Serology Test: What It Is, How It Works, and When You Need One

Consider scheduling an appointment at the clinic after you recover from a fever or any viral illness. You feel much better; all your symptoms have vanished, yet your doctor recommends a blood test. "It is only necessary for us to test your antibodies." But do you know what exactly they refer to? Well, in most cases, they recommend performing a serology test. 

Blood tests may be conducted in various cases to assess your cholesterol, blood glucose, and other levels. But another class of tests works slightly differently: instead of focusing on the components of the blood, these tests detect changes that occurred in response to a specific factor. In other words, such blood tests evaluate your immune response, which is the purpose of serology. By conducting a serological test, your doctors try to determine whether your body has encountered a certain disease, is currently fighting it, or has become immune to it.

Did you ever have your blood examined during a COVID-19 diagnosis or for syphilis, HIV, dengue fever, or hepatitis? Or did you have your blood examined before receiving a blood transfusion? Then you know what serological testing is.

What Is A Serology Test?

A serology test is a blood test that checks your blood's serum for the presence of any antibodies or antigens. An antibody is a protein that your immune system makes when there is an external threat, such as a virus or bacterium. An antigen, on the other hand, is a foreign substance that triggers the production of those antibodies.

Once the foreign object infects the body, your immune system starts producing antibodies against it. The antibodies then remain in your blood for many years, sometimes even for your entire lifetime. The reason serology tests are performed is to determine whether an individual has been infected, has developed immunity from past infections or vaccines, or is currently infected.

Types of Antibodies a Serology Test Detects

When your body reacts to infection, it generates different kinds of antibodies at various phases. Some commonly detected antibodies include:

• IgM Antibodies: They are the earliest to appear, within days after contracting an infection. An IgM-positive test result usually indicates an active or very recently contracted infection.

• IgG Antibodies: They usually appear 2-3 weeks after a previous reaction. An IgG test result is usually positive when there has been a past infection or vaccination-induced immunity.
• IgA Antibodies: Usually appear in the mucosal regions and are tested under special circumstances.

Common Conditions Diagnosed Using Serology Tests

Serology testing is not restricted to any particular disease; it applies broadly to various diseases and immune system disorders. Physicians conduct this test to determine not only whether the pathogen is present in the body but also how the body has responded to it over time.

Infections commonly diagnosed through serology: 

• COVID-19 (antibody testing post-infection or post-vaccination)
• Dengue fever (NS1 antigen and IgM/IgG antibodies)
• Hepatitis B and C
• HIV/AIDS (initial screening via ELISA serology)
• Syphilis and other sexually transmitted infections
• Typhoid fever (Widal test: a type of serology test)
• Rubella (tested during pregnancy screening)
• Toxoplasmosis
• Epstein-Barr virus (EBV/mononucleosis)
• Lyme disease

How Is A Serology Test Done? 

Serology testing is a basic test and similar to routine blood tests. The blood sample collected will be examined in a lab.

Steps involved:

• A healthcare professional collects a blood sample from the vein.

• It is sent to the lab for examination.

• The lab searches for certain antibodies or antigens.

• Test results are obtained within a few hours or one to two days.

How to Prepare for a Serology Test

No special preparation is necessary in most situations, except if your physician tells you otherwise. 

Basic preparation tips:

• Stay hydrated
• Inform about medications
• Follow fasting instructions if advised
• Relax during the procedure  

Common Laboratory Techniques Used in Serology: 

• ELISA (enzyme-linked immunosorbent assay): The most common technique for detecting antibodies. Very sensitive and used to test HIV, hepatitis, and COVID-19.

• Western blotting: A confirmatory test after a positive ELISA. Common in HIV diagnosis.

• Agglutination tests: The sample is combined with an antigen, and agglutination indicates the presence of antibodies. Useful for blood typing and typhoid testing. 

• Immunofluorescence: Fluorescently labeled antibodies are used to identify specific pathogens using microscopy.

• Rapid immunochromatographic tests: Rapid diagnostic techniques similar to at-home pregnancy tests in format, used for dengue, malaria, and COVID-19. 

Understanding Your Serology Test Results

The results of a serology test are usually categorized as positive or negative, but their meaning depends on the type of antibody detected.

• Reactive/Positive: Detection of either antibodies or antigens. Depending on the antibody type, this result could indicate a current infection, a past infection, or vaccine-induced immunity.

• Non-reactive / Negative: There are no detections at all. This is usually an indication that there has been no prior exposure or that the test was taken too early for antibodies to develop.

• Equivocal / Indeterminate: The result is borderline. Your doctor may ask you to retest after a few weeks. 

What Is A Serology Titer?

