With PSA, it is overall trends and results in a certain timescale that provide better indications of the way things are going rather than one or two PSA tests in isolation. Infections can affect results as can other things, sex not long before blood is taken for a test or certain strenuous activities for example. Then like good cells, some cancer cells die and divide, sometimes further mutating in the process. Furthermore, some cancer cells give rise to greater or lesser PSA at any particular time and this can vary. The upshot of all of this is that quite often even men with a fairly stable PSA history may find that it fluctuates, ie PSA rises and falls from one test to the next and in itself may not be of any significance. However, a large one off increase where infection has been ruled out as the cause, may well need to be further investigated.