within a week or two of getting off Prednisone for the first time in 12 years, I was on 60 mg again for uveitis. It was a devastating turn of events.

When my ophthalmologist wanted me on a TNF inhibitor instead of Actemra, I reluctantly agreed. I was on Humira and Prednisone for a few months. When I tapered down to 15 mg of Prednisone while I was still on Humira ... I had a PMR relapse.

A long discussion with my rheumatolgist ensued. He said multiple autoimmune disorder are sometimes impossible to adequately treat with biologics. A biologic that works for one autoimmune condition doesn't work for the other autoimmune condition. I was given a choice between Humira or Actemra but not both. The other option I was given was to continue with more Prednisone.

It didn't take me long to decide and I chose Actemra. That was when my rheumatologist told me to do weekly injections instead of every 2 weeks. Weekly injections worked well and I tapered off Prednisone in 3 months the second time.

Monthly infusions have worked even better. Now my rheumatologist controls my Actemra dose and adjusts it according to labs and how I feel. I'm currently only getting 500 mg every month with no relapses of anything.
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My cortisol level prevented me from tapering off Prednisone the first time. I had to stay on 3 mg of Prednisone and I was told not to go any lower. It took time but after my cortisol level improved, an endocrinologist said I could just discontinue Prednisone if I didn't need prednisone to control PMR. My endocrinolgist said 3 mg was a low dose and there was no need to taper as long as my cortisol level was "adequate." She wouldn't guarantee my cortisol level would be adequate every day going forward. She said my symptoms might was and wane and to expect good and bad days. She also said unless something catastrophic happened, I shouldn't have an adrenal crisis. Fortunately nothing catastrophic happened and I have been of Prednisone now for almost 4 years.