Serologic tests are blood tests that look for antibodies in your blood. They can involve a number of laboratory techniques. Different types of serologic tests are used to diagnose various disease conditions.

Serologic tests have one thing in common. They all focus on proteins made by your immune system. This vital body system helps keep you healthy by destroying foreign invaders that can make you ill. The process for having the test is the same regardless of which technique the laboratory uses during serologic testing.

It’s helpful to know a little about the immune system and why we get sick to understand serologic tests and why they’re useful.

Antigens are substances that provoke a response from the immune system. They’re usually too small to see with the naked eye. They can enter the human body through the mouth, through broken skin, or through the nasal passages. Antigens that commonly affect people include the following:

  • bacteria
  • fungi
  • viruses
  • parasites

The immune system defends against antigens by producing antibodies. These antibodies are particles that attach to the antigens and deactivate them. When your doctor tests your blood, they can identify the type of antibodies and antigens that are in your blood sample, and identify the type of infection you have.

Sometimes the body mistakes its own healthy tissue for outside invaders and produces unnecessary antibodies. This is known as an autoimmune disorder. Serologic testing can detect these antibodies and help your doctor diagnose an autoimmune disorder.

A blood sample is all that the laboratory needs to conduct serologic testing.

The test will occur in your doctor’s office. Your doctor will insert a needle into your vein and collect blood for a sample. The doctor may simply pierce the skin with a lancet if conducting serologic testing on a young child.

The testing procedure is quick. The pain level for most people isn’t severe. Excessive bleeding and infection may occur, but the risk of either of these is low.

Antibodies are diverse. So, there are various tests for detecting the presence of different types of antibodies. These include:

  • An agglutination assay shows whether antibodies exposed to certain antigens will cause particle clumping.
  • A precipitation test shows whether the antigens are similar by measuring for the presence of antibody in body fluids.
  • The Western blot test identifies the presence of antimicrobial antibodies in your blood by their reaction with target antigens.

Normal test results

Your body produces antibodies in response to antigens. If testing shows no antibodies, it indicates you don’t have an infection. Results that show there are no antibodies in the blood sample are normal.

Abnormal test results

Antibodies in the blood sample often mean you’ve had an immune system response to an antigen from either current or past exposure to a disease or foreign protein.

Testing may also help your doctor diagnose an autoimmune disorder by finding out if antibodies to normal or non-foreign proteins or antigens are present in the blood.

The presence of certain types of antibodies can also mean that you’re immune to one or more antigen. This means that future exposure to the antigen or antigens won’t result in illness.

Serologic testing can diagnose multiple illnesses, including:

The care and treatment provided after serologic testing can vary. It often depends on whether antibodies were found. It may also depend on the nature of your immune response and its severity.

An antibiotic or another type of medication may help your body fight the infection. Even if your results were normal, your doctor might order an additional test if they still think you might have an infection.

The bacteria, virus, parasite, or fungus in your body will multiply over time. In response, your immune system will produce more antibodies. This makes the antibodies easier to detect as the infection gets worse.

The test results may also show the presence of antibodies related to chronic conditions, such autoimmune disorders.

Your doctor will explain your test results and your next steps.