Resource Examination of the joint fluid If there is a possibility that your child has an acute bacterial infection in a joint they will need to be admitted to hospital and have a sample of fluid taken from the joint. This usually happens under a general anaesthetic and the joint is ‘washed out’ at the same time. Print When infection is suspectedIf there is any chance (from the history or examination) that your child has an acute bacterial infection in a joint they will need to be admitted to hospital and have a sample of fluid taken from the joint which is then sent to the laboratory to test for infection. Usually, an orthopaedic specialist does this under a general anaesthetic in theatre and the joint is ‘washed out’ at the same time. An infection may be indicated by sudden onset (one or two days only) of pain, swelling, heat and an inability to move the joint and is often accompanied by high fevers, going off food and feeling generally unwell.When arthritis is suspectedIf arthritis is strongly suspected and there are only a few joints affected, a steroid injection is commonly recommended. Sometimes when this is done for the first time the clinician will send a sample of fluid to the laboratory. This is to look for very rare conditions such as gout or tuberculosis and to check that the fluid looks as expected under the microscope.