4.1 Quality of the experiment

4.1.2 Calibration curve
The calibration curve clearly demonstrates that RLU-values increase with higher exposure concentration of 2,3,7,8-TCDD. Secondly, luciferase production cannot increase indefinitely. At very high 2,3,7,8-TCDD concentrations, the so-called “dose-response curve” shows a maximum, because one or more factors in the test system become rate-limiting. This is typical for any biological response. Finally, the calibration curve has a clear S-shape when RLUs are plotted against the concentrations on a logarithmic axis. In the middle of the curve there is a “turning point” where the slope of the curve no longer increases with increasing dose. Given the symmetrical shape of the curve, this turning point is exactly half-way the slope, in other words where the light production is 50% of the maximum. The concentration that describes the location of this point is called the “50% effect concentration” or “EC₅₀”.

The dose-dependent light production can be described with a calibration curve as

with Y = measured RLU; x is 2,3,7,8-TCDD concentration (pM); a₀ = the maximum RLU response; a₁ = EC₅₀; a₂ = a slope parameter. Parameters a₀, a₁, and a₂ can be calculated using least square regression analysis. We will not go into detail how this is done, but we provide you here with the estimated values of a₀ = 25.38, a₁ = 9.42, and a₂ = -1.32.