A standard curve (aka calibration curve) for the protein of interest is constructed by plotting the mean absorbance (y-axis) against the protein concentration (x-axis) and choosing the best fit curve for the data points. Based on the standard curve, we interpolate the sample absorbances to compute the sample concentrations. We will be discussing the steps in more detail below.
The Standard Curve
After obtaining raw data from the ELISA reader, the ELISA results are ready for statistical analysis. We suggest using an ELISA data analysis software for the analysis. Our lab works with CurveExpert 1.4, but many other curve fitting software and tools are available, such as GraphPad Prism. Microsoft Excel can also be used to analyze ELISA results, but it may not offer as many options or flexibility as other programs for scientists.
For this standard curve example, we will be using CurveExpert 1.4 to explain the process.
1. Enter ELISA data into software
Categorize the ELISA raw data into three sections:
- Absorbance of the blank well
- Absorbance of standards with known concentrations
- Absorbance of samples with unknown concentrations
It is important to run the blank well with sample diluent to determine the background absorbance. Even without the presence of the protein, the buffer will still have an OD value. The absorbance of the blank wells should be subtracted from all standard and sample absorbances for accurate OD readings.
Open “CurveExpert 1.4” to see the interface below:
Enter the standard concentration in the x-axis column and the corresponding OD values in the y-axis column. The data plot will be presented in the bottom right corner.
2. Select the best fitting curve
Click the [Run] button in the top menu bar to allow the software to examine the data and choose the best possible curve fit. The window below will show up:
Click [All On] to include all model families for calculation. However, if you prefer not to include all of them, specific model families can also be selected for calculation. If “Polynomials” is checked, you will be asked to input the polynomial constraint, which we recommend setting as “4”.
Press [OK] to run the calculation.
The resulting curve fits will be ranked based on the standard error and correlation coefficient. Double click on each model to see the corresponding curve.
Choose the curve that meets the following criteria:
- The equation with the higher R value
- The curve should rise smoothly and closely resemble a straight line
Right click and select “Copy” to paste the graph into an excel sheet or word document.
Our lab and most companies generally recommend using a 4-parameter algorithm for the best standard curve fit. [Why?]
In this example, we have chosen the quadratic fit curve. Apart from the polynomial fit, the quadratic curve has the highest R value and closely resembles a straight line that rises smoothly. [Why aren’t we using the polynomial fit curve?]
3. Calculate target protein concentration
The calculation can be performed in the software or with Excel. If the samples were diluted before the ELISA, make sure to multiply the computed sample concentrations by the sample dilution factor.
Using software (CurveExpert 1.4) to find the sample concentrations
Using software will enable the user to easily find the x- and y-values, differentiate, and integrate the curve fit. Right click on the chosen curve fit graph for the graphing features menu and choose “Analyze”. For ELISA analysis, we would navigate to the “Find x=f(y)” tab and enter the sample OD value (y value) in the “At Y =” field. Click [Calculate] to obtain the x value (the target protein concentration) at the specified y value.
Using MS Excel to find the sample concentrations
Click the [Info] button in the top left corner of the graph. This will provide the model information for the curve fit along with other statistical information for the model.
The “Coefficients” tab displays the model function and the values of the coefficients a, b, c, etc. Press [Copy] and paste the function and coefficient values in an Excel sheet.
In the Excel sheet, input the corresponding coefficient values and y values (OD value) into the formula to calculate the sample concentrations.