Question: What do the "BUN" and "creatinine" lab tests mean? -R.W., Philadelphia

Answer: BUN stands for "Blood Urea Nitrogen," and represents a breakdown product of protein digestion. Protein is digested into amino acids. Amino acids contain nitrogen, which is split off to form ammonia waste, while the rest of the amino acid is used to provide calories (fuel) for your body.

The liver helps in this protein breakdown, eventually combining the ammonia waste to form the main waste product of protein breakdown: urea. Urea is released by the liver into the blood stream as blood urea nitrogen (BUN). It's then filtered and removed by the kidneys, ending up in our urine.

Creatinine has to do with muscle. With use, muscle cells constantly die and are replaced with new ones. Under normal situations, muscle cells die at a pretty constant and predictable rate. When muscle cells break down, creatinine is the end waste product. Under normal conditions, creatinine formation and its blood concentration are actually pretty constant.

Doctors use these BUN and serum creatinine numbers to evaluate kidney function. The BUN level can rise or fall independent of how well the kidneys are functioning, so doctors look not only at the BUN and creatinine values, but also the ratio of BUN to creatinine.

BUN elevation out of proportion to the creatinine level might indicate a dehydrated state; gastrointestinal bleeding; heart attack; urinary tract obstruction from prostate tumor or kidney stone; shock; excessive protein intake; excessive protein breakdown from starvation; or congestive heart failure. Lower than normal BUN values are seen in liver failure, malnutrition; low protein diet; and over-hydration.

Creatinine is produced at a fairly constant rate, so a sudden rise in its level indicates a large decline in kidney function. The normal serum creatinine level can vary depending upon how muscular a person is.

A creatinine level of 1.4 in a bodybuilder may be normal, but may represent markedly decreased kidney function in a petite elderly woman who might be expected to have a creatinine level of 1.0 or less. Because kidneys try their best to get rid of waste when kidney function declines over time, a lot of kidney function can be lost before you'll see a rise in the creatinine and BUN levels.

Question: Have you ever heard of a condition called "double crush syndrome"? I was recently at my chiropractor and he thinks I have it. -J. H., Marietta, Ga.

Answer: Double crush syndrome is an interesting and controversial concept where one nerve condition is linked to another nerve condition. For example, someone may develop the wrist condition of carpal tunnel syndrome not just because of direct compression of the median nerve at the wrist, but because of a previous or co-existing compressed/pinched nerve condition in the neck that is affecting overall function along the nerve branches in that limb. The belief is that if a pinched nerve in the neck or a herniated disc problem is not corrected and the nerve is not healed, the flow of nutrition and other chemicals down the nerve is impaired. For this reason, it's important to look beyond the obvious injury and explore for any spinal condition in the neck or back that might be related. That's where treatment approaches like chiropractic care, physical therapy with modalities like neck traction, epidural steroid injection or neurosurgical evaluation can be helpful on a case-by-case basis.

Dr. Mitchell Hecht is a physician specializing in internal medicine. Send questions to him at: "Ask Dr. H," P.O. Box 767787, Atlanta, GA 30076.