ALT level in cat

Question
1 month ago my 7 year old cat who has absolutely no symptoms of liver disease had an ALT level of 262. The veterinarian had me change her diet to Hills L/D dry and wet. Three days ago on Friday, we retested her and the ALT was 320 and he said that we needed to do further blood tests on monday.
The very next day (Saturday)I went to another vet and her ALT level was 208.
I confirmed with the second vet that there are sometimes differences in points between machines but that a difference in a mear 24 hour period that went from 320 to 208 was impossible.
The second vet gave me antibiotics and "zendofil" to clean the liver and told me to come back in a week to retest.
I dont know which ALT level is accurate, I dont know what to think or who to trust.
Please help me.  

Answer
I would not trust IN HOUSE lab results if this is the problem.

Ask for blood to be sent to a lab (they are all country wide- Antech Diagnostics being the main one)

Denosyl (aka- Sam E ) is great for the liver.

which vet did you like more? thats what can define where you want to continue to stay.