The following tests are routinely performed in the evaluation of a patient with suspected Sjogren’s syndrome.
a. SS-A (Ro) and SS-B (La) antibodies. Antibodies are a large family of proteins in the blood that are capable of binding to molecular targets, termed antigens, on the surfaces of microorganisms, cells, and blood proteins. Each antibody protein has a binding site that can attach to only a very specific molecular target. However, the large array of such antibody proteins in our blood ensures that a diverse and large number of molecular targets can be recognized. Binding of such molecular targets is an essential step in our defense against infections and cancer. Cells that produce antibodies against molecular targets on our own tissues are actively eliminated from our bodies, particularly early in life. If this mechanism goes awry, then we may form antibodies to self-material and be susceptible to autoimmune disease. SS-A and SS-B antibodies are examples of two antibodies that bind molecular targets within our own cells. SS-A antibodies are found in approximately 60-80% of Sjögren’s patients. SS-B antibodies are less common, being found in approximately 30-50% of Sjögren’s patients. SS-A antibodies thus occur commonly by themselves; however, it is very uncommon for SS-B antibodies to occur alone. Accordingly, 20-40% of Sjögren’s patients may lack SS-A and/or SS-B antibodies. The finding of SS-A and/or SS-B antibodies is not specific to Sjögren’s syndrome patients. These antibodies may also be found in patients with systemic lupus and occasionally in other autoimmune diseases, such as myositis. Additionally, they may be found in approximately 1 in 200 healthy women.
b. The antinuclear antibody (ANA) test is positive in the majority of Sjogren’s syndrome patients. However, a positive ANA test is also common in healthy individuals. The ANA test is most commonly performed using a technique that involves immunofluorescent staining of human cells grown in tissue culture (Figure 3). With this test, a positive result is listed as the last dilution of serum that results in visible staining of the nucleus of the cultured human cell. Typical dilutions are 1:40, 1:80, 1:160, 1:320 and 1:640. Positive ANA test results of 1:80 and 1:160 may be seen in up to 15% and 5% of healthy individuals, respectively. A negative ANA test does not exclude the diagnosis of Sjogren’s syndrome; some of these individuals may still have SS-A and/or SS-B antibodies. With the immunofluorescent staining test, the pattern of nuclear staining is reported. Most Sjogren’s patients have either a speckled or homogeneous pattern. However, some patients may have a centromere pattern, denoting staining of centromere proteins in the mitotic spindle of dividing cells.
c. The rheumatoid factor test detects immunoglobulins in your blood that bind to other immunoglobulins, resulting in large protein complexes. Rheumatoid factor is a characteristic feature of rheumatoid arthritis, being found in up to 80% of affected patients. However, it is also common in Sjögren’s patients and in this setting does not indicate the presence of or predict the later development of rheumatoid arthritis.
d. The complete blood count (CBC) test determines the number of red cells (RBC), white cells (WBC), and platelets as well as the level of hemoglobin (oxygen-carrying protein in red cells) in the blood. A low number of white blood cells (leucopenia) can be found in approximately 15-20% of Sjögren’s syndrome patients, but is usually not associated with an increased risk of infection. Anemia, defined by a low hemoglobin level and low number of red blood cells, is less common.
e. The complete metabolic panel (i.e. blood chemistries) measures the electrolytes as well as chemicals or proteins in your blood that reflect the function of the kidneys, liver, and immune system. In Sjögren’s syndrome, the total protein level may be elevated, reflecting the increase in antibody (or immunoglobulin) levels in your blood. The serum bicarbonate or carbon dioxide level may be low, reflecting a disorder of the kidney that may occur in Sjögren’s syndrome, known as renal tubular acidosis. An elevation of the creatinine indicates the presence of kidney dysfunction, a rare complication of Sjögren’s syndrome. Elevation of the liver enzymes, alkaline phosphatase, aspartate aminotransferase (AST) and alanine aminotransferase (ALT), can be seen in liver diseases, some of which occur more commonly in Sjögren’s syndrome (such as primary biliary cholangitis).
f. The erythrocyte sedimentation rate (ESR) measures the degree to which whole blood, collected in tubes containing a chemical that prevents clotting (anticoagulant), separates into plasma (the upper layer) and packed red cells (the lower layer) over the course of one hour. The rate at which red cells settle to the bottom of the tube depends largely on the amount of fibrinogen protein in the blood. Fibrinogen binds to the surface of red cells and decreases their negative electrostatic charges, allowing the cells to aggregate and settle more quickly. The amount of fibrinogen produced by the liver increases with systemic inflammation. Thus the ESR is a simple test for measuring the degree of inflammation. The ESR can also increase when there are higher levels of immunoglobulins in the blood, as is the case for some Sjögren’s syndrome patients.
g. The C-reactive protein (CRP) test measures systemic inflammation. It is elevated most commonly in the setting of an infection or tissue injury, such as a myocardial infarction (heart attack). In the setting of autoimmune disease, the CRP may be elevated, but usually to a much lesser extent than during an infection.
h. The urinalysis detects a variety of abnormalities in the kidney or genitourinary system, including the bladder. Protein in the urine (proteinuria) is an indication of kidney disease, either stemming from a disorder of the glomerulus (the filtering structure) or the tubules (the structures within the kidney responsible for reabsorbing filtered salts, water, and acids from the initial blood filtrate). Blood in the urine may arise from a glomerular disease or from a disorder in the urine collecting system or bladder (such as a kidney stone or tumor). White blood cells in the urine denote the presence of inflammation or infection anywhere in the kidneys, urine collecting system, bladder, or urethra. The specific gravity is an indirect measure of the concentration of the urine. The pH indicates the extent of urine acidification. In Sjogren’s syndrome, the most common abnormality is inflammation in the tissue surrounding the tubules (interstitial nephritis), and this may lead to poorly concentrated urine (with a low specific gravity), protein in the urine, and occasionally a high urine pH. Inflammation or damage in the glomerulus (glomerulonephritis) is a less common form of kidney disease in Sjogren’s syndrome and is characterized by protein in the urine, occasionally with red and/or white cells.
i. The immunoglobulin quantitation test measures the levels of the most common types of immunoglobulins in the blood, known as IgG, IgM, and IgA. Some patients with Sjogren’s syndrome have elevated levels of these immunoglobulins, termed hyperglobulinemia, reflecting overactivity of the immune system.
j. The serum protein electrophoresis and the serum immunofixation electrophoresis tests analyze the types of proteins present in the liquid phase of the blood (serum). These tests rely on an electric field to separate the proteins based on their electrical charge. In the diagnostic evaluation of a patient with Sjogren’s syndrome, particular attention is paid to the immunoglobulins, proteins produced by plasma cells. Each of these cells creates a unique antibody, with a structure different from that produced by most other plasma cells. In approximately 10% of Sjogren’s syndrome patients, a clone of plasma cells proliferates and produces increased amounts of one immunoglobulin, with identical structure and specificity. This is called a monoclonal protein. This monoclonal immunoglobulin protein can be detected and characterized with electrophoretic techniques that rely on its distinctive properties in an electrical field.
k. The complement proteins, C3 and C4, are part of a group of proteins that mediate tissue inflammation and damage in certain immunologic diseases, including Sjogren’s syndrome. Low levels of these proteins generally reflect the ongoing utilization of these proteins in a process triggered by the binding of immunoglobulins to molecular targets in the serum (thereby forming immune complexes) or to targets on cellular surfaces. Very low levels of C4 can also be a reflection of the genetic absence of this protein.