OH had op on prostate in Aug. (all now well after a nasty 8 weeks) - he was sent to specialist after a blood-test ordered by his GP. He had, for a long time, answered the standard question about 'getting up in the night' very honestly with a 'No'. This turned out to be because his bladder had stretched enormously to accommodate the blocked flow of urine - it was about to burst and he had to be catheterised until the op.. Not a nice experience, but life-saving.
So, answer is that a blood-test indicated that something had to be done although I'm not an expert on exactly what it showed up. We are hugely glad that that blood-test was done.