Titer is an estimate of the amount of antibodies in the body. A higher titer indicates a stronger immune response. It is especially applicable in vaccine studies, such as measuring hepatitis B immunity after vaccination.

When Does a Doctor Order a Serology Test?  

The doctor may order a serology test in a variety of conditions,  

• You suspect an infection, but tests taken directly from your body were negative.

• You have recently been ill, and your physician wants to confirm the diagnosis. 

• You are being tested before receiving a blood transfusion or an organ transplant.

• Antenatal testing for rubella, syphilis, hepatitis B, and HIV. 

• Determining if your vaccinations have worked (post-vaccination titer testing)

• Occupational medicine testing (particularly for healthcare professionals)

• A travel medicine evaluation assesses your immunity status before traveling to high-risk locations. 

• Assessing autoimmune disorders such as lupus or rheumatoid arthritis. 

Limitation of Serology Testing

While serological testing is a powerful tool, it does have limitations that are important to understand: 

• Window period: It takes some time for the body to start producing antibodies after becoming infected. A test conducted early in the first few days of infection will yield a false negative result.

• Cross-reactivity: Certain antibodies may react with multiple pathogens. This might sometimes lead to inaccurate results, in which case a confirmatory test is normally required.

• Fails to detect active infection in some cases: The serological test detects the presence of antibodies, not necessarily an active pathogen. In some cases, an additional PCR test becomes necessary to confirm a live infection. 

• Waning antibodies. Some antibody levels decline over time, meaning older infections may not always be detected.

Prevention and Health Monitoring

While serology tests help detect infections, prevention remains the best approach to staying healthy.

• Preventive steps include:
• Maintaining proper hygiene
• Getting vaccinated on time
• Eating a balanced diet
• Regular health checkups
• Avoiding exposure to infections

Regular monitoring can help detect issues early and prevent complications.

Prevention and Health Monitoring

Although serological tests aid in identifying infections, prevention is the key to maintaining good health.

Preventive steps include:

• Maintain proper hygiene
• Timely vaccination
• Eating a balanced diet
• Routine health examinations
• Avoiding exposure to infections

Conclusion

A serological test is a valuable tool that helps provide insights into how your immune system responds to an infection. It is beneficial for both understanding your immune status and identifying the presence of other illnesses.

If you have been experiencing symptoms lately or need information about past infections, consulting a doctor for a serology test can be a wise step. Routine testing also helps you stay well-informed about your overall condition and enables timely action when needed. 

Summary of key points:

• Serological tests only detect the antibodies produced by the immune system, not the actual infection.
• Effective in diagnosing any past or current infections
• Aids in assessing immunity and the history of diseases
• Safe, simple, and widely used in medical practice 

FAQs

What is a serology test used for?

The purpose of a serology test is to look for antibodies in the bloodstream to see if you have been exposed to any form of infection. Serological tests can be performed to detect diseases such as dengue fever, hepatitis, and COVID-19. The results of a serology test may help diagnose infections or determine whether you have had prior exposure to the disease. 

Is a serology test the same as a blood test?

Serology tests are blood tests, but not all blood tests are serology tests. Blood tests can include routine tests such as checking blood sugar or hemoglobin levels, but serology tests specifically look at antibodies in the bloodstream. 

How long does it take to get serology test results?

The results of serological tests can be obtained within a day or two, depending on the  laboratory and type of test. Some serological tests can take several days to deliver results, but they generally provide faster results than blood tests. It is always advisable to consult your doctor for an accurate interpretation once the results are available.

Can a serology test detect current infections?

It might be possible to detect current infections using serologic tests by detecting the presence of IgM antibodies. Nonetheless, serology tests are most useful when diagnosing infections that occurred previously. In cases of current infections, doctors recommend conducting other tests, such as PCR or antigen tests, for more precise and timely diagnosis.

Do I need to fast before a serology test?

Generally, serologic tests do not require fasting, as they detect antibodies rather than metabolic indicators. However, if serologic testing is combined with other blood tests, your doctor may recommend fasting. You should adhere to the instructions from your healthcare specialist to ensure the testing procedure is effective and accurate.

Are serology tests accurate?

The reliability of serological tests is not in question; however, their effectiveness may depend on the timing of testing and the type of infection. For instance, if the tests are conducted early, there may be insufficient antibodies present in the bloodstream to identify. Thus, doctors usually combine serology tests with other indicators to avoid misdiagnosis.

When should I take a serology test? 

If you experience prolonged or unusual symptoms, need to check for a previous infection, assess immunity following vaccination, or if your preliminary tests are inconclusive, you should consider taking a serology test. Consulting a healthcare professional will help you choose the right time to test and select an appropriate serological test.