Free PSA
What it measures, and what changes the number.
PSA (prostate specific antigen) is a protein made by the prostate gland. In the blood, most PSA is bound to other proteins, while a smaller portion circulates unbound. The free PSA test measures only that unbound portion.
Free PSA is almost always ordered with total PSA, because it is mainly used to calculate the free to total PSA ratio. It is rarely read on its own.
Free PSA is less stable than total PSA, so labs usually separate and analyse the sample promptly. Recent ejaculation, vigorous cycling, or a prostate examination before the blood draw can change PSA levels. Different assays can give different results, which is why serial results are easiest to compare when they come from the same lab and method. Spanish language reports may print it as PSA libre.
Often read alongside: Total PSA, Free/total PSA ratio