Fibrous dysplasia (FD) is an infrequent non-hereditary bone disease caused by a somatic mutation of the GNAS gene. Periostin is a novel marker that increases during tissue healing and fibrous or inflammatory diseases. We conducted an exploratory case-control study to evaluate sensitivity of periostin as a biomarker of FD. The study comprised 15 patients with FD, and healthy age- and sex-matched subjects (controls). Serum periostin levels were assessed and comparisons were established between FD patients and controls, and between patients with the monostotic and the polyostotic form of FD. No statistically significant differences in serum periostin levels were observed between the cohort of FD patients studied here and the control group (FD: 51.1±10ng/ml vs.

control:

44.2±15ng/ml; p=0.15), or between the clinical forms of FD (polyostotic: 51.8±9.1ng/ml vs.

monostotic:

49.6±13 ng/ml; p=0.66). A sub-analysis performed to compare serum levels of periostin in FD patients with and without a history of fractures showed no statistically significant differences [fracture patients (n=4): 41.2±17ng/ml vs. non-fracture patients (n=11): 49.9±11 ng/ml; p=0.47].Lastly, sensitivity of periostin as a biomarker of FD was analyzed, and was found to have low sensitivity to estimate disease activity [ROC curve; cut-off points: 39.625(0.867-0.467)]. To conclude, in the cohort of FD patients studied here, periostin serum levels did not differ significantly from those of the control group or between the two forms of the disease, and showed low sensitivity as a biomarker of the disease. (AU)