Low urine specific gravity in dogs: what urine says about the kidney

What this marker measures

Urine specific gravity compares the concentration of the urine with that of pure water. It is measured on a refractometer, on a few drops. It reflects one precise function: the kidney's ability to hold on to water or to let it go, under the control of an antidiuretic hormone. A well hydrated dog that has not drunk for several hours normally produces concentrated urine. So dilute urine can mean two opposite things: either the dog has drunk a lot and the kidney is clearing the surplus, which is its job, or the kidney has lost its ability to concentrate and the dog drinks to compensate. This value only has meaning alongside the context of the sample.

What vets look into when the value sits outside the range

Ordered from the most common to the rarest.

The first two rows explain a large part of isolated low specific gravities. They are cleared by repeating the sample under good conditions, which avoids unnecessary tests.

What this value does not tell you on its own

A low specific gravity does not say whether the dog drinks too much because it passes too much urine, or passes a lot of urine because it drinks a lot. That is the whole question, and the report does not settle it.

What is read at the same time:

• When the sample was taken. A specific gravity measured on the first urine of the morning, before food and before any fluids, does not carry the same weight as one measured at the end of a consultation.
• Creatinine, urea and SDMA from the same panel, to place the kidney.
• The full urine dipstick: sugar, protein, blood, pH, plus examination of the sediment and a culture if the context justifies it. Sugar in the urine immediately changes the direction of the enquiry.
• Blood glucose, blood calcium, electrolytes, liver enzymes and alkaline phosphatase.
• The urine protein:creatinine ratio and blood pressure.
• Neutering status and the date of the last season in a bitch, with an ultrasound scan of the uterus at the slightest doubt.
• How much water is drunk over several days, measured at home. This is often the single most decisive piece of information, and it cannot be obtained at the practice.

A point of method. One low specific gravity is not enough to conclude anything, especially if the collection conditions are not known. Vets repeat the measurement on several morning samples, and compare with the reference values their laboratory gives for the species.

Questions to ask your vet

1. When was the urine collected, and had the dog drunk or been given fluids beforehand?
2. Should we repeat the measurement on several first-of-the-morning samples before going further?
3. What do the dipstick, the urine sediment and, if needed, the culture show?
4. Which blood tests would rule out diabetes, hyperadrenocorticism and a high calcium?
5. Could the medicines my dog is given explain this dilution, and how would we check that?

My dog had just had a drink, does that distort the result?

Yes, and it is the first explanation to rule out. A dog that has just drunk, that is on a drip, or that eats wet food produces dilute urine without anything being wrong. The most informative sample is the first one of the morning, before the food bowl.

Low specific gravity with a normal creatinine, is that reassuring?

That combination makes advanced kidney damage less likely but does not explain the dilution. The vet then looks at blood sugar, calcium, the liver, hormones and a possible upper urinary tract infection. Persistently dilute urine in a dog that drinks a lot is always documented, even with a clean blood panel.

How do I measure what my dog drinks at home?

Fill the bowl with a known amount, note what is left at the same time the next day, and repeat over three consecutive days, allowing for any other access to water. That figure, set against body weight, is data the vet cannot obtain in the consulting room, and it guides a great deal of what follows.