The time it takes for the serum PSA to double (PSA doubling time, PSADT) can be helpful in monitoring a patient with prostate disease. The assumption is that the increase in PSA follows a positive exponential course.
The patient should have several measurements (4 or 5) over a period of time with several months between each measurement.
serum PSA now =
= (initial PSA at time 0) * EXP(k * (time interval in days))
If the serum PSA doubles then
(serum PSA now) / (initial PSA at time 0) = 2 =
= EXP(k * (doubling time in days))
and
LN(2) = k * (doubling time in days)
So that:
PSA doubling time =
= LN(2) / SLOPE(line for plot of LN(PSA) plotted as y axis vs time in days)
where:
• For the doubling time of CEA in Chapter 27 I used LOG10.
Alternatively:
k =
= (LN(final PSA) - LN(initial PSA)) / (time interval in months between the 2 values)
k =
= LN(2) / (doubling time in months)
If these 2 equations are combined:
PSA doubling time in months =
= LN(2) * (time interval in months) / (LN(final PSA) - LN(initial PSA))
Limitations:
• PSA assays may be variable, especially after an interval of several months.
• The variability in the rate of PSA increase may affect the results.
