Diagnosing lupus: Types of blood tests a doctor may order

Blood tests play a vital role in helping doctors to diagnose and manage lupus. These tests detect changes in specific components of blood or detect antibodies. Antibodies are proteins that the immune system makes to protect the body.

Doctors may order tests to detect the following in the blood:

However, blood tests alone cannot diagnose lupus. Other tests — urine, biopsy, kidney function, and imaging — are also necessary.

Learn more about lupus.

A person with ANA antibodies in their blood can have an autoimmune condition, such as lupus. Unlike other antibodies — that instruct immune cells to attack foreign cells in the body — ANAs tell the immune cells to attack the nuclei of the body’s own cells.

This test is usually one of the first tests a doctor orders if they suspect lupus. Around 97 in 100 people who have SLE receive a positive result.

Evaluation

A positive result means a high number of ANAs is present in the blood. The test reports these results as a titer (ratio), indicating the amount of ANAs in the blood. While a titer of 1:640 can suggest a person has an autoimmune condition, a doctor will assess the results further.

A positive result does not necessarily mean a person has lupus, but a negative result may indicate that lupus is less likely.

When ANA results are positive, a doctor orders more tests to look for specific antibodies, including:

• Anti-double-stranded deoxyribonucleic acid (Anti-dsDNA): This refers to antibodies against the genetic material of the cell. Presenting with anti-dsDNA usually indicates lupus.
• Anti-Sm: This may occur in people with lupus.
• Anti-U1 ribonucleoprotein (RNP): This antibody occurs in autoimmune diseases, including lupus.
• Anti-Ro/SSA and anti-La/SSB: If these antibodies are present, it can indicate that someone is having lupus complications.
• Antiphospholipid antibodies: About a third of people with lupus test positive for these.

Blood consists of the following:

• red blood cells (RBC)
• white blood cells (WBC)
• platelets
• serum

A CBC shows how much of these components is in the blood. It also looks for the amount of hemoglobin, a protein that is present in red blood cells. CBC is a routine blood test.

When blood components are not within the typical range, such as having a low WBC count and low hemoglobin, it can indicate something is wrong.

There are several results from a CBC that potentially indicate lupus:

• Low hemoglobin: This sometimes indicates anemia, a common condition for people with lupus. Results less than 13.5 grams per deciliter (g/dL) in males or less than 12 g/dL in females can indicate anemia.
• Low white blood cell count (leukopenia): Lupus can cause a person to have a low WBC count due to antibodies that destroy these cells. Figures may vary between labs, but a WBC count below 4,000 cells per microliter of blood is typically considered low.
• Low platelets: Platelets are responsible for causing blood to clot. Lupus can cause a low platelet count of less than 150,000 platelets per microliter of blood.

The ESR generally shows how quickly RBCs separate from other blood components and settle at the bottom of a test tube. Other names for it are sedimentation rate, or “sed rate.”

If the test shows a high ESR, it indicates inflammation in the body, possibly due to lupus. This is likely because inflammation causes the RBCs to clump together faster. Typical ranges for ESR are as follows:

• less than or equal to 20 millimeters per hour (mm/hr) for females 50 years old and younger
• less than or equal to 15 mm/hr for males 50 years old and younger
• less than or equal to 10 mm/hr for a child

A faster sed rate does not mean a person has lupus. It can be high for other reasons, such as:

• infection
• arthritis
• certain cancers
• heart disease
• other autoimmune diseases

However, alongside other test results, this can be a good indicator of lupus.

Doctors may use this test to detect inflammation. The liver produces CRP during an inflammatory process, meaning a higher amount is likely because inflammation is present in the body.

High levels of CRP proteins are common for people with lupus.

According to a 2023 article, a moderate elevation of 1–10 mg/dL may indicate systemic inflammation. Results from other tests are necessary to determine if the elevation is due to lupus.

A doctor will order various tests to diagnose a person with lupus. However, none of these tests alone can give a definitive positive indication of the condition.

Blood tests with other tests — such as urine tests and imaging — are necessary to gather information that may lead to a lupus diagnosis. Generally, blood tests that detect ANA, elevations in ESR and CRP, low WBCs, and low hemoglobin can help the diagnostic process.