You test two patient sera in parallel with twofold serial dilutions starting at 1:20. Serum A is positive through 1:640 but negative at 1:1280. Serum B is positive through 1:160 but negative at 1:320. What is the fold difference in antibody titer between Serum A and Serum B and what does this imply?

• A. Serum A has a 4-fold higher titer (1:640 vs 1:160), implying a stronger antibody response in Serum A.
• B. Fold differences cannot be calculated from endpoint titers produced by serial dilutions; only ELISA absorbance values permit fold-change interpretation.
• C. Serum A has a 2-fold higher titer and therefore indistinguishable clinically from Serum B because twofold differences are within assay variability.
• D. Serum B actually has the higher titer because lower denominators indicate stronger antibody concentration; therefore B is two times higher than A.

Answer: (A)