Lupus is a difficult disease to diagnose, because its symptoms can be vague. And unlike some other diseases, it cannot be diagnosed with a single lab test. However, when certain clinical criteria are met, lab tests can help confirm a diagnosis of lupus. Blood work and other tests can also help monitor the disease and show the effects of treatment.
WebMD takes a look at the uses and limitations of the tests that are commonly used to diagnose and monitor lupus.
Blood Tests for Lupus
Antinuclear Antibody (ANA)
- What it is: ANA is a type of antibody directed against the cells' nuclei.
- Why the test is used: ANA is present in nearly everybody with active lupus. Doctors often use the ANA test as a screening tool. Plus, looking at patterns of the antibodies can sometimes help doctors determine the specific disease a person has. That, in turn, helps determine which treatment would be most appropriate.
- Limitations of the test: Although almost all people with lupus have the antibody, a positive result doesn't necessarily indicate lupus. Positive results are often seen with some other diseases and in a smaller percentage of people without lupus or other autoimmune disorders. So a positive ANA by itself is not enough for a lupus diagnosis. Doctors must consider the result of this test along with other criteria.
Antiphospholipid Antibodies (APLs)
- What it is: APLs are a type of antibody directed against phospholipids.
- Why the test is used: APLs are present in up to 60% of people with lupus. Their presence can help confirm a diagnosis. A positive test is also used to help identify women with lupus that have certain risks that require preventive treatment and monitoring. Those risks include blood clots, miscarriage, or preterm birth.
- Limitations of the test: APLs may also occur in people without lupus. Their presence alone is not enough for a lupus diagnosis.
- What it is: Anti-Sm is an antibody directed against Sm, a specific protein found in the cell nucleus.
- Why the test is used: The protein is found in up to 30% of people with lupus. It's rarely found in people without lupus. So a positive test can help confirm a lupus diagnosis.
- Limitations of the test: Up to only 30% of people with lupus have a positive anti-Sm test. So relying on an anti-Sm result alone would miss a large majority of people with lupus.
- What it is: Anti-dsDNA is a protein directed against double-stranded DNA. DNA is the material that makes up the body's genetic code.
- Why the test is used: Between 75% and 90% of people with lupus have a positive anti-dsDNA test. Also, the test is very specific for lupus. Therefore, a positive test can be useful in confirming a diagnosis. For many people, the titer, or level, of the antibodies rises as the disease becomes more active. So, doctors can also use it to help measure disease activity. Also, the presence of anti-dsDNA indicates a greater risk of lupus nephritis, a kidney inflammation that occurs with lupus. So a positive test can alert doctors to the need to monitor the kidneys.
- Limitations of the test: Up to 25% of people with lupus have a negative test. So, a negative test doesn't mean a person doesn't have lupus.
Anti-Ro(SSA) and Anti-La(SSB)
- What it is: Anti-Ro(SSA) and Anti-La(SSB) are two antibodies that are commonly found together. They are specific against ribonucleic acid (RNA) proteins.
- Why the test is used: Anti-Ro is found in anywhere from 24% to 60% of lupus patients. It's also found in 70% of people with another autoimmune disorder called Sjögren's syndrome. Anti-La is found in 35% of people with Sjögren's syndrome. For this reason, their presence may be useful in diagnosing one of these disorders. Both antibodies are associated with neonatal lupus, a rare but potentially serious problem in newborns. In pregnant women, a positive Anti-Ro(SSA) or Anti-La(SSB) warns doctors of the need to monitor the unborn baby.
- Limitations of the test: Like other antibodies, the fact that the test is not positive in many people with lupus means it can't be used to diagnose lupus. Also, it is more indicative of Sjögren's syndrome than of lupus.
C-Reactive Protein (CRP)
- What it is: CRP is a protein in the body that can be a marker of inflammation.
- Why the test is used:The test looks for inflammation, which could indicate active lupus. In some cases, the test could be used to monitor inflammation. Results of the test could indicate changes in disease activity or in response to treatment.
- Limitations of the test: Because there are many causes for an elevated result, including infection, the test is not diagnostic for lupus. Nor can it distinguish a lupus flare from an infection. Also, the level of CRP doesn't directly correlate with lupus disease activity. So it isn't necessarily useful for monitoring disease activity.
- What it is: Complement proteins are involved in inflammation. The test can look for levels of specific complement proteins or for total complement.
- Why the test is used: Complement levels are often low in patients with active disease, especially kidney disease. So doctors may use the test to gauge or monitor disease activity.
- Limitations of the test: Like other tests, complement must be taken in the context of clinical findings and other test results. A low complement in itself is not diagnostic of lupus.
Erythrocyte Sedimentation Rate (ESR)
- What it is: ESR measures the speed of red blood cells moving toward the bottom of a test tube. When inflammation is present, blood proteins stick together and fall and collect more quickly as sediment. The more quickly the blood cells fall, the greater the inflammation.
- Why the test is used: ESR is used as a marker of inflammation. Inflammation could indicate lupus activity. This test could be used to monitor inflammation, which could indicate changes in disease activity or response to treatment.
- Limitations of the test: Like CRP, the ESR is not specific to lupus. Because there are many causes for a positive result, including infection, the test is not diagnostic for lupus. Nor can it distinguish a lupus flare from an infection. Also, the level doesn't directly correlate with lupus disease activity. So it isn't necessarily useful for monitoring disease activity.
Complete Blood Cell Count (CBC)
- What it is: CBC is a test to measure levels of different blood cells.
- Why the test is used: Abnormalities in blood cell counts, including white blood cells and red blood cells, may occur in people with lupus. This may be related to the lupus, lupus treatments, or infection. For example, leukopenia, a decrease in the number of white blood cells, is found in about 50% of people with lupus. Thrombocytopenia, or a low platelet count, occurs in about 50% of people with lupus, as well. Doctors can use this test to monitor these potentially serious problems.
- Limitations of the test: Many other medical conditions can cause abnormalities in blood cell counts. So the test by itself is not specific to a lupus diagnosis.
- What it is: A chemistry panel is a test to assess kidney function and liver function. It also gives information on electrolytes, blood sugar, cholesterol, and triglyceride levels.
- Why the test is used: Abnormalities may indicate the development of complications from lupus. They could also result from treatments for such conditions as kidney disease, elevated blood sugar levels, elevated cholesterol levels, and liver disease.
Urine Tests for Lupus
Besides blood tests used to diagnose and monitor lupus, doctors use urine tests to diagnose and monitor the effects of lupus on the kidneys. These tests include the following:
- Measurement of Glomerular Filtration Rate and Proteinuria.This test measures how effective the kidneys are at filtering the blood to eliminate waste products. It is conducted on urine collected over a 24-hour period.
- Protein/Creatinine Ratio. This test is performed on a one-time urine sample. It measures for protein loss, which is an indicator of kidney function.
- Urinalysis: Urinalysis can be used in screening for kidney disease. The presence of protein, red blood cells, white blood cells, and cellular casts may all indicate kidney disease.