Free/total PSA ratio
What it measures, and what changes the number.
This value compares the amount of free, unbound PSA with the total PSA in the same sample. It is usually expressed as a percent. It is a calculated result, not a separate measurement.
It is read together with total PSA and free PSA, which are the two numbers used to produce it. It is most often reported when total PSA falls in a range where extra information is sought.
Because free PSA degrades faster than bound PSA, a delay in processing the sample can shift the ratio. Ejaculation, cycling or a prostate examination shortly before the draw can also affect PSA values. Both measurements need to come from compatible assays, so ratios from different labs may not match. Spanish reports may print it as Índice PSA libre / PSA total.