What are the different ELISA sample preparation methods?

ELISA sample preparation methods include: 

• Cell Culture Supernatant: Cell media is centrifuged for 10 minutes at 4°C and 1,500 rpm, supernatant is immediately aliquoted and stored at -80°C. Freeze/thaw cycles are minimized.
• Cell Extract: Tissue culture plates are placed on ice. After removing the medium, cells are washed with ice-cold PBS. PBS is removed and 0.5 mL extraction buffer is added per 100 mm plate, then cells are scraped and transferred to a pre-chilled tube. Sample is then vortexed briefly, incubated on ice for 15-30 minutes, and centrifuged for 10 minutes at 15,000-17,000 x g and 4°C. The supernatant (soluble cell extract) is collected in chilled tubes and stored at -80°C. Freeze/thaw cycles are minimized.
• Conditioned Media: After cells have reached desired level of confluence in complete serum-containing growth medium, growth medium is removed and washed gently and repeatedly with warm PBS. After removing the last PBS wash, serum-free growth medium is added and incubated for 1 to 2 days. The medium is then pipetted into a centrifuge tube and centrifuged at 300 x g for 10 min at 4°C. Supernatant is immediately aliquoted and samples stored at -80°C. Freeze/thaw cycles are minimized.
• Serum: Whole blood is collected without anticoagulant, incubated at room temperature for 30 minutes and centrifuge for 10 minutes at 3,000 rpm and 4°C. The supernatant (serum) Is immediately aliquoted is stored at -80°C. For accurate results, hemolysis is avoided while collecting samples. 
• Plasma: Whole blood is collected into tubes containing anticoagulant, centrifuged for 10 minutes at 3,000 rpm and 4°C. Supernatant is aliquoted immediately and stored at -80°C. Freeze/thaw cycles are minimized.
• Milk: Milk samples are centrifuged for 15 minutes at 1500 x g and 4°C and the supernatant or aqueous fraction is collected, filtered through a 0.2 µm filter and immediately stored at -80°C. Freeze/thaw cycles are minimized.
• Tissue Extract: Tissue is homogenized in extraction buffer, agitated, and centrifuged for 20 minutes at 15,000-17,000 x g at 4°C to obtain a protein extract. The supernatant (soluble protein extract) is aliquoted and stored at -80°C. Freeze/thaw cycles are minimized.
• Saliva: Collected saliva samples are centrifuged for 2 minutes at 10,000 x g and 4°C. Supernatant is immediately aliquoted and stored at -80°C. Non-protein binding collection devices are used to avoid interference. Freeze/thaw cycles are avoided. 
• Urine: Fresh urine samples are centrifuged for 1 minute at 10,000 x g and the supernatant is stored at -80°C to avoid bacterial contamination and false-positive results. Freeze/thaw cycles are avoided.
• Bone Tissue: De-mineralized bone samples are extracted using 4M Guanidine-HCl and protease inhibitors, then dissolved in 2M Guanidine-HCL to prepare the final sample for analysis.