More precisely rheumatoid factor
For many years, rheumatoid factor was considered the main laboratory sign of rheumatoid arthritis. The problem is that it is not specific enough: a positive result occurs in chronic infections, hepatitis, Sjögren's syndrome, other autoimmune diseases, and simply in some healthy older people. The diagnostic value of such a marker is limited.
Antibodies to cyclic citrullinated peptide have a specificity approaching 95 percent: their detection in a person with joint complaints makes the diagnosis of rheumatoid arthritis highly likely. Moreover, they are often detected months and even years before the first symptoms appear, which makes it possible to classify the patient as a risk group and begin monitoring in advance.
The second important property is prognostic. High levels of antibodies to CCP are associated with a more aggressive course of the disease, early development of erosions of the articular surfaces and worse functional outcome. This directly affects the tactics: if the result is positive, basic therapy begins earlier and more actively, since it is the first months of the disease that determine whether the joints can be saved. The analysis is included in the modern classification criteria for rheumatoid arthritis.
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.