Can you help me interpret the results?

[Alfred001]

This was a C13 test, so I understand what's measured is the amount of C13 in the breath, I don't understand why the values are given with a - before them, can someone explain that?

Is it unusual that the value at T30 was lower than at T0? Could this indicate the test wasn't done properly?

[Helico_expert]

The measurement is quantative, not qualitative.
So the measurement is not accurate down to decimals.

Therefore, we normally just round up negative value to zero.
That's why some laboratory they want to release the raw data. Because that will only add confusion to doctors and patients.

if you are positive, then the difference will be very big.