Investigations are often unnecessary in patients with rheumatic complaints. In patients with tennis elbow, osteoarthritis and many other conditions, the diagnosis can be made on the basis of history and examination findings. There are no diagnostic tests in osteoarthritis and tests are only requested to exclude some other condition.
ERYTHROCYTE SEDIMENTATION RATE (ESR) AND C-REACTIVE PROTEIN (CRP)
These provide a guide to the activity of inflammation and are characteristically raised in inflammatory conditions such as rheumatoid arthritis but are normal in osteoarthritis.
TESTS FOR RHEUMATOID FACTOR
Rheumatoid factors are autoantibodies found in the serum, usually of the IgM class, which are directed against human IgG. They are detected by agglutination of either latex particles (the latex test) or sheep red cells (the Rose- Waaler test) (Fig. 8.2). The major value of rheumatoid factor tests is in the diagnosis of rheumatoid arthritis. The latex test is quicker and easier to perform; it is more sensitive, and therefore more often positive, but is less specific than the sheep red cell test.
Antinuclear antibodies in the serum are detected using immunofluorescent staining of the nuclei of a tissue such as rat liver or human cells in tissue culture. A low titre of less than 1 : 40 is weakly positive and of little significance. Antibodies to other nuclear antigens A variety of antinuclear antibodies have been described with particular disease associations that are summarized. It seems likely that the pattern of disease is determined by the nature of the autoantibodies produced.
Antibodies against double-stranded DNA (dsDNA) These can be detected using the Farr test, a radioimmunoassay measuring the percentage antibody binding of added labelled dsDNA. Antibodies are found in about 50% of cases of systemic lupus erythematosus (SLE) but seldom in any other condition. They are therefore a much more specific test than antinuclear antibodies. They are also associated with more severe disease and renal involvement.
Antibodies against extractable nuclear antigen (EN A)
IgG class antibodies against soluble nuclear antigens are characteristic of mixed connective-tissue disease, the overlap syndrome described on p. 405, but are also found in patients with SLE.