How to Calculate the Percentage of PFS With Kaplan-Meier Curves

In medical statistics, "PFS" stands for "progression-free survival." This term states the probability of a given patient surviving with a disease that has stopped progressing. Biostatisticians can estimate the percentage of patients who experience progression-free survival using Kaplan-Meier curves, which use real data to estimate the survival curves for a set of patients.

Instructions

1. Plot the Kaplan-Meier curve. Set the curve on axes for which the X-axis represents time, usually in years, and the Y-axis represents the progression-free survival.

2. Choose the time for which you want to compute the percentage of progression-free survival. If you have no specific time preference and want to compute a general percentage for survival, look at the graph to find a time value at which the curve has a long horizontal run, or no vertical changes.

3. Draw a vertical line from the time value on the X-axis upward to the curve. Place a point at the place where your drawn vertical line intersects the curve. This point of intersection will correspond to a percentage of progression-free survival.

4. Draw a horizontal line from the point of intersection. Draw this line, starting from the point, all the way to the Y-axis.

5. Locate the value on the Y-axis at which your horizontal line hits. Multiple the value by 100. This is the percentage of progression-free survival.

References

• "Using and Understanding Medical Statistics"; David Matthews, et al.