Qualitative vs. Quantitative Antibody Tests: What's the Difference?

December 9, 2020

When searching for SARS-CoV-2 antibodies, doctors have several assay options they can choose to employ in the fight against COVID-19. Determining which kind of test best applies to each individual case depends on the doctor’s reasons for ordering it. Here’s a quick rundown on what the different types of assays and when it’s most appropriate to use:

 

Qualitative testing

 

A qualitative test is often the first port of call in general COVID-19 testing, and the most traditional type of testing used in infectious disease. By detecting SARS-CoV-2 antibodies in a blood sample, this diagnostic assay provides a simple “yes or no” answer as to whether or not antibodies are present. However, qualitative testing cannot measure how many SARS-CoV-2 antibodies are present, and it doesn’t indicate the strength of the immune response. In some cases, just knowing that a patient has developed an immune response to the virus is all the clinical information that’s needed.

 

Quantitative and semi-quantitative testing

 

A qualitative test determines the “if;” quantitative and semi-quantitative tests answer the question “how much?” By measuring the level of SARS-CoV-2 antibodies in a patient’s blood sample, quantitative and semi-quantitative tests give a more definitive answer about the body’s immune response to the virus. Quantitative tests are traceable to a specific international standard that ensures the result produced correlates to a numeric quantity of antibodies. Due to the current lack of an international standard, the FDA has not granted EUA to any quantitative tests. Semi-quantitative tests estimate an approximate concentration of SARS-CoV-2 antibodies in a patient’s blood sample, and can be traceable to a specific standard to ensure consistent results. These tests may be deployed if a patient has already received a positive qualitative result, or if the doctor suspects a patient has previously been infected with COVID-19.

Quantitative and semi-quantitative tests are poised to play a vital role in the development of COVID-19 vaccines that aim to trigger a pre-emptive antibody response. By measuring the body’s immune response over time, scientists can use these tests to determine which vaccines are most effective and how long that effectiveness will last.

Additionally, quantitative and semi-quantitative testing can help guide the allocation of plasma donations from recovered COVID-19 patients to current patients by identifying which donors have the highest level of antibodies. Plasma from those donors is likely to offer the greatest benefit to patients who are currently sick.


Because so much is still unknown about the behavior of this novel coronavirus, it’s important to note that the basic presence of SARS-CoV-2 antibodies is not currently a reliable indicator of immunity to repeat or future infections.