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.
Decreased Urine Specific Gravity
Contrarily, USG can be abnormally low as opposed to abnormally high, resulting in clear-colored urine. Now, the urine may be essentially clear-colored if you've had too much water to drink, but in this case, the specific gravity may or may not be within the normal range. Therefore, solely using urine color as an estimate of proper or improper specific gravity is bound to mislead you at one point or another. That's why we run all of these tests instead!
If on the test, the USG comes back as abnormally low, this may be due to:
Diabetes insipidus. This is not related to diabetes mellitus, so don't get confused. Diabetes insipidus is a condition where large volumes of dilute urine are passed as a result of inadequate levels of or lack of a kidney response to antidiuretic hormone. Since vasopressin concentrates urine, its absence or lack of effect causes water to just pour out of the body.
Another reason for low specific gravity is primary kidney disease, such as glomerulonephritis, pyelonephritis, and acute tubular necrosis. In these kidneys, internal kidney disease disables the kidneys' abilities to properly reabsorb water, resulting in dilute urine.
With this lesson in your pocket, you're an expert in using urine specific gravity to help diagnose medical problems! USG is a test that measures the concentration of solutes in the urine. Normal USG is approximately 1.005-1.030.
Reasons for abnormally high USG include dehydration, congestive heart failure, diabetes mellitus, and SIADH, or syndrome of inappropriate antidiuretic hormone secretion.
On the other hand, abnormally low USG is a result of excessive intake of water, primary kidney disease, or diabetes insipidus, a condition where large volumes of dilute urine are passed as a result of inadequate levels of or lack of a kidney response to antidiuretic hormone.
After this lesson, you'll have the ability to:
- Explain what the urine specific gravity, or USG, test measures
- Identify normal USG levels
- Describe reasons for abnormally high and abnormally low USG