BACKGROUND The objective of this study was to determine the minimum number of cultures needed to detect Cutibacterium acnes in primary reverse shoulder arthroplasties (RSAs). METHODS It is a prospective study including 160 primary RSAs. Exclusion criteria included an active infection, an invasive shoulder treatment in the last 6 months before surgery, an Arthro-SCAN or Arthro-MRI in the last 6 months before surgery, previous shoulder surgeries and revision cases. In 90 cases, 11 cultures were obtained. Another 10 cultures were obtained in the other 70 cases (culture 10 was a sterile sponge to detect false positives). To determine the minimum number of cultures needed to detect C acnes, the prevalence of C acnes contamination of the 160 patients included was determined. RESULTS There were 128 females and 32 males, with a mean age of 74 years. There were 1690 cultures obtained from the 160 primary RSA surgeries, and 132 of them turned out to be positive for C acnes. There were 42 patients with positive cultures. Twenty of them were males and 22 females. When considering the skin and the deep tissue cultures altogether, the prevalence of positive cultures for C acnes was of 26.25%. If only deep tissues cultures were considered, the prevalence of positive cultures for C acnes was of 23.13%. When considering the skin and the deep tissue cultures together, the sensitivity to detect the C acnes is 19% if only 1 culture is obtained, 31% if 2 cultures are obtained, 50% if 3 cultures are obtained, 59.5% if 4 cultures are obtained, 66.7% if 5 cultures are obtained, 73.8% if 6 cultures are obtained, 85.7% if 7 cultures are obtained, and 92.9% if 8 cultures are obtained. When considering only the deep tissue cultures, if only 1 culture is obtained, the sensitivity to detect the C acnes is 24.3%, 40.5% if 2 cultures are obtained, 54.1% if 3 cultures are obtained, 73% if 4 cultures are obtained, 89.2% if 5 cultures are obtained, and 97.3% if 6 or 7 cultures are obtained. CONCLUSIONS A minimum number of 8 cultures are needed to detect C acnes in skin. Moreover, a minimum of 6 cultures are needed to detect it in deep tissues when performing an RSA.