Screening for systemic diseases
Antinuclear antibodies are autoantibodies directed against components of the cell nucleus. Their appearance is characteristic of systemic connective tissue diseases: systemic lupus erythematosus, scleroderma, Sjogren's syndrome, dermatomyositis, mixed connective tissue disease, as well as autoimmune hepatitis.
The test is prescribed for a combination of symptoms that do not fit into one picture: prolonged low temperature without an obvious cause, pain and swelling of several joints, a butterfly-type rash on the face, increased sensitivity to the sun, hair loss, dry eyes and mouth, Raynaud's syndrome, unexplained changes in blood and urine tests. In systemic lupus erythematosus, ANAs are positive in almost all patients, making a negative result a strong argument against this diagnosis.
However, a positive result in no way equals a diagnosis. Low titers of antinuclear antibodies occur in healthy people, especially with age and in women, as well as in chronic infections, thyroid diseases, cancer, and while taking a number of medications. Therefore, positive screening requires clarification: the type of glow is determined and antibodies to specific antigens are tested, and the result must be interpreted by a rheumatologist.
How to read reference values
Each form contains a reference interval - a range of values typical for healthy people. It depends on the methodology and equipment of a particular laboratory, so it is incorrect to directly compare the results of different laboratories; To monitor over time, it is better to take tests in one place.
The reference interval covers most healthy people, but not all: a slight deviation may be an individual norm. Conversely, a value within the interval is sometimes a deviation for a particular person if he previously had different numbers.
What distorts the result
The most common causes of errors are not related to the laboratory, but to preparation. Eating changes the levels of glucose, lipids and a number of enzymes. Physical activity the day before increases creatine kinase and transaminases. Stress affects hormones and glucose.
The sampling technique is also important: prolonged application of a tourniquet increases potassium and protein fractions, and hemolysis of the sample makes several indicators unreliable. If the result is questionable, it is better to retake the test rather than start treatment immediately.
Express methods and their place
Express tests provide an answer in minutes and are indispensable where speed is important: upon admission to a hospital, before emergency surgery, when examining contact persons. Their sensitivity is lower than laboratory methods, so a positive result always requires confirmation.
A negative rapid test early after possible infection also does not exclude infection: during the serological window, antigens and antibodies have not yet reached detectable levels.