The most common and most likely answer to this crossword clue is the 8 letter word BIOASSAY
B
I
O
A
S
S
A
Y
Buddy explains!

The straight part
We know that "Lab test" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lab test = bioassay.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BIOASSAY mean?
BIOASSAY noun- The analysis of the biological activity of a substance, with reference to a standard preparation, using a culture of living cells, or a living creature.
- To carry out such an analysis.