In biology, hierarchical clustering is used to separate studied samples into different groups, thus if successful, the clustering criteria can be seen to "discriminate" between group A vs. B.
I am stuck trying to find an appropriate word to convey that two samples are so similar that they are "indiscriminable". It appears as that's not a word featured in dictionaries. I have tried "indiscernable" and that doesn't sound right either, neither does "inseparable".
What is a good adjective to use in this case? Should I try to rephrase my sentence?
PS: Please consider that this question refers to academic writing (in other words, "formal register" only).