Wouldn't that make the entire conceit of hair cell regeneration moot? If you don't know how much hair cells you regenerated, then why would any "upgrades" matter to people at all? If you can't tell, then you wouldn't know whether you've been duped or if you've actually regenerated your hearing.
Could 20,000 Hz be that new form of measurement? Only children can hear up to 20,000 Hz and it requires reaching to the end of the cochlea. Maybe that is what counts for "100% hearing"? That way you know for certain that you have all your hearing back (I would think).
I could hear 20,000 Hz at 28, and still had hearing in noise issues at that age. But, I digress.
The problem is establishing a reliable baseline to specifically know that certain cells regenerated from previous dead areas of the cochlea.
With the current testing, a baseline can be established with common clinical measures. Including: Audiogram (250 Hz - 8 kHz, Extended High-Frequency Audiogram (8 kHz - 16 kHz), Word-In-Quiet Score (50 Words), WIN/SIN tests (Words in varied noisy/multi-talk backgrounds). All of these figures can allow a clinician to conclude that a patient's hearing performance is normal, or some range below normal -> mild, moderate, severe in a number of categories: Pure Tone, Word Recognition, Words in Noise.
However, NONE of these tests can truly tell the clinician that the outer hair cells on the 250th-500th rows on the cochlea from the base are damaged and need targeted for regeneration. Or that perhaps the cells are fine, but the synapses are what are no longer connected to the cells. Or, perhaps that there is a combination of cell loss and synapse loss throughout the cochlea, but the synapse loss is greater, so the patient will benefit from a synaptophathy repair first. Or, perhaps there is no dead cell damage at all, or synaptopathy damage, but the patient has some highly worn cells in a key location that need to be observed. Really, what makes sense is a way to image the cochlea like we do the brain for patients with MS or other neurological conditions.
Back to the 20,000 Hz measurement problem. Currently, intratympanic drugs enter into the base of the cochlea first, which hits the highest frequencies first, and then work their way towards the apex, where low frequency detection exists. You might say, well wait a minute, so let's make 200 Hz the magic number. Well, problem is, some people's hearing wears from the high-frequency down, while others from various locations and outward. Since the condition is extremely heterogeneous and highly individual, so is the success criteria.
See example below: