problem with diagnosing kidney disease, and then classifying symptoms accodring to gfr values and stages, is that it leaves out the fact that GFR measures the glomerular filtration of creatinine (or Cystatin C) ONLY.
I think almost all the symptoms associated with CKD and kidney damage are rather caused by the reduced clearance of uremic toxins, which are mainly cleared through tubular secretion, NOT glomerular filtration.
For example, in CKD-associated pruritus the activation of PAR-2 has been recently purported to be one main factor behind the subjective symptoms that are often reported to be felt in CKD patients, to a varying degree:
Uremic solutes of indoxyl sulfate and p-cresol enhance protease-activated receptor-2 expression in vitro and in vivo in keratinocytes
Increased Levels of Total p-Cresylsulfate Are Associated with Pruritus in Patients with Chronic Kidney Disease
Turns out water-based uremic solutes such as p-cresylsulfate and indoxyl sulfate are excreted through renal tubular secretion, therefore have little to do with glomerular filtration, which filters most of the serum creatinine and cystatin C, and hence, the GFR value and subsequent CKD stage).
In generalized declined CKD, most often glomerular functions and tubular secretory abilities goes more or less together as far as their capacity for excreting the renal metabolites into the urine is progressively reduced (classified into CKD stages and corresponding symptoms).
But, what in the case where tubular secretion mainly is greatky diminished, such as as a result of acute tubular injuries going chronic (from nephrotoxic drugs), or tubulointerstitial fibrosis, but GFR remains high, because the glomeruli would have been relatively spared from the drug-induced tubular damage (common and main mechanism of drug-induced nephrotoxicity)?
Ofc I'm setting myself as a possible example (lol), but I've been allegedly suffering from a couple drug-induced nephrotoxic injuries (not related to aas or hypertension); as a result I have persisting symptoms such as generalized prutius (mild), unspecified lethargy/fatigue that I developped, metallic taste in mouth...,
despite GFR being consistently normal on tests (I've done creatinine and cystatin C) over the years, as well as consistently normal 24-hour urine, consistently normal albumin-to-creatinine ratio, and other flurries of tests repeatedly done,..., being normal as well,
I might still be suffering from pruritus, in the possible eventuality it's my tubular secretion functions that are more greatly reduced from the injuries, rather than overall glomerular function, therefore allowing some uremic toxins to remain unfiltered in the blood (tubular secretion into urine), thus allowing pruritus symptoms to be felt in me, despite all my other lab values being normal.
Only my anaecdotal hypothesis obviously, but could be a thing.