Hi Laura,

I can definitely help you with this! The four parameter logistic equation is a common method for curve fitting in ELISA assays. It allows you to estimate the concentration of an unknown sample based on the response of a calibration curve.

Here are the steps to find x-values according to a four parameter logistic equation:

1. Enter your a, b, c, and d values into Excel. You can put them in separate cells or in one cell separated by commas.
2. Create a column for your x-values. These should be the concentrations of your calibration standards.
3. In the next column, enter the four parameter logistic equation using cell references for your a, b, c, d, and x values. For example, if your a, b, c, and d values are in cells A1, A2, A3, and A4, respectively, and your x values are in column B starting in cell B2, the equation would be:
   
   
   Formula:

   =(A1-A4)/(1+(B2/A3)^A2)+A4 

4. Copy the equation down the column for all of your x values.
5. Now, when you have a response value (y) from your assay, you can use the solver function in Excel to find the corresponding x-value (concentration). To do this, go to the Data tab and click on Solver. Set the objective to the cell containing your four parameter logistic equation, and set the value to your assay response (y). Set the variable cells to the column of x-values you just created. Set the constraints to make sure the x-values are positive and within the range of your calibration standards. Then click Solve.
6. The solver will find the x-value that corresponds to your assay response (y) based on the four parameter logistic equation.