Imaging of bones and joints
There are three main types of investigations: imaging of bones and joints, blood tests, and synovial fluid analysis.
A plain x-ray of the affected joint is one of the most useful investigations. Changes that occur on plain x-ray can be characteristic of specific musculoskeletal diseases such as rheumatoid arthritis, osteoarthritis and gout (see Figure 22). Most changes occur over a prolonged period of time and x-rays can therefore provide a useful historical record. Ultrasound is becoming increasingly widely used, particularly in identifying early joint inflammation (see Figure 23). Other investigations including magnetic resonance imaging (MRI), computerized tomography (CT) scanning, isotope bone scans and dual-energy x-ray absorptiometry (DEXA) scans (for osteoporosis) all have an important role.
Blood tests can be useful in indicating the degree of inflammation and in monitoring response to therapy. The erythrocyte sedimentation rate (ESR) is one of the best-known inflammatory markers and indicates what has been happening over the last few days or longer. It is non-specific and influenced by many things including anaemia. C-reactive protein responds more rapidly to changes in inflammation – normally within days.
A specific biochemical test which may be useful is for serum uric acid, which may be raised in gout, although it may be unreliable during an acute episode.
Many of the inflammatory arthropathies are associated with increased titres of a number of autoantibodies, although their significance is not always clear. Tests for rheumatoid factor, and the more recently used anti-CCP (cyclic citrullinated peptide) antibody test, for example, are both often strongly positive in patients with rheumatoid arthritis. However, rheumatoid factor may also be positive in other disease states and in the elderly, and is therefore not highly specific. It is also important to consider infection as a cause of an arthropathy, particularly in the case of a single joint – blood cultures for infection should be taken even if there is no fever.
Synovial fluid analysis
Obtaining a sample of synovial fluid for analysis is an important skill to learn, and is vital to perform in order to exclude infection of a joint. Synovial fluid should be sent for culture and gram staining. If gout or other crystals are considered as a cause of the problem, the fluid is examined for crystals under a polarizing light microscope.