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lascivious_boasts t1_j9pxpbj wrote

The short answer is that you are correct. Routine tests, including the rate of urine production, the ability of the body to clear waste products (creatinine, eGFR, urea) may all be normal in the case of only a single kidney working.

It's not very rare to find someone with one kid ey by accident when doing a scan. These are caused by congenital issues that led to a solitary kidney or an issue in early childhood that damaged one kidney and not the other.

Occasionally there will be mild derangement of some of those values that lead to the suspicion that a single kidney is not working and the other is working much better.

In this case there are some tests that can be used. As one of the main reasons for kidneys to fail is blockages in the arteries that feed blood into them, an arteriogram can show if one kidney is getting blood while the other isn't. This can be done with an intravascular catheter, but is more commonly done with CT and arterial phase contrast. If one kidney is getting lots of blood, and something is blocking the artery to the other (renal artery stenosis) then it's a fair bet that the one not getting blood isn't working well.

Equally, structural differences can show if there is a blockage in the outflow of one kidney and not the other (unilateral hydronephrosis, where the kidney retains urine due to the pressure needed to push it through a narrow ureter).

Finally, the test that really shows this is a differential renal function test. This can be done in a variety of ways. One includes injecting a dye and seeing how much comes out of each kidney by CT scanning and looking at how much dye is in each kidney. Another involves the differential clearance of a radioactive marker.

As to why we have two of some organs and one of others: it's just evolution in action. These body shapes and functions arose and persisted because animals with these survived and thrived when others didn't.

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FineLetMeSayIt OP t1_j9qbfjw wrote

Thank you for such a wonderfully detailed response! I guess barring an anomaly on your tests, doctors would never think to look for single sided kidney failure. I don't know enough about kidney diseases, but I thought about this because of a coworker/acquaintance who currently has to undergo dialysis. In laymen's logic, it would seem unlikely BOTH of her kidneys failed at the same time. Which means at some point one kidney was doing the job of two, but nobody noticed.

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lascivious_boasts t1_j9qe4vg wrote

It's actually much more common for both kidneys to fail together.

The underlying causes of kidney failure tend to be systemic (that is affecting the whole body) rather than local.

The big one is diabetes, and barring a blockage in blood supply to one kidney, the main damage is in all the tiny blood vessels that feed each nephron. This usually happens equally between both sides (although occasionally the renal artery stenosis is more of a problem, in which case one side can get a stent to try to maintain its function).

Other big causes are inflammatory/autoimmune. This means anywhere there's a kidney cell/structure the damage occurs. Broadly, this affects both sides at the same time.

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fiendishrabbit t1_j9tmiei wrote

Though not always. Had a colleague with hereditary PKD and an asymmetric progression (left kidney, non-functional and a huge amount of scar tissue. Enough that they had to remove it when transplanting. Right kidney had been pulling all the weight for the last few years and was now failing as well).

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puahaha t1_j9qgvnh wrote

Kidney failure is also sinister because you can be asymptomatic even while function is declining until it's really bad, like less than 20~30% functionality. So not only is compensatory action like one kidney fulfilling the role of the other at play, both will have to fail to a very significant degree before you even notice.

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paceminterris t1_j9t2ctd wrote

While it is true that it is rare to "sense" kidney function decline, it is a trivial matter to detect with blood and urine tests. If you get a blood test every year as part of routine screening, you will catch kidney disease as soon as it begins.

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wannabe-physiologist t1_j9tkqe3 wrote

Unilateral kidney failure is possible in cases of unilateral nephrolithiasis (kidney stones) however this would be painful enough to prompt someone to go to the ED where the imaging and labs others have mentioned will be done.

In the case of unilateral nephrolithiasis you would expect to see a patient with unilateral flank pain, CVA tenderness, and the urine studies would show red blood cells in the urine. This isn’t technically kidney failure per se, but is one of the ways someone without access to care may develop it.

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