Recommended buffers, solutions
Note: Do not use sodium azide in these preparations. Sodium azide inactivates the horseradish peroxidase enzyme.
The BD OptEIA™ Reagent Set B (Cat. No. 550534) containing Coating Buffer, Assay Diluent, Substrate Reagents A and B, Stop Solution and 20X Wash Buffer Concentrate is recommended.
1. Coating Buffer- 0.1 M Carbonate, pH 9.5; 8.40 g NaHCO3, 3.56 g Na2CO3; q.s. to 1.0 L; pH to 9.5. Freshly prepare or use within 7 days of preparation, stored at 2-8°C.
2. Assay Diluent- PBS* with 10% FBS#, pH 7.0. The BD Pharmingen™ Assay Diluent (Cat. No. 555213) is recommended.
*Phosphate-Buffered Saline: 80.0 g NaCl, 11.6 g Na2HPO4, 2.0 g KH2PO4, 2.0g KCl, q.s. to 10 L; pH to 7.0.
#Fetal Bovine Serum: Hyclone Cat. No. SH30088 (heat-inactivated) recommended.
Freshly prepare or use within 3 days of preparation, with 2-8°C storage.
3. Wash Buffer - PBS* with 0.05% Tween-20 Freshly prepare or use within 3 days of preparation, with 2-8°C storage.
4. Substrate Solution - Tetramethylbenzidine (TMB) and Hydrogen Peroxide. The BD Pharmingen™ TMB Substrate Reagent Set (Cat. No. 555214) is recommended.
5. Stop Solution - 1 M H3PO4 or 2 N H2SO4
Additional Materials Required
1. Microwell plates; BD Falcon™ ELISA Plates (Cat. No. 353279) recommended
2. Microplate reader capable of measuring absorbance at 450 nm
3. Precision pipettes
4. Graduated cylinder, one liter
5. Deionized or distilled water
6. Wash bottle or automated washer
7. Log-log graph paper or automated data reduction
8. Tubes to prepare standard dilutions
9. Laboratory timer
10. Plate sealers or parafilm
Specimen Collection and Handling: Specimens should be clear, non-hemolyzed and non-lipemic.
Cell culture supernatants: Remove any particulate material by centrifugation and assay immediately or store samples at ≤ -20°C. Avoid repeated freeze-thaw cycles.
Serum: Use a serum separator tube and allow samples to clot for 30 minutes, then centrifuge for 10 minutes at 1000 x g. Remove serum and assay immediately or store samples at ≤ -20° C. Avoid repeated freezethaw cycles.
Plasma: Collect plasma using citrate, EDTA, or heparin as anticoagulant. Centrifuge for 10 minutes at 1000 x g within 30 minutes of collection. Assay immediately or store samples at ≤ -20° C. Avoid repeated freezethaw cycles.
Standards Preparation and Handling
1. Reconstitution: After warming to room temperature, carefully open vial to avoid loss of material. Reconstitute lyophilized Standard with 1.0 mL of deionized water to yield a stock standard. Allow the standard to equilibrate for at least 15 minutes before making dilutions. Vortex gently to mix.
2. Storage/ handling of reconstituted standard: After reconstitution, immediately** aliquot standard stock at 50 μl per vial and freeze at ≤ -70°C for up to 6 months. Do not leave reconstituted standard at room temperature. If necessary, store at 2-8ºC for up to 8 hr prior to aliquoting/freezing.
3. Standards Preparation for Assay:
a. Prepare a 500 pg/mL standard from the stock standard. Vortex to mix.
b. Add 300 μL Assay Diluent to 6 tubes. Label as 250 pg/mL, 125 pg/mL, 62.5 pg/mL, 31.3 pg/mL, 15.6 pg/mL, and 7.8 pg/mL.
c. Perform serial dilutions by adding 300 μL of each standard to the next tube and vortexing between each transfer. Assay Diluent serves as the zero standard (0 pg/mL).
