1. A computer-implemented method of determining a cycle threshold (Ct) value for a real-time polymerase chain reaction (PCR) data curve having a parabolic shape, the method comprising the steps, implemented in a computer system, of: receiving a data set representing a PCR growth curve, the data set having a plurality of coordinate values (x,y), where x is a cycle number and y is an indicator of accumulated growth; approximating the data set using a two segment piecewise linear function having a first linear segment ending at x* and a second linear segment beginning at x*, wherein the two segment piecewise linear function has the form: f ⁡ ( x ) = { ax + b , x ≤ x * c ⁡ ( x - x * ) + ax * + b , x > x * , where a, b, and c are parameters for the approximation; determining, with the computer system: a) whether an R 2 value of a quadratic fit to the data points beginning with x* to the end of the data set is above a predetermined threshold value of 0.90; and b) whether an R 2 value of the two segment piecewise fit over the data set is above a threshold value of 0.85; and c) whether the slope of the second segment is greater than the slope of the first segment if both slopes are greater than zero; and if a), b)and c) are true: estimating, with the computer system, the Ct value of the data set using: Ct = x * + mean i = 1 ⁢ ⁢ … ⁢ ⁢ x * ⁢ ⁡ ( y i - ax i - b ) c - a ; and outputting the Ct value for display or further processing.