General approach to microbiological diagnosis of osteomyelitis

A microbiological diagnosis should be pursued in all patients with osteomyelitis. Take samples for culture before administering antibiotics, if possible. If antibiotics have already been started, it may still be appropriate to take samples, because culture results often remain positive.

For patients with acute osteomyelitis or osteomyelitis associated with sepsis or septic shock, collect blood samples for culture, because a significant number of patients will have concurrent bacteraemia. Isolation of a pathogen from the bloodstream helps direct subsequent managementChen 2021. To increase blood culture yield, adults should have 2 sets of blood cultures (ie 4 bottles) collectedNickerson 2016. For children, the usual practice is to collect a single set of blood cultures at presentation, unless clinical features warrant further sampling. Blood cultures are less useful in chronic osteomyelitis but may still be appropriate depending on the clinical presentationFantoni 2019

If blood cultures yield a pathogen, bone biopsy is generally unnecessary. If no organism is identified, or the organism that is identified is of questionable significance (eg a commensal organism such as a coagulase-negative staphylococcus), consider collecting suitable samples of bone or pus for culture and histopathology. These samples may be collected by radiologically guided percutaneous biopsy or open biopsy. However, bone biopsy may not be necessary if there is a good response to empirical therapy, even if blood culture results are negative. This is particularly true for children.

Negative bone-biopsy culture results are most often due to sampling error; however, antibiotic administration in the days before sampling can also reduce culture yield. If suspicion of osteomyelitis remains high despite negative culture results, bone biopsy may need to be repeated. For example, if a patient with vertebral osteomyelitis has a negative bone-biopsy culture result after needle biopsy, open surgical biopsy may be trialled as it is more likely to result in a diagnosis. If culture results are still negative and alternative pathology (eg malignancy, tuberculosis) is excluded, empirical therapy is often indicated.