Diagnosis of lupus, especially systemic lupus erythematosus (SLE), is difficult and sometimes takes years. There is no single definitive test for SLE. Symptoms vary widely among patients and can suggest many other conditions.

SLE is sometimes called "the great impostor" because it can be mistaken for many other diseases, such as rheumatoid arthritis, polymyalgia rheumatica, chronic fatigue syndrome, fibromyalgia, multiple sclerosis or Sjogren's syndrome. A physician may need to test for and rule out other conditions.

An accurate and complete medical history is crucial in diagnosing and monitoring lupus. The U.S. National Institutes of Health (NIH) suggests that individuals keep a checklist of symptoms (e.g., joint pain, rash) and bring it to a physician appointment. The list should include types of symptoms, sites where they occur, when they were first noticed, frequency of occurrence and recent dates.

The physician will likely conduct a physical examination with a focus on the joints, skin, muscles and other areas affected by pain, rashes or other symptoms. Diagnostic tests may include:

• Blood tests. The ANA test detects antinuclear antibodies (immune cells that attacks foreign substances), which, according to the American College of Rheumatology, are present in virtually everyone who has lupus. This test is not definitive for lupus, however, because antinuclear antibodies can also be present with other immune diseases such as rheumatoid arthritis, polymyositis or scleroderma. Also, 25 percent of the population may be ANA positive but never develop lupus.

More specific tests, including the anti-double strand DNA (dsDNA) and anti-smith antibodies (Sm), can help confirm a diagnosis. A number of other immune system blood tests may also help in diagnosis, especially in patients with a negative ANA test.

Other blood tests used in diagnosing lupus may include:

• • Complement. Can help suggest lupus by indicating depressed levels of proteins that attack foreign substances. Scientists have linked abnormal levels of certain complement proteins on red blood cells (e.g., complement 4, complement receptor 1) to lupus.
    
    
  • Complete blood count. Detects low red blood cell count (anemia), low white blood cell count or low platelet count that may be due to lupus.
    
    
  • Blood differential. Measures the number of white blood cells, which may be altered because of lupus or other diseases.
    
    
  • Sedimentation rate or or C-reactive protein test. Nonspecific indicators of inflammation.
    
    
  • Rheumatoid factor test. Measures an antibody called rheumatoid factor that can indicate lupus, rheumatoid arthritis or other autoimmune diseases.
    
    
  • Waste product test. Can reveal kidney damage that could be due to SLE.
    
    
• Urine tests. Urinalysis may reveal protein in the urine or evidence of kidney inflammation, both of which can result from SLE. A special microalbuminuria test is needed to find the small amounts of protein that are an early sign of kidney damage. Blood in urine (hematuria) may indicate kidney damage caused by SLE or other disorders. A creatinine urine test can also reveal such kidney damage.
  
  
• Biopsy. Skin tissue samples examined under a microscope can help diagnose discoid (cutaneous) lupus, and a skin or kidney biopsy may reveal tissue damage characteristic of SLE.
  
  
• Imaging tests. Techniques such as MRI, CAT scan, ultrasound, radionuclide imaging or arthrography may be used to detect organ and tissue damage that may be due to lupus.

A primary care physician who suspects lupus may refer a patient to a rheumatologist (specialist in diseases of the joints, muscles and bones) or specialist in autoimmune disorders (immunologist) for additional expertise in diagnosis and treatment. Patients with discoid (cutaneous) lupus may be referred to a dermatologist.