Prostate specific antigen (PSA) has a half-life in the serum. Sometimes it is useful to measure the decline in total PSA concentrations over time.
Situations where the half-life of PSA may be of interest:
(1) after total prostatectomy: after complete removal of prostate tissue the PSA should reach 0. If there is persistence of PSA then a metastasis may be present, or the resection was incomplete.
(2) after an insult causing a transient rise in PSA (digital rectal exam, exercise, needle biopsy, etc)
The kinetics of PSA clearance has been described as:
(1) first-order
(2) two-compartment (biphasic), with alpha and beta phases
(2a) for free PSA the alpha phase is about 1 hour and beta phase 19 hours
(2b) for total PSA the alpha phase is 1.5 hour and beta phase 2.7 days
Parameters:
(1) number of days since event
(2) half-life of total PSA (2 to 3 day)
number of half-lives =
= (number of days) / (half-life)
percent of additional PSA present =
= EXP((-1) * (0.693 * (number of half-lives since event)
current total PSA =
= (background total PSA) * (1 + (percent additional PSA))
background total PSA =
= (current total PSA) / (1 + (percent additional PSA))
Normally it takes 2-3 weeks after an event before the serum PSA level has totally cleared.
