Since autoimmune diseases can affect multiple organs and produce highly variable signs and symptoms that can change in severity over time, the recognition and diagnosis of autoimmune diseases can be difficult. Signs and symptoms can be vague and slow to present and can cause incorrect diagnosis. In addition, laboratory tests used to diagnose autoimmune disorders often depend on the particular disorder suspected. Blood tests that indicate the presence of inflammation are usually performed to help diagnose an autoimmune disorder. When autoimmune disease is suspected, the common laboratory evaluation will serve as an initial red flag to pursue further testing. Initially, it is typical that one or more of the following laboratory tests may be performed:
Characteristic findings on a complete blood count (CBC) can include a normochromic, normocytic anemia indicating the chronicity or severity of a disease and can also include an elevated or decreased platelet count and/or white blood cell count. Leukopenia and thrombocytopenia are common in patients with systemic lupus erythematosus (SLE).
Depending on the specific autoimmune disease, testing for serum levels of specific organ enzymes or abnormalities in metabolic processes may show elevation of transaminases (ALT and AST), creatinine kinase, bilirubin, and other serum proteins (immunoglobulins).
Coagulation studies may show elevations in the activated partial thromboplastin time (aPTT) and/or the prothrombin time (PT), which could suggest an inhibitor of the clotting process as seen in certain autoimmune disorders such as the antiphospholipid syndrome.
In autoimmune disorders, the urinalysis is commonly used to assess renal injury (glomerulonephritis, interstitial nephritis) and can show proteinuria, hematuria, or active sediment (white blood cell casts or red blood cell casts).
Inflammatory markers are serum proteins that are produced in response to inflammation. These proteins are mainly produced by the liver in response to stress and are also called acute phase reactants. These inflammatory markers are not diagnostic of inflammation but reflect abnormalities that are seen in autoimmune diseases, infections, malignancies, and other illnesses.
The following are some inflammatory markers used to diagnose autoimmune diseases:
Erythrocyte sedimentation rate (ESR)
The ESR, the measure of the quantity of red blood cells (RBC) that precipitate in a tube, is typically elevated in inflammation. Multiple factors can influence the ESR, including the patient's age, gender, RBC morphology, hemoglobin concentration, and serum levels of immunoglobulin. While the ESR is not a diagnostic test, it can be used to monitor disease activity and treatment response and signal that inflammatory or infectious stress is present.
C-reactive protein (CRP)
CRP (including CRP-high sensitivity) was discovered and named for its reactivity to the C polysaccharide in the cell wall of S. pneumoniae. CRP helps in the process of phagocytosis and activates the complement system. CRP production is under the control of IL-1, IL-6, and TNF-alpha. Changes in serum CRP concentration occur faster than ESR. Therefore, CRP may be a better reflection of current inflammation. Unlike the ESR, CRP is a fairly stable serum protein whose measurement is not time-sensitive and is not affected by other serum components. In addition, the magnitude of inflammation directly relates to the concentration of CRP. More recently, the use of high-sensitivity CRP has been utilized and may better quantify lower levels of inflammation.
Rheumatoid factor (RF) and anti-cyclic citrullinated peptide (CCP)
RF is an autoantibody that reacts to the Fc portion of polyclonal IgG and to other classes of immunoglobulin (most assays detect the IgM rheumatoid factor). RF is helpful in evaluating patients who may have rheumatoid arthritis (RA), although it is absent in about 15% of patients with rheumatoid arthritis. RF positive patients are more likely to have progressive, erosive arthritis with loss of joint mobility and also extraarticular manifestations, including rheumatoid nodules and vasculitis. On the other hand, RF can be present in other autoimmune disorders. A new biomarker for RA has recently emerged. The presence of autoantibodies against cyclic citrullinated peptide (anti-CCP) present in serum has been shown to be approximately 95% specific for the diagnosis of RA. Testing for both RF and anti-CCP may be beneficial when excluding the diagnosis of RA.
Antinuclear antibody (ANA)
Autoantibodies to nuclear antigens (ANAs) are a diverse group of antibodies that react against nuclear, nucleolar, or perinuclear antigens. These antigens represent cellular components such as nucleic acid, histone, chromatin, nuclear and ribonuclear proteins. Testing for ANA is commonly performed to help diagnose many autoimmune diseases.
The next section of this course will concentrate on the ANA test, including its methods, staining patterns, and interpretations.