A fascinating explanation of how titers are figured, written by a mom on the ISO board on babycenter.com

When we run titers we grade the dilution and the strenth of the reaction. We basically add a set amount of your serum with a set amount of donor cells that have the corresponding antigen to your antibody then incubate then spin in a centrifuge.(we do serial dilutions on these specimens) We then take each test tube with the dilutions and gently shake the congealed cells at the bottom. If the cells stay clumped then the tube is positive. We grade the clump----4+ is a clump that stays in one piece, 3+ is 2 large pieces---we go all the way down to microscopic clumps. Each grade is also given a number called it's strength. Once we know the dilution then we add up all the strength numbers to come up with one number which we also report. This is to normalize the titers since titer levels can vary depending on what lab does it and what tech does it since it is all a manual procedure and thus leads to variations. A titer that rises by 2 dilutions and with a strength that increases by 10 is considered significant. Even if your titer goes up or down by a dilution if your strength doesn't increase or decrease by 10 then that would be consistent with no change. Hope this makes some sense since it is kind of a long winded explanation.