Utility of specific investigations for rheumatological diseases

A small number of rheumatological diseases can be confirmed by a specific investigation. Examples include:

• for septic arthritis—Gram stain and culture of synovial fluid
• for gout or calcium pyrophosphate deposition—polarised light microscopy of synovial fluid
• for idiopathic inflammatory myopathies, giant cell arteritis or other systemic vasculitides—tissue biopsy.

Certain investigation results are important to support the diagnosis of some rheumatological diseases; however, these diseases cannot be definitively diagnosed by a positive test result alone because healthy individuals can also have positive resultsAustralian Rheumatology Association (ARA), 2018. Examples include:

• antinuclear antibody (ANA) is positive in more than 90% of people with SLE, as well as 5 to 10% of healthy individualsNashi, 2021Selmi, 2016
• human leucocyte antigen B27 (HLA B27) is positive in more than 95% of people with axial spondyloarthritis, but also in 5 to 20% of healthy individuals (according to ethnicity), many of whom will have back painSheehan, 2004
• antiphospholipid (aPL) antibodies must be present for a diagnosis of antiphospholipid syndrome, but may also be present in 1 to 5% of the general population.

For the diagnosis of viral arthritis or reactive arthritis, preceding infection may be confirmed by specific microbiological tests, although confirmation of an infection is often not possible (or practical) retrospectively. Examples of specific microbiological tests include:

• viral serology on paired sera
• antistreptolysin-O titre
• stool culture
• urethral swabs or urine nucleic acid amplification testing (NAAT) (eg polymerase chain reaction [PCR]) for Chlamydia trachomatis or Neisseria gonorrhoeae.