I need a suggestion belongs to haemolysin liquid assay for s.epi to make a quantitative data with rabbit blood and haemolysis buffer

1- supernatant added PSFM with overnight growth after measured OD600

2- working with triplicate samples

3- the dilution 10ul supernatant +965 HB + 25 rabbit blood / or 100 ul supernatant +875 HB +25 rabbit blood

4- 15 min incubation @ 37 then centrifuge

5- positive control with 100X triton and negative control with PBS

6- measured OD 530

My question OD530 reading value each time gave me negative!

even if I make the negative control a blank then I read, the reading for triplicate different

any suggestion for the negative value for the samples I tried with 20 samples

I used 0.5 and 0.05 concentration for OD600 as growth

the colour of blood dose effect of the growth factors?

THANKS

CHEERS