Qualitative vs Quantitative Most labs are doing qualitative tests to see if a person has antibodies in their plasma which is in the blood sample they submit. A qualitative test (think 'quality' of blood) yields an answer of either yes, positive, the amount of antibodies is above a preset threshold OR no, negative, the amount of antibodies is not above that preset threshold. Hospitals and programs that want to acquire plasma to donate to patients want to make sure that the number of antibodies is sufficient for their purpose. They need a quantitative test (think 'quantity') which yields a numerical result (see below) regarding antibodies, not just a '+' or '-'.
The Meaning of Antibodies Researchers still don't know what the number of antibodies means for the COVID-19 virus. How high does a person's number have to be (ie, how many antibodies in the blood's plasma) in order for his immune system to protect him? Does the number reduce over time? Is the memory of having the antibodies enough to recreate them when called to duty in the future even though the number reduces?
Titers Assigning the numerical value in a quantitative test is NOT a matter of counting the total number of antibodies in the plasma. It is the counting of the last dilution of a blood sample where the antibodies are still detectable (at a predetermined concentration)! If the blood is diluted 102 times and there are still antibodies in there, but there are not detectable antibodies in the next dilution, the quantitative result will be a titer of 102. A titer is the amount of a substance detectable in a solution.
Understanding Titer Numbers The protocol for how many times to dilute a blood sample and then check for antibodies is determined by the lab/program. Rather than checking every progressive dilution, they may find it more efficient to only check a handful of preset dilutions. The Mt. Sinai plasma donation program, for example, first checks at 80 dilutions. If there is a concentration of at least 10% antibodies, then they check at 160 dilutions. If there is no reaction in that dilution, ie less than 10% concentration of COVID-19 antibodies, then the person is said to have a titer of 80. If there is a reaction at 160, they continue this process of checking at levels of 320, 960 and lastly 2880 dilutions. Their titer would be at least 160, but would be higher if detected in progressive dilutions. A person could potentially have a quantitative result of much higher than only 2880 dilutions, but since it is not tested they would never know. 2880 is the highest dilution of which I am aware. I hope this clarifies why some people have numbers of 102, in the 200s or in the 2000s.
