That’s a real tightrope walk between the two.
I would really look at both extremes like sex or heavy bike riding before your blood draw.
If you can manage (walk in, no appt) to have the blood drawn 36-48 hrs AFTER either of these two GI extremes, you will probably be OK.
BTW, have you tried the FODMAP diet for IBS? It REALLY works but you MUST adhere to it very closely - especially portion control.
Phil
Phil