EXTRAPOLATED AREA

You have found the area under the curve up until the last recorded time point (the last measurement was taken at 12 hours).

However, you still do not have the TOTAL area under the curve because the curve itself is not complete. To complete the curve, you need to extrapolate the graph as shown below.

The total area under the curve is the sum of the area up to and including the final recorded time point PLUS the area of the last extrapolated section - the final trapezoid to infinity.