Where you need a quick answer
The surface antigen of the hepatitis B virus, HBsAg, is the main marker of infection: it appears in the blood before symptoms and before antibodies, and its persistence for more than six months means chronic infection. The rapid test determines it from a drop of blood on the spot, without sending the material to a laboratory.
The main area of application is situations where a decision needs to be made immediately: hospitalization, preparation for emergency surgery and other invasive interventions, examination for trauma with contact with someone else's blood, screening before dental and endoscopic procedures. A quick result allows you to organize the necessary infection safety measures in a timely manner.
Limitations must be taken into account. The sensitivity of the express method is lower than laboratory ELISA and LAI: with a low concentration of antigen, a false negative result is possible, and in the early period after infection the antigen has not yet reached a detectable level. Therefore, any positive rapid test requires confirmation by a quantitative laboratory method and determination of the DNA of the virus, and a negative test with a clear risk of infection requires repeated examination over time.
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.