The standard curve is the reference for every sample on the plate. If the standards are wrong, every concentration you report is wrong by the same amount, and no curve fit will detect it. This guide shows how to prepare a serial dilution that gives a clean, reproducible curve.

1. Read the kit insert first

Before you calculate anything, take these values from the kit insert or certificate of analysis:

Stock concentration after reconstitution (for example 10 ng/mL)
Recommended top standard (for example 1000 pg/mL)
Assay range and number of standard points
Standard diluent: use the diluent the kit specifies, ideally the same one used for samples

Reconstitute lyophilized standards exactly as instructed, mix gently by inversion and let them stand for the time stated (often 10 to 15 minutes). Do not vortex hard, because proteins can denature or foam.

2. Choose the dilution factor and number of points

Most ELISA kits use a 2-fold series with 7 standards plus a zero. Other options:

Dilution factor	Points needed for ~2 log units	Typical use
2-fold	7	Most sandwich ELISAs
2.5-fold	6	Wider range, fewer wells
3-fold	5	Broad-range or competitive assays

The goal is to cover both plateaus of the sigmoidal curve and place several points on the steep middle part. That is where a 4PL or 5PL fit gets most of its information.

3. Calculate the volumes

For each step you transfer a volume of the previous standard into fresh diluent:

DF = (V_transfer + V_diluent) / V_transfer

C_n = C_top / DF^n

C_n is the concentration of the standard n steps below the top.

Make enough volume for duplicate or triplicate wells plus some extra. With 100 µL per well in duplicate, 250 µL per tube leaves a safe margin.

4. Worked example: 2-fold series from a 10 ng/mL stock

Target: top standard 1000 pg/mL, 7 points, 250 µL final volume per tube.

Step 1: top standard (S1)

Stock = 10 ng/mL = 10000 pg/mL
Dilution needed = 10000 / 1000 = 10-fold

Volume of S1 needed = 250 µL (kept) + 250 µL (transferred to S2) = 500 µL
Stock:    500 / 10 = 50 µL
Diluent:  500 - 50 = 450 µL

Steps 2 to 7: 2-fold series

Put 250 µL diluent into tubes S2 to S7. Transfer 250 µL from S1 into S2, mix, then 250 µL from S2 into S3, and so on. Discard 250 µL from S7 so that every tube ends with 250 µL.

Tube	Concentration (pg/mL)
S1	1000
S2	500
S3	250
S4	125
S5	62.5
S6	31.25
S7	15.63
S0	0 (diluent only)

S0 is the zero standard. It defines the background, but it is not a point on the logarithmic curve, because log(0) is undefined. Your lowest quantifiable standard is S7.

5. Pipetting technique that actually matters

Change tips at every step. Carry-over on the outside of a tip compounds through the whole series.
Mix thoroughly before each transfer: pipette up and down 5 to 10 times, or invert the tube. Poor mixing is the most common cause of a flat or noisy low end.
Pre-wet the tip once in the solution before aspirating for the transfer.
Use calibrated pipettes in their accurate range. Avoid pipetting volumes below about 10% of the pipette's maximum.
Use low-binding tubes for very low protein concentrations, so the analyte does not stick to the plastic.
Prepare standards fresh for every plate unless the kit explicitly allows storage of diluted standards.

6. Plate layout for standards

Run every standard at least in duplicate, ideally triplicate.
Place replicates in adjacent wells or spread them across the plate. Avoid putting the whole curve in edge wells only.
Load standards and samples within the same time window, so that incubation times are comparable.

7. Check the curve after reading

When the plate is read, check:

Monotonic signal: OD should rise (sandwich ELISA) or fall (competitive ELISA) consistently across the series.
Replicate %CV below about 10% for standards.
Back-calculated recovery of each standard within 80 to 120% of nominal.
Dynamic range: a reasonable OD difference between the top and lowest standard.

A single standard that is far off while its neighbours are fine usually points to a pipetting or mixing error in that tube. A shift of the entire curve points to a stock or reconstitution problem.

Common mistakes

Reconstituting the stock with the wrong volume. Every standard is then wrong by the same factor.
Reusing one tip for the whole series.
Not mixing before transferring.
Using a different diluent for standards and samples.
Treating the zero standard as the lowest point of the curve.
Storing diluted standards that are not stable.

Doing it in Labreador

In the ELISA module you enter the top concentration and dilution factor, and the standard concentrations are filled in for you. The curve is then fitted with 4PL or 5PL, every standard is back-calculated with its recovery, and Method Health warns you about low dynamic range or high replicate %CV. Everything runs in your browser, so no data leaves your machine.

If you use Labreador in a publication, please cite:

Helczman, M. (2026). Labreador - Bioassay Analysis Platform (Version 1.6.0) [Computer software]. Zenodo. https://doi.org/10.5281/zenodo.21676846

How to Prepare an ELISA Standard Curve: Serial Dilution Step by Step