Orthopedic infections with fungi and mycobacteria are rare. Only about 1 % of all prosthetic joint infections are caused by fungi, with the vast majority being Candida spp [1]. A large systematic review of fungal osteomyelitis from 1072 cases showed Aspergillus spp. to be the most common pathogen (26.5 %), followed by Candida spp. (20.7 %) and Mucor spp. (16.8 %) [2]. The anatomic areas most frequently involved were the extremities (31.5 %) and vertebrae (29.7 %). Similarly, rates of mycobacterial orthopedic infections in the U.S. are extremely low. In a 2021 comprehensive literature review of 50 published articles, there were only 107 reported prosthetic joint infections with Mycobacterium tuberculosis [3]. >25 % of cases had prior tuberculosis infections and almost 10 % of patients had lived or traveled to endemic countries. Osteoarticular tuberculosis occurs in about 1-3 % of all patients with active tuberculosis, with spinal infections being the most common [4]. In 2021, the CDC reported 7882 cases of tuberculosis with 243 involving bone or joint infections [5]. There is no national surveillance system to track extra-pulmonary non-tuberculosis mycobacterial (NTM) infections, so it is difficult to know their true prevalence. An evaluation of extra-pulmonary NTM infections in hospitalized patients showed an annual incidence of 1.5 cases/100,000 population; of the 992 cases evaluated, only 26 (2.6 %) were from bone or joint infections [6].

The Infectious Diseases Society of America (IDSA) guidelines on native vertebral osteomyelitis recommend obtaining mycobacterial cultures in immunocompromised patients, or patients with significant exposure to tuberculosis [7]. These guidelines also recommend obtaining fungal cultures in patients with significant immunosuppression (including chronic corticosteroids), presence of a long-term indwelling venous catheter, or injection drug use. Both fungal and mycobacterial cultures are recommended in cases of culture-negative or recurrent vertebral osteomyelitis. There are no specific recommendations for non-vertebral osteomyelitis. Although the American Academy of Orthopedic Surgeons guideline on periprosthetic joint infections suggests that there is no evidence to support routine fungal and mycobacterial cultures of synovial fluid, other guidelines and expert reviews recommend sending them in select cases based on clinical suspicion or patient-specific risk factors [[7], [8], [9]].

Processing fungal and mycobacterial cultures is time and labor intensive and is associated with significant healthcare costs. At our institution, the cost per reportable fungal culture result is $11.02 and $43.70 per mycobacterial culture result. It takes about 10 minutes to process and read initial fungal cultures and about 45 minutes per mycobacterial culture. Each subsequent culture examination requires additional time throughout the incubation and testing period, as each fungal culture is held for 21 days and each mycobacterial culture is held for 42 days (or until growth occurs). Cultures with growth then require the necessary testing for microbial identification. Because most cultures are negative, however, there is little value in return for the time, effort, and cost associated with them. This strain on resources recently prompted the microbiology staff and antimicrobial stewardship team to question when these cultures are clinically warranted and if there were ways to optimize use of these diagnostic tests. The objective of this study was to determine the utility and diagnostic yield of fungal and mycobacterial cultures when obtained during orthopedic surgical procedures at an academic, safety-net hospital.