The most sensitive test that obstruction and renal limitation is occurring is the urine concentrated specific gravity in a first morning specimen with a density below 1.010, serious when1.006.

Hello. The glomerular filtration rate (GFR) is accepted as the best index of kidney function.

Sincerely.

The mechanism of renal failure in BPH is obstructive uropathy causing bilateral hydronephrosis in the kidneys . This occurs in the late stages of BPH . The other mechanism could be acute pyelonephritis secondary to lower urinary tract infection due to obstructive uropathy . If there is renal failure in BPH & is not due to obstruction or UTI , then other causes should be excluded ( such as Diabetes or Hypertension ) .

The most sensitive test of renal function is the urine specific gravity when pushed to its highest by need for fluid conservation, best measured in the first morning sample. As long as it is 1.016 or higher, all is well. If it close to 1006, severe obstruction is present and the urine will have lost its color. This colorless urine should be the tip off to the patient, the specific gravity to the urologist.1

Obstructive uropathy due to BPH produces chronic interstitial nephritis with renal tubular dysfunction . The tubules lose the ability to concentrate urine by not responding to ADH , due to nephrogenic diabetes insipidus . The specific gravity of 1010 suggests an urine osmolality of 300 mosm . The normal urine osmolality ranges from 100 - 1200 mosm . In addition , the kidneys also lose the ability to conserve salt , producing salt losing nephropathy . I agree with Richards P. Lyon about the role of measuring urine specific gravity , which is more simple than measuring urine osmolality .

  The daily osmolar load on kidney is 10 mosm / kg coming from protein metabolism . Therefore , in a 60kg individual , the osmolar load would be 600 mosm . If the urine osm is 300 mosm ( specific gravity : 1010 ) , he would require 2 litres of urine output to eliminate the osmolar load . ( 600/300 = 2 L ) . If it is 100 mosm , it would need 6 L ( 600/100 + 6 L ) . The patient would present with polyuria . The patient loses the ability to conserve water in summer & would develop hypernatremia .

  Chronic Interstitial nephritis causes CKD due to decine in GFR & renal function has to be measured by e-GFR to stage severity of CKD .

Osmolarity has always been a confusing gimmick used by "experts" to stay so. In clinical use urine specific gravity or densiity is its equivalent and doable, as long as an accurate refrctometer is used. But the meaning of a random 1.006 density is always believed to indicate that the renal tubule has the power to dilute is and that a fixed 1.010 is always the endpoint of renal function is missing the fact that with obstructive, not real vascular disease, 1.006  or less is the end point ofrenal function as with serious Posterior Urethral Valves.The texts still have not recognized this. Will they ever?

All the replies before are obviously correct, but I think that the suggested many of the suggested indices could represent already stages of kidney damage du to the progressive obstruction ,  whose appearance I believe should be better to avoid, even it's well known the very frequent full recovery of renal function also in advanced stage of damage once the obstruction would be removed. So I believe that once a prostatic hypertrophy had been ascertained and submitted to a medical therapy, over the many suggested controls, a periodic renal echography  should be suggested, having  care to particularly control the  calyces conditions, whose enlargement in presence of a correct medical conservative treatment  should  have to suggest an urological surgical  treatment to eliminate the obstructive progressing damage .     .