Anti-nuclear antibodies (ANA)
This test is most useful in situations where SLE is suspected – as ANA negative SLE is rare. ANA is also associated with a number of connective tissue diseases and therefore high titre ANA results may be considered supportive evidence of this. However, positive ANA (particularly at low titre) may also be seen after infection or even in asymptomatic individuals (especially older people and females).
SLE in pregnancy or with planned pregnancy
These patients should have anti-cardiolipin (ACL) antibodies tested and their Ro antibody status determined. Ro antibodies are specific for a type of extractable nuclear antigen (ENA), and are relevant in pregnancy because they are associated with congenital heart block.
Anti-neutrophil cytoplasmic antibodies (ANCA)
This test is most useful in situations where small vessel vasculitis is suspected. ANCA have different specificities, of which the most clinically relevant are MPO and PR3 antibodies. These are particularly associated with microscopic polyangiitis, eosinophilic granulomatosis with polyangiitis (EGPA) and granulomatosis with polyangiitis (GPA). However, clinicians should be mindful that vasculitis is not excluded by a negative ANCA result.
Specific IgE tests to individual allergens can support a diagnosis of allergy in a patient who has compatible symptoms. However, specific IgE antibodies are not useful as a screening test because they can be positive in asymptomatic patients. These patients may be sensitized but not necessarily allergic. These tests only have a role in type I hypersensitivity reactions, which are characterised by a rapid onset of symptoms following exposure.
Immunoglobulins should be measured (this includes levels of IgG, IgA and IgM). Electrophoresis is performed to identify whether there is a paraprotein. If present then the type and size of the paraprotein is determined by immunofixation and densitometry respectively. Not all myeloma patients will secrete an intact paraprotein and therefore kappa and lambda serum free light chains are also typically requested