Question:

Here is information about the Sloan-Kettering online calculator. It is possible that it applies to other PSA doubling-time calculators.: Who Can Use This Tool? For Those Who Have Not Been Treated The tool is designed to be used by patients who have had a biopsy of the prostate confirming the presence of cancer. The biopsy must have had a Gleason grade assigned to it. The patient must also have had a PSA blood test performed, either prior to or not within 10 days following the biopsy, and not after the start of hormonal therapy if hormones have been started. The patient must also have been assigned a clinical stage using either the 1992 or 1997 clinical staging systems. The tool can also be used by patients who are considering hormone refractory therapy. The tool cannot be used for patients whose prostate cancer is believed to have spread to other parts of the body, such as bones or lymph nodes. For Patients Who Have Been or Are Receiving Treatment The tool can be used by patients who have received prostatectomy to treat the prostate cancer within the past 6 months. This tool is not for patients who have received hormonal therapy or radiation therapy either prior to or following surgery. This tool is also not for the patient who had surgery longer than 6 months ago.

Response:

On Christmas Eve, Warren inquired: Is there some kind of formula that is used to determine one’s PSA doubling time? Well, a kinda scary response has been posted by MAJ. I don’t think it’s all that complicated, and requires little but basic arithmetic skill.

I’m afraid it is exactly as complicated—or simple, depending on your perspective—as MAJ described.   Anyone with a scientific calculator with a log key can use the formula he gave for two measurements.   For three or more measurements it is more complicated.   As he says, if the PSA is following the model, then you could use a least squares line for log P, but most lay people wouldn’t know what that means and wouldn’t have the tools to do it.   An alternative would be to use the formula with different pairs of values and estimate the doubling time for each such pair.  Then you could compare the values you got.  If they showed very different values, then that would indicate the model probably didn’t apply.  If they showed similar but different values,  that could give you some estimate of the approximate doubling time. It is important to remember in all this, as my urologist likes to say, it isn’t rocket science.   Gross differences in doubling times can mean something, but relatively small differences are probably meaningless. Your doctor should have software tools available to indicate a doubling time if he is going to use it in diagnosis.   But it would only be one tool and more likely than not, just a glance at the figures would give him enough information, together with the other data, to help draw a conclusion. Here’s the definition of PSADT from the website of the Prostate Cancer Research Institute: "PSA doubling time (PSADT): the calculation of the time it takes for the PSA value to double based on at least three values separated by at least three months each; before diagnosis, a PSADT of less than 10 years may be an indication of the presence of PC."

This is a definition, but it doesn’t tell you how to find the doubling time from values if the ratio of the values during the time interval is not a doubling. – Hide quoted text — Show quoted text – And from "What Every Doctor Who Treats Male Patients Should Know," a May 2005 paper by Strum and Pogliano that all of us should copy and give to our primary care physicians. "PSA velocity (PSAV) and PSA doubling time (PSADT) are important markers that can indicate the existence of prostate cancer. Blood sampling for PSA determinations, done at least three months apart, and by the same laboratory using the same testing procedure, are necessary to establish PSAV and PSADT. The validity of such determinations is increased if such testing involves at least three determinations over an 18-month span of time. However, a progressive and serial increase in PSA values should raise concern that prostate cancer is present and that a greater degree of vigilance is mandatory.