I just went with the flow.
My platelets were low for awhile, but neither the GI or GP thought much of it. At a certain point, my GI thought I should see a hemotologist just to rule anything else out. When I explained everything to the hemotologist, he immediately said, "It's the sulfasalzine." He did a quick physical exam to make sure my spleen wasn't hard or enlarged, and it wasn't. That was basically the end of it, no treatment.
They told me not to worry about it, and I didn't.
After years of sulfasalazine, I've been mostly off it for about six months now (for other reasons), and the platelets have crept back up. The sulfasalazine worked great, especially for joint pain, and I'd still take it despite the low platelets if I didn't seem to have developed an intolerance to it.