Need Help Interpreting Specific Gravity

[willowita, ADN, RN]

I would expect a high SG for someone with a worsening kidney disease. Especially if proteins are present in the urine. But is there a reason they would have SG within the normal range? I don't understand why SG would be normal.

[hodgieRN]

You are right that the SG would be higher with kidney disease. SG is basically a measurement of water vs. particles. It could be argued that the kidneys are not only excreting water, but also not excreting proteins, solutes, electrolytes. So, there is a kind of a "false" normal. SG doesn't specifically test kidney function compared to the Glomerular Filtration Rate (GFR). SG only measures the ratio of solutes in a solution...the gravity of the particles in urine. If the particles aren't even going through, then the SG could be normal (that's what I'm thinking).

There's many different ranges of kidneys disease and there are some pt's who mainly produce water, but lack the capabilites of excreting proteins such as pt's with chronic renal insuff. vs. a dialysis pt with full blown failure. Water can pass via osmosis b/c H2O molecules are much smaller than the larger creatinine cells or myoglobin cells. The filter can only handle smaller particles and that might give a specific gravity much lower or even normal than expected. Hope this helps

[Esme12, ASN, BSN, RN]

Here is a great site explaining lab tests.......Lab Tests Online: Welcome!

[nurseprnRN, BSN, RN]

Remember also that renal failure comes in stages-- when the kidneys just begin to fail, they lose their ability to concentrate urine. A lot of water passes through them and into the bladder, so there's a lot of urine but it's of low quality, if that makes any sense. You'll see a very low SG in that case.

Another great book is Joyce LeFever Kee's classic "Laboratory and Diagnostic Tests With Nursing Implications." That last bit is important, because you will need to know the whys and wherefores for NCLEX...and for work as a nurse.