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BUN-to-creatinine ratio

Talked to my meds doc today, and he said that I should be looking at the BUN (blood urea nitrogen) in my lab results, in addition to the creatinine, GFR, and sodium levels. He says that the BUN is a good measure of dehydration, and that the creatinine level should be considered in relation to it, rather than by itself.

I know nothing about BUN, but WebMD says,
A blood urea nitrogen (BUN) test measures the amount of nitrogen in your blood that comes from the waste product urea. Urea is made when protein is broken down in your body. Urea is made in the liver and passed out of your body in the urine.

A BUN test is done to see how well your kidneys are working. If your kidneys are not able to remove urea from the blood normally, your BUN level rises. Heart failure, dehydration, or a diet high in protein can also make your BUN level higher. Liver disease or damage can lower your BUN level. A low BUN level can occur normally in the second or third trimester of pregnancy.

Blood urea nitrogen to creatinine ratio (BUN:creatinine)
A BUN test may be done with a blood creatinine test. The level of creatinine in your blood also tells how well your kidneys are working—a high creatinine level may mean your kidneys are not working properly. Blood urea nitrogen (BUN) and creatinine tests can be used together to find the BUN-to-creatinine ratio (BUN:creatinine). A BUN-to-creatinine ratio can help your doctor check for problems, such as dehydration, that may cause abnormal BUN and creatinine levels.

Why It Is Done
A blood urea nitrogen (BUN) test is done to:
  • See if your kidneys are working normally.
  • See if your kidney disease is getting worse.
  • See if treatment of your kidney disease is working.
  • Check for severe dehydration. Dehydration generally causes BUN levels to rise more than creatinine levels. This causes a high BUN-to-creatinine ratio. Kidney disease or blockage of the flow of urine from your kidney causes both BUN and creatinine levels to go up.

So now I'm going back to look at those old lab results & listing them again, this time with the BUN info included.

Lab Result Timeline

8/30/10: creatinine 1.19 (a little high), eGFR 50 (low), blood sodium 143 (normal), BUN 11 (normal), BUN-to-creatinine ratio 9 (low normal)
3/9/11: creatinine 1.25, eGFR 47, blood sodium 141, BUN 12 (normal) , BUN-to-creatinine ratio 10 (low normal)
8/17/11: creatinine 1.24, eGFR 54, blood sodium 141, BUN 14 (normal), BUN-to-creatinine ratio 11 (low normal)
12/20/11: creatinine 1.26, eGFR 53, blood sodium 141, BUN 12, BUN-to-creatinine ratio 10 (low normal)
6/26/12: creatinine 1.23, eGFR 54, blood sodium 138, BUN 15 (normal), BUN-to-creatinine ratio 12 (lowish normal)
4/11/13: creatinine 1.53, eGFR 41, blood sodium 142, BUN 15, BUN-to-creatinine ratio 10 (low normal)
6/3/13: creatinine 1.41, eGFR 46, blood sodium 139, BUN 13, BUN-to-creatinine ratio 9 (low normal)
6/11/13: creatinine 1.65 (quite high), eGFR 38 (quite low), blood sodium 139 (normal), BUN 15, BUN-to-creatinine ratio 9 (low normal)

* reduced lithium dosage *

6/21/13: creatinine 1.40, eGFR 46, blood sodium 138, BUN 11 (normal), BUN-to-creatinine ratio 8 (low)

* completely off lithium *

8/8/13: creatinine 1.33, eGFR 49, blood sodium 139, urine sodium <30 (low), BUN 14, BUN-to-creatinine ratio 11 (low normal)
12/30/13: creatinine 1.57, eGFR 40, blood sodium 143, BUN 10, BUN-to-creatinine ratio 6 (quite low!)
1/7/14: creatinine 1.60 (quite high), eGFR 39 (quite low), blood sodium 142 (normal), BUN 9, BUN-to-creatinine ratio 6 (quite low!)

Okay, so, here I do indeed see a dehydration-related change that may be worth investigating. In June 2013, when I was still on lithium, my creatinine was high (1.65), but my BUN was perfectly normal, and my BUN-to-creatinine ratio was still (barely) within the normal range, but now (as of 1/7/14) my BUN-to-creatinine ratio is very low. So my sodium levels (both blood and urine) may be the same now as they were then, but my BUN-to-creatinine ratio has definitely changed, so my level of dehydration is different. (Maybe my nephrologist could have told me this?)

Regarding the BUN-to-creatinine ratio, Wikipedia says:
The principle behind this ratio is the fact that both urea (BUN) and creatinine are freely filtered by the glomerulus, however urea reabsorbed by the tubules can be regulated (increased or decreased) whereas creatinine reabsorption remains the same (minimal reabsorption).
Um. Okay?

Wikipedia also describes a BUN-to-creatinine ratio of less than 10 to be indicative of "renal damage [which] causes reduced reabsorption of BUN, therefore lowering the BUN:Cr ratio."

WebMD says, "A low BUN-to-creatinine ratio may be caused by a diet low in protein, a severe muscle injury called rhabdomyolysis, pregnancy, cirrhosis, or syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH sometimes occurs with lung disease, cancer, diseases of the central nervous system, and the use of certain medicines."

WebMD also says, "Dehydration generally causes BUN levels to rise more than creatinine levels. This causes a high BUN-to-creatinine ratio. Kidney disease or blockage of the flow of urine from your kidney causes both BUN and creatinine levels to rise." Okay, so I'm supposedly dehydrated, and yet my BUN levels have remained pretty stable while my creatinine has gone up, and my BUN-to-creatinine ratio has thus gone down. Huh?

The Quest Diagnostics website says, "In most cases of chronic renal disease the ratio remains relatively normal." Hmm. Mine is not. I wonder why.

I'm still terribly confused. My meds doc said that he noticed I wasn't actually getting particularly anxious about the whole thing, which impressed him mightily. I'm frustrated & confused & concerned, but I don't feel particularly anxious.

And may I just say that it sucks trying to figure out all this garbage on my own? Sheesh. Where's a doctor when you need one? Oh, right, she's just telling you to eat salt.

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