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Urine Specific Gravity: Normal Range, Fluctuations, and Causes

Urine Specific Gravity: Normal Range, Fluctuations, and Causes
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  • 0:01 The Color of Urine
  • 0:31 What Is Urine Specific…
  • 1:42 Increased Urine…
  • 3:41 Decreased Urine…
  • 5:06 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will discuss a test known as urine specific gravity. You'll learn the normal values of urine specific gravity as well as the major causes of its abnormal elevation or decrease.

The Color of Urine

Believe it or not, urine, normally a light straw color, is something that can come in many different colors, from almost as clear as water to dark yellow to white to red to almost black. Of course, as you can only imagine, most of those colors signify a big problem is going on.

It's important that you learn that even seemingly normal-looking urine, in terms of its color, may sometimes actually hide a big problem that can be diagnosed with a specific test we're going to go over.

What Is Urine Specific Gravity?

This test is known as urine specific gravity, and it is a test that measures the concentration of solutes in the urine. Solutes, in case you didn't know, are substances that are dissolved within something. For example, sugar becomes a solute when dissolved in water. In fact, sugar can actually sometimes be found in urine, as per the famous disease called diabetes mellitus.

Be that as it may, normal urine specific gravity, more quickly abbreviated as USG or SG, is approximately 1.005-1.030.

At this point, you should be wondering where in the world these numbers came from. It's simple, really. This test is comparing the density of urine to the density of distilled water, the latter of which is set at 1.000. So, water has a specific gravity of 1.000, but because urine is mainly water with some additional stuff like electrolytes dissolved within it, thereby increasing its density compared to pure water, USG can never be less than 1.000.

Increased Urine Specific Gravity

As a rough guide here, if you've ever had dark-colored urine, it's probably because your urine was denser or more concentrated, thereby increasing urine specific gravity.

The causes for a higher-than-normal specific gravity include:

Dehydration, which may be a result of improper fluid intake or loss due to vomiting or diarrhea. In these cases, the kidneys do everything they can to reabsorb as much water as possible back into circulation in order to maintain blood pressure. This leaves less water per amount of solute being excreted by the kidneys, thereby raising specific gravity.

Another cause of higher-than-normal specific gravity includes congestive heart failure. Since the blood isn't pumping blood properly in this case, the kidneys don't get enough blood delivered to them. This tricks the kidneys to (most likely) erroneously believe there's not enough fluid in the vasculature to maintain blood pressure. This activates an important system, called the renin-angiotensin-aldosterone system, which then forces the kidneys to reabsorb as much water back into circulation as possible, resulting in very concentrated urine.

Furthermore, diabetes mellitus can cause higher-than-normal specific gravity as a result of increased levels of glucose being excreted into the urine, thereby increasing urine concentration.

Finally, SIADH, or syndrome of inappropriate antidiuretic hormone secretion, can also cause elevated specific gravity. This is a condition where excessive antidiuretic hormone (aka vasopressin), a hormone that promotes water retention, causes USG to increase.

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