HBsAg: Screen & Test

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Hepatitis B Virus can cause severe illness, leading the cirrhosis and cancer of the liver. The HBsAg screen is used to help detect and diagnose HBV infection in patients and the blood supply.

What is HBsAg?

At first, 'HBsAg' looks like a bunch of random, meaningless letters. Let's break down this string of letters to see what they mean. 'HB' refers to hepatitis B, a virus that infects liver cells. The 's' refers to surface, and the 'Ag' means antigen. So, HBsAg is a surface antigen on the hepatitis B virus (HBV).

That's great, but what is an antigen? Simply put, an antigen is anything that generates an immune response. An antigen is made by a pathogen invading a host. So, the surface antigen on HBV is something on the outside of the virus that tells the body that something is wrong, and causes you to start fighting the infection.

The HBsAg test looks for the presence of the HBsAg in a person's blood. If the HBsAg is present, it means the virus is also present. If it is negative, it means the patient is not currently infected with HBV.

HBV particle, showing surface antigen
HBV Virus

HBV Infection

Hepatitis B virus infects human liver cells. It is a blood-borne pathogen, meaning it is passed through sexual contact, sharing needles, and mother-to-child during birth. HBV is a major public health concern and is the most prevalent chronic virus infection, according to the World Health Organization.

In most people, HBV infection is quickly taken care of by the body. However, in about 5-10% of people, HBV infection becomes chronic, meaning it just doesn't go away. These people with chronic HBV infection can either become carriers, meaning they have no sign of disease, but can spread the virus, or become very ill, developing cirrhosis and liver cancer. It is estimated that 350,000,000 people are chronically infected with HBV worldwide, and 1,000,000 people will die due to HBV infection.

Early symptoms of HBV infection are vague and may include headache, fatigue, and jaundice. Because someone can carry and transmit HBV without knowing it, people should protect themselves from the infection. A safe, effective vaccine against HBV is readily available.

When is the HBsAg Test Used?

As mentioned above, HBV is an infection that you'd rather just avoid getting. Are there any groups of people that you think you might especially want to protect from this blood-borne pathogen? If you said people getting blood transfusions, congratulations, you're correct! The HBsAg test is used on blood donations, because the surface antigen can be detected weeks before symptoms of HBV infection have even begun.

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