Is it true that chronic sinusitis flare up would cause only minor increase in the wbc of a cbc?

Yes: Chronic sinusits is just that, chronic. One could have this condition and not have a response of raising the white blood cell count. The body is handling the infections on a chronic basis and does not respond by sending out the numbers of white cells to raise the total circulating count.

Yes: Chronic sinusitis may cause little or no change in the WBC count. A CBC is of no value in diagnosing chronic sinusitis.