I have a bitch who has been unsuccessfully bred twice before so we are trying progesterone testing to optimise our chances of a litter, She was tested Monday (not 100% confident on the result as they said they had made a mistake and had to rerun test) and the levels were undetectable. Had her then tested by different person yesterday and she had shot up to 17.3. Retest today was 17.4. How would you interpret this? Retest when?

Bitches ovulate around 18nmol, so she probably has just ovulated.

Peak fertility would be 2 - 4 days after that level.

Some bitches stall at pre ovulation levels, but that is pretty much ovulation so, no need to retest, unless you want to after mating to ensure levels are continuing to rise to maintain a pregnancy.

If you've tried to breed her twice before and missed, she might be doing something atypical really (like hanging at a certain number and not actually ovulating). If I were you, and if the stud dog is nearby and available, I'd try to get a mating in 2 days time and also re-test her then.

Then you will hopefully get a definite 'ovulation occurred' result.

This is important because ovulation occurs between 12.7 up to 25.4nmol - it can vary, dog by dog. The only way we know it did occur, is that there was a 9.5 point rise in 24 hours. You are looking for that massive sudden leap in progesterone, not a specific number. So you will only get that if you test again.