ADS131A02: Using async slave mode without interrupts

Tool/software:

We're using the ADS131A02 in async slave mode. However, we would prefer not to use an interrupt; we would rather just let the device convert at a reasonable rate (1 or 2kHz) and then perform a transaction (NULL command) to read it when we need the readings. This seems to be working, however occasionally we find the F_DRDY bit is set in the returned STAT_1 register.

What exactly does this bit signify? When is it set, are the associated ADC readings still valid (i.e. from the most recent complete conversion - in which case we can ignore this status bit), or might some bits be from one conversion and others be from a different conversion (in which case we will need to perform another transaction, or re-use the previous reading we took)?