Written by Krista O'Connell | Published on June 8, 2012
Medically Reviewed by George Krucik, MD

What Are Serologic Tests?

Serologic tests can involve a number of laboratory techniques. Different types of serologic tests can be used to diagnose various health issues. Tests have one thing in common: they all focus on proteins made by the body’s immune system. This vital body system helps keep us healthy by destroying foreign invaders that can make us ill. It doesn’t matter what type of technique is used in the laboratory during serologic testing—the process for the patient is the same.

Serologic Testing—Antigens and Antibodies

To understand serologic tests and why they are useful, it is helpful to know a little about the immune system and why we get sick.

Antigens are substances that provoke a response from the immune system. They are most often too small to be seen with the naked eye. They can enter the human body in a number of ways—through the mouth, through broken skin, or by the way of the nasal passages. Antigens that commonly affect people include the following:

  • bacteria
  • fungi
  • viruses
  • parasites

The immune system defends against antigens by producing antibodies in response to the presence of antigens. These antibodies are particles that attach to the antigens to deactivate them. When your blood is tested, doctors can identify the type(s) 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 these unnecessary antibodies. This is called an autoimmune disorder. Serologic testing can detected these antibodies to help your doctor diagnose an autoimmune disorder.

What to Expect If Your Doctor Orders Serologic Testing

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

The test will be done in your doctor’s office. A needle will be inserted into your vein, and the blood will be collected in a tube. If serologic testing is being conducted on a younger child, the skin may simply be pierced with a lancet. A small amount of blood is then collected.

The testing procedure is very quick. The pain level for most people is not severe. Excessive bleeding and infection may occur. However, the risk of either of these is very low.

Identify Types of Antigens

Because antibodies are so diverse, various tests are useful for detecting the presence of different types.

An agglutination assay shows whether antibodies exposed to antigens will cause “clumping.”

A precipitation test will show whether the antigens are similar.

Understanding Your Serologic Test Results

Normal Test Results

Antibodies are produced in response to antigens. No antibodies indicate a lack of a current or past infection. Results that show there are no antibodies in the blood sample are considered normal.

Abnormal Results—Meaning

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

The testing may also be done to diagnose an autoimmune disorder. In that case, antibodies to normal or non-foreign proteins (antigens) would be present in the blood.

The presence of certain types of antibodies can also mean that the patient has immunity against one or more antigens. As a result, future exposure to the antigen or antigens is not likely to result in illness.

Abnormal Results—Diagnostic Value

Illnesses that can be diagnosed through serologic tests include the following:

  • brucellosis (caused by a bacteria)
  • amebiasis (caused by a parasite)
  • measles and rubella (both caused by viruses)
  • human immunodeficiency virus (HIV)
  • syphilis
  • fungal infections

Follow-Up Care After Serologic Testing

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

An antibiotic or other type of medication may be prescribed to help the body fight off the infection. An additional test may be scheduled even if no antibodies were detected. This is because a specific type of infection might still be suspected.

The bacteria, virus, parasite, or fungus in the body will multiply over time. In response, the immune system will produce more antibodies. As a result, they will be easier to detect as time goes on and the infection gets worse.

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

Your doctor will explain your test results and what the next steps will be.

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