Diagnosis of lupus can be challenging, especially because of its wide range of symptoms that may come on slowly and change over time. It is usually made by the clinical evaluation of physical signs and symptoms in combination with tests that can help to confirm the diagnosis or rule out other causes of a person's signs and symptoms.
The following laboratory tests may be useful in the diagnosis of lupus:
- Autoantibody testing, such as:
- Anti-nuclear antibody (ANA) – positive in almost all people with SLE, although can also be positive in those with other autoimmune diseases as it indicates a stimulated immune system
- Anti-Sm antibody – usually seen only in those with SLE
- Anti-double stranded DNA (Anti-dsDNA) – high results are characteristic of active SLE
- Anti-SSA and Anti-SSB – may also be positive
- Anti-RNP – may be positive
- Anti-chromatin antibodies – may be present in people with SLE who are positive for ANA but negative for anti-dsDNA
- Histone antibodies – for drug-induced lupus
- Antiphospholipid antibodies – such as lupus anticoagulant, anticardiolipin, anti-β2 glycoprotein I
- Complement 3 (C-3) and C4, CH50 – often decreased; C-3 is one of nine major complement proteins associated with some diseases. Decreased amounts may be associated with lupus, but also gram-negative septicemia, shock, and malaria.
Other tests that may be useful for evaluating someone with or suspected of having lupus:
- Urinalysis – may show blood, urinary casts, or protein in the urine, which can indicate kidney involvement
- Complete blood count (CBC) – may reveal anemia and decreased numbers of white blood cells and platelets, which can occur with lupus
- Comprehensive metabolic panel (CMP) – indicates current status of the kidneys and liver as well as electrolyte and acid/base balance and levels of blood glucose and blood proteins
- Rheumatoid factor (RF) – may be positive or negative
- Serum protein electrophoresis (SPEP) – increased gamma globulin proteins
- Erythrocyte sedimentation rate (ESR) – increased with inflammation, such as with lupus as well as other inflammatory conditions
- C-reactive protein (CRP) – another marker of inflammation that may be elevated with lupus
- Cryoglobulin – frequently positive; cryoglobulins are abnormal proteins in the blood that will precipitate when the body temperature drops below normal, causing blockage of the blood vessels.
X-rays or other imaging tests may be ordered to examine organs potentially affected by lupus.
In addition, a kidney biopsy may be performed. This involves taking a piece of kidney tissue for examination to detect any changes in the tissue that could indicate lupus and help guide treatment.
The American College of Rheumatology and the Systemic Lupus International Collaborating Clinics (SLICC) have validated classification criteria to aid health practitioners in making a diagnosis, in particular, of SLE.