When I opsonized the S. aureus in autologous serum, the bacteria always clump together?

(some strains totally clumped, some not).

Could it due to the serum I used is isolated from buffy coat?

Basically, I want to isolate neutrophil from buffy coat and prepare S. aureus for bactericidal assay. But the bacteria always clumped.

I saw papers use serum from blood without anticoagulant. But what I can have should have anticoagulant. could this be the reason for clumping?

thank you