Urine Specific Gravity, Phosphorus, and Testing for Kidney Disease

Myrna Loy

The vet also suggested that it is possible that Myrna might have a normally low specific gravity, low due to her heart meds that encourage excretion as well as the effects of the aspirin. The theory is that since her blood work has for the past year or so shown her albumin to be high, that indicated a bodily dehydration level. At the same time the urine specific gravity was high. It may have been high because the kidneys were concentrating-keeping-the fluid from the body in the kidneys for a few hours before forcing her to urinate.  But lately, the blood work has shown a more normal level of albumin meaning that she’s well hydrated.  But with the body well hydrated-supposedly due to drinking a lot of water due to the high sodium in SO-the kidneys did not need to concentrate the urine and normal excretion occurred every 2 hours. 

The role of phosphorus is also very important indicator for kidney function, as mentioned earlier when discussing the GFR test.  Phos in the body along with calcium keeps bones and teeth strong.  It does not cause a problem until kidney disease is present. Kidney function is measured by the estimated glomerular filtration rate.  As kidney failure progresses, the amount of fluid the kidney can process slows and at this point, the kidneys can't adequately filter out phosphorus. High phosphate levels can cause calcium and phosphorus to form deposits in the heart, lungs, joints and skin that affect organ functioning.

One thing I learned that surprised me was that one cannot test to see if kidney disease is developing.  It’s either there or not there.  One cannot catch it and stop it or limit it. One can only discover its existence and then mitigate the situation based on what the cat needs. Blood work and scans will tell you how far along is the disease but won’t show you that it’s just beginning. By the time it shows up in blood work, the kidneys have lost about 70% of their function.  So, today kidney disease may not be present. Tomorrow, it may suddenly be there.  The best we can do is get the base readings from the tests last week at MSU and use them to compare against for the future if we run them again next year or any time we think Myrna might be suffering from kidney disease. 

So, there is a lot to think about concerning Myrna: change her food but not do away with SO entirely; possibly cease aspirin but discuss it with the cardiologist first; keep an eye out for hepatic failure due to Valium; work to somehow keep up hydration levels without SO; and watch for other signs in blood work and other tests for kidney failure.  I learned a lot about how the kidneys work and more about the relationship and meaning of blood values.  The MSU vet also suggested we test Myrna’s urine over the next few weeks at the same time and then at different times of the day to see if the urine specific gravity alters at different points in the day or remains the same and is the same low level now or becomes higher later.  And if higher, is she also presenting with a high blood albumin that shows she’s dehydrated or is the albumin normal.