Four parameter logistic (4PL) curve is a regression model often used to analyze bioassays such as ELISA. They follow a sigmoidal, or "s", shaped curve. This type of curve is particularly useful for characterizing bioassays because bioassays are often only linear across a specific range of concentration magnitudes. Beyond this linear range, the responses quickly plateau and approach the minimum and maximum.

Four parameter logistic curve refers to the following four parameters:

1. Minimum: the point of smallest response; can be baseline response, control or response when treatment concentration is zero.
2. Maximum: the point of greatest response
3. Inflection point: the dose at which the curvature of the response line changes; where the rate of change switches signs; often referred to as the IC50 or EC50
4. Hill coefficient: the slope of the curve at the inflection point

Note that four parameter logistic (4PL) curves are symmetric in nature around the inflection point. For asymmetric logistic curves, a five parameter logistic (5PL) curve is required.

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