Does platelet rich therapy help with tendonitis?

Possible. Ultrasound-guided dry needling, and autologous blood injection(platelets) may stimulate a healing response in chronic tendinopathy. In a study, high-level athletes were able to return to full sporting activity after receiving these treatments. But this modality may be tried after the other initial therapies failed.
PRP. Some reports have claimed success with prp but just as many state there is no effect. There is some discussion that "all prp is not hte same" and this may be so but to date there have not been the overall results consistently to state it really works. It is a work in progress. Prp follows scott's parabola http://www.Aao.Org/publications/eyenet/201103/upload/opinion-mar-2011.Pdf.