Hi Hyw987
There is some confusion about what 'undetectable' means: 'Undetectable' does not mean it is zero. The accuracy of the measured PSA depends on the laboratory/equipment which is used. Ideally a laboratory, when quoting a PSA, should use the sign '<' (less than); this indicates the accuracy of the measuring instrument. Typically, when I get my result it states < 0.003, signifying that the equipment they use is accurate to three decimal places. Quite often the term 'undetectable' is used quite loosely, implying that it is zero.
In your case the rise from 'undetectable' to 0.1 does not give you enough information to make a judgement. Apart from the importance of the '<' sign, it is important to watch the trend, obviously a single reading can be quite misleading. Also it is important that every time you have a PSA test the blood sample should be analysed by the same laboratory, which may not always be the case.
Incidentally, depending on the consultants/hospitals, PSA of < 0.1 or < 0.2 is often used an indicator for further treatment.
Generally the NHS measurements are not as accurate as those provided by some of the private clinics which are super sensitive.
I suggest you have you PSA measured every three to six months and observe the trend.