Working Detector Preparation
Note: One-step incubation of Biotin/Streptavidin reagents.) Add required volume of Detection Antibody to Assay Diluent. Within 15 minutes prior to use, add required quantity of Enzyme Reagent, vortex or mix well. For a full 96-well plate, prepare 12 mL of Working Detector.
Warnings and Precautions
1. Reagents which contain preservative may be toxic if ingested, inhaled, or in contact with skin.
2. Handle all serum and plasma specimens in accordance with NCCLS guidelines for preventing transmission of blood-borne infections.
3. Capture Antibody contains < 0.1% sodium azide. Sodium azide yields highly toxic hydrazoic acid under acidic conditions. Dilute azide compounds in running water before discarding to avoid accumulation of potentially explosive deposits in plumbing.
4. Detection Antibody contains 1% BSA and ProClin™-150 as a preservative.
5. Enzyme Reagent contains 1% BSA and ProClin™-300 as preservative.
6. Source of all serum proteins is from USDA inspected abattoirs located in the United States.
Recommended Assay Procedure
1. Coat microwells with 100 μL per well of Capture Antibody diluted in Coating Buffer. For recommended antibody coating dilution, see lot-specific Instruction/ Analysis Certificate. Seal plate and incubate overnight at 4° C
2. Aspirate wells and wash 3 times with ≥ 300 μL/well Wash Buffer. After last wash, invert plate and blot on absorbent paper to remove any residual buffer.
3. Block plates with ≥ 200 μL/well Assay Diluent. Incubate at room temperature for 1 hour.
4. Aspirate/wash as in step 2.
5. Prepare standard and sample dilutions in Assay Diluent. See "Standards Preparation and Handling".
6. Pipette 100 μL of each standard, sample, and control into appropriate wells. Seal plate and incubate for 2 hours at room temperature.
7. Aspirate/ wash as in step 2, but with 5 total washes.
8. Add 100 μL of prepared Working Detector (Detection Antibody + SAv-HRP reagent) to each well. Seal plate and incubate for 1 hour at room temperature.
9. Aspirate/ wash as in step 2, but with 7 total washes. NOTE: In this final wash step, soak wells in wash buffer for 30 seconds to 1 minute for each wash.
10. Add 100 μL of Substrate Solution to each well. Incubate plate (without plate sealer) for 30 minutes at room temperature in the dark.
11. Add 50 μL of Stop Solution to each well.
12. Read absorbance at 450 nm within 30 minutes of stopping reaction. If wavelength correction is available, subtract absorbance 570 nm from absorbance 450 nm.
Assay Procedure Summary
1. Add 100 μL diluted Capture Ab to each well. Incubate overnight at 4° C
2. Aspirate and wash 3 times.
3. Block plates: 200 μL Assay Diluent to each well. Incubate 1 hr. RT
4. Aspirate and wash 3 times
5. Add 100 μL standard or sample to each well. Incubate 2 hrs. RT
6. Aspirate and wash 5 times
7. Add 100 μL Working Detector (Detection Ab + SAv-HRP) to each well. Incubate 1 hr. RT
8. Aspirate and wash 7 times (with 30 sec to 1 min soaks)
9. Add 100 μL Substrate Solution to each well. Incubate 30 min. RT in dark
10. Add 50 μL Stop Solution to each well. Read at 450 nm within 30 min. λ correction 570 nm.
Calculation of Results
Calculate the mean absorbance for each set of duplicate standards, controls and samples. Subtract the mean zero standard absorbance from each.
Plot the standard curve on log-log graph paper, with IL-15 concentration on the x-axis and absorbance on the y-axis. Draw the best fit curve through the standard points.
To determine the IL-15 concentration of the unknowns, find the unknown's mean absorbance value on the y-axis and draw a horizontal line to the standard curve. At the point of intersection, draw a vertical line to the x-axis and read the IL-15 concentration. If samples were diluted, multiply the IL-15 concentration by the dilution factor.
Computer data reduction may also be employed, utilizing linear regression analysis.
Cross Reactivity: The following factors were tested in the OptEIA™ assay at 10 ng/mL and no cross-reactivity was identified.