Urine SG Misconceptions and Myths

From episode 75 on the medicine feed. With Prof Jill Maddison.

I recently had a case that reminded me of these very important pearls about what I used to believe USG was telling me, and what we can actually deduce from it: 

• "Normal" USG for a dog can be anything, depending on the physiological state the animal is in.

• In an azotaemic animal, to be absolutely sure that the kidney is functioning normally, the SG needs to be:  

  • Dog: > 1.030 – 1.035  

  • Cat: > 1.035 – 1.040

• If we have azotaemia with an appropriate SG the animal is either:  

  • Dehydrated – most likely. 

  • Has something affecting renal perfusion (e.g. heart failure)  

  • Has something affecting the osmolarity (e.g. some diabetics where the glucosuria affects the osmolarity)  

  • Is a greyhound 

• ISOSTHENURIA: 1.008 – 1.012. Means that the glomerular filtrate hasn't been concentrated nor diluted during its passage through the kidney. This MIGHT indicate renal disease. (Listen to the full episode to understand why she says MIGHT. There are some very important exceptions.) 

  • MYTH: For it to be renal failure, the SG HAS to be isosthenuric.

  • FACT: You can have dog that have renal failure with a specific gravity of 1.020. You can have cats with renal failure with a specific gravity up to 1.030, where there is some degree of concentration, but it may not be appropriately concentrated in that animal.

• HYPOSTHENURIA USG < 1.008

  • MYTH: An animal with dilute urine (hyposthenuria) can have structural renal disease.

  • FACT: It takes the same number of functional nephrons to actively dilute urine as it does to concentrate it. If an animal is PUPD or is azotaemic with  a USG  of less than 1.008, the animal does not have structural renal disease.